embedded SQLin C C ECPG This chapter describes the embedded SQL package for PostgreSQL. It was written by Linus Tolke (linus@epact.se) and Michael Meskes (meskes@postgresql.org). Originally it was written to work with C. It also works with C++, but it does not recognize all C++ constructs yet. This documentation is quite incomplete. But since this interface is standardized, additional information can be found in many resources about SQL. An embedded SQL program consists of code written in an ordinary programming language, in this case C, mixed with SQL commands in specially marked sections. To build the program, the source code is first passed through the embedded SQL preprocessor, which converts it to an ordinary C program, and afterwards it can be processed by a C compiler. Embedded SQL has advantages over other methods for handling SQL commands from C code. First, it takes care of the tedious passing of information to and from variables in your C program. Second, the SQL code in the program is checked at build time for syntactical correctness. Third, embedded SQL in C is specified in the SQL standard and supported by many other SQL database systems. The PostgreSQL implementation is designed to match this standard as much as possible, and it is usually possible to port embedded SQL programs written for other SQL databases to PostgreSQL with relative ease. As already stated, programs written for the embedded SQL interface are normal C programs with special code inserted to perform database-related actions. This special code always has the form: EXEC SQL ...; These statements syntactically take the place of a C statement. Depending on the particular statement, they can appear at the global level or within a function. Embedded SQL statements follow the case-sensitivity rules of normal SQL code, and not those of C. The following sections explain all the embedded SQL statements. One connects to a database using the following statement: EXEC SQL CONNECT TO target AS connection-name USER user-name; The target can be specified in the following ways: dbname@hostname:port tcp:postgresql://hostname:port/dbname?options unix:postgresql://hostname:port/dbname?options an SQL string literal containing one of the above forms a reference to a character variable containing one of the above forms (see examples) DEFAULT If you specify the connection target literally (that is, not through a variable reference) and you don't quote the value, then the case-insensitivity rules of normal SQL are applied. In that case you can also double-quote the individual parameters separately as needed. In practice, it is probably less error-prone to use a (single-quoted) string literal or a variable reference. The connection target DEFAULT initiates a connection to the default database under the default user name. No separate user name or connection name can be specified in that case. There are also different ways to specify the user name: username username/password username IDENTIFIED BY password username USING password As above, the parameters username and password can be an SQL identifier, an SQL string literal, or a reference to a character variable. The connection-name is used to handle multiple connections in one program. It can be omitted if a program uses only one connection. The most recently opened connection becomes the current connection, which is used by default when an SQL statement is to be executed (see later in this chapter). Here are some examples of CONNECT statements: EXEC SQL CONNECT TO mydb@sql.mydomain.com; EXEC SQL CONNECT TO unix:postgresql://sql.mydomain.com/mydb AS myconnection USER john; EXEC SQL BEGIN DECLARE SECTION; const char *target = "mydb@sql.mydomain.com"; const char *user = "john"; EXEC SQL END DECLARE SECTION; ... EXEC SQL CONNECT TO :target USER :user; The last form makes use of the variant referred to above as character variable reference. You will see in later sections how C variables can be used in SQL statements when you prefix them with a colon. Be advised that the format of the connection target is not specified in the SQL standard. So if you want to develop portable applications, you might want to use something based on the last example above to encapsulate the connection target string somewhere. To close a connection, use the following statement: EXEC SQL DISCONNECT connection; The connection can be specified in the following ways: connection-name DEFAULT CURRENT ALL If no connection name is specified, the current connection is closed. It is good style that an application always explicitly disconnect from every connection it opened. Any SQL command can be run from within an embedded SQL application. Below are some examples of how to do that. Creating a table: EXEC SQL CREATE TABLE foo (number integer, ascii char(16)); EXEC SQL CREATE UNIQUE INDEX num1 ON foo(number); EXEC SQL COMMIT; Inserting rows: EXEC SQL INSERT INTO foo (number, ascii) VALUES (9999, 'doodad'); EXEC SQL COMMIT; Deleting rows: EXEC SQL DELETE FROM foo WHERE number = 9999; EXEC SQL COMMIT; Single-row select: EXEC SQL SELECT foo INTO :FooBar FROM table1 WHERE ascii = 'doodad'; Select using cursors: EXEC SQL DECLARE foo_bar CURSOR FOR SELECT number, ascii FROM foo ORDER BY ascii; EXEC SQL OPEN foo_bar; EXEC SQL FETCH foo_bar INTO :FooBar, DooDad; ... EXEC SQL CLOSE foo_bar; EXEC SQL COMMIT; Updates: EXEC SQL UPDATE foo SET ascii = 'foobar' WHERE number = 9999; EXEC SQL COMMIT; The tokens of the form :something are host variables, that is, they refer to variables in the C program. They are explained in . In the default mode, statements are committed only when EXEC SQL COMMIT is issued. The embedded SQL interface also supports autocommit of transactions (similar to libpq behavior) via the -t command-line option to ecpg (see below) or via the EXEC SQL SET AUTOCOMMIT TO ON statement. In autocommit mode, each command is automatically committed unless it is inside an explicit transaction block. This mode can be explicitly turned off using EXEC SQL SET AUTOCOMMIT TO OFF. The SQL statements shown in the previous section are executed on the current connection, that is, the most recently opened one. If an application needs to manage multiple connections, then there are two ways to handle this. The first option is to explicitly choose a connection for each SQL statement, for example: EXEC SQL AT connection-name SELECT ...; This option is particularly suitable if the application needs to use several connections in mixed order. If your application uses multiple threads of execution, they cannot share a connection concurrently. You must either explicitly control access to the connection (using mutexes) or use a connection for each thread. If each thread uses its own connection, you will need to use the AT clause to specify which connection the thread will use. The second option is to execute a statement to switch the current connection. That statement is: EXEC SQL SET CONNECTION connection-name; This option is particularly convenient if many statements are to be executed on the same connection. It is not thread-aware. In you saw how you can execute SQL statements from an embedded SQL program. Some of those statements only used fixed values and did not provide a way to insert user-supplied values into statements or have the program process the values returned by the query. Those kinds of statements are not really useful in real applications. This section explains in detail how you can pass data between your C program and the embedded SQL statements using a simple mechanism called host variables. In an embedded SQL program we consider the SQL statements to be guests in the C program code which is the host language. Therefore the variables of the C program are called host variables. Passing data between the C program and the SQL statements is particularly simple in embedded SQL. Instead of having the program paste the data into the statement, which entails various complications, such as properly quoting the value, you can simply write the name of a C variable into the SQL statement, prefixed by a colon. For example: EXEC SQL INSERT INTO sometable VALUES (:v1, 'foo', :v2); This statements refers to two C variables named v1 and v2 and also uses a regular SQL string literal, to illustrate that you are not restricted to use one kind of data or the other. This style of inserting C variables in SQL statements works anywhere a value expression is expected in an SQL statement. To pass data from the program to the database, for example as parameters in a query, or to pass data from the database back to the program, the C variables that are intended to contain this data need to be declared in specially marked sections, so the embedded SQL preprocessor is made aware of them. This section starts with: EXEC SQL BEGIN DECLARE SECTION; and ends with: EXEC SQL END DECLARE SECTION; Between those lines, there must be normal C variable declarations, such as: int x = 4; char foo[16], bar[16]; As you can see, you can optionally assign an initial value to the variable. The variable's scope is determined by the location of its declaring section within the program. You can also declare variables with the following syntax which implicitly creates a declare section: EXEC SQL int i = 4; You can have as many declare sections in a program as you like. The declarations are also echoed to the output file as normal C variables, so there's no need to declare them again. Variables that are not intended to be used in SQL commands can be declared normally outside these special sections. The definition of a structure or union also must be listed inside a DECLARE section. Otherwise the preprocessor cannot handle these types since it does not know the definition. As a host variable you can also use arrays, typedefs, structs and pointers. Moreover there are special types of host variables that exist only in ECPG. A few examples on host variables: Arrays One of the most common uses of an array declaration is probably the allocation of a char array as in: EXEC SQL BEGIN DECLARE SECTION; char str[50]; EXEC SQL END DECLARE SECTION; Note that you have to take care of the length for yourself. If you use this host variable as the target variable of a query which returns a string with more than 49 characters, a buffer overflow occurs. Typedefs Use the typedef keyword to map new types to already existing types. EXEC SQL BEGIN DECLARE SECTION; typedef char mychartype[40]; typedef long serial_t; EXEC SQL END DECLARE SECTION; Note that you could also use: EXEC SQL TYPE serial_t IS long; This declaration does not need to be part of a declare section. Pointers You can declare pointers to the most common types. Note however that you cannot use pointers as target variables of queries without auto-allocation. See for more information on auto-allocation. EXEC SQL BEGIN DECLARE SECTION; int *intp; char **charp; EXEC SQL END DECLARE SECTION; Special types of variables ECPG contains some special types that help you to interact easily with data from the SQL server. For example it has implemented support for the varchar, numeric, date, timestamp, and interval types. contains basic functions to deal with those types, such that you do not need to send a query to the SQL server just for adding an interval to a timestamp for example. The special type VARCHAR is converted into a named struct for every variable. A declaration like: VARCHAR var[180]; is converted into: struct varchar_var { int len; char arr[180]; } var; This structure is suitable for interfacing with SQL datums of type varchar. Now you should be able to pass data generated by your program into an SQL command. But how do you retrieve the results of a query? For that purpose, embedded SQL provides special variants of the usual commands SELECT and FETCH. These commands have a special INTO clause that specifies which host variables the retrieved values are to be stored in. Here is an example: /* * assume this table: * CREATE TABLE test1 (a int, b varchar(50)); */ EXEC SQL BEGIN DECLARE SECTION; int v1; VARCHAR v2; EXEC SQL END DECLARE SECTION; ... EXEC SQL SELECT a, b INTO :v1, :v2 FROM test; So the INTO clause appears between the select list and the FROM clause. The number of elements in the select list and the list after INTO (also called the target list) must be equal. Here is an example using the command FETCH: EXEC SQL BEGIN DECLARE SECTION; int v1; VARCHAR v2; EXEC SQL END DECLARE SECTION; ... EXEC SQL DECLARE foo CURSOR FOR SELECT a, b FROM test; ... do { ... EXEC SQL FETCH NEXT FROM foo INTO :v1, :v2; ... } while (...); Here the INTO clause appears after all the normal clauses. Both of these methods only allow retrieving one row at a time. If you need to process result sets that potentially contain more than one row, you need to use a cursor, as shown in the second example. The examples above do not handle null values. In fact, the retrieval examples will raise an error if they fetch a null value from the database. To be able to pass null values to the database or retrieve null values from the database, you need to append a second host variable specification to each host variable that contains data. This second host variable is called the indicator and contains a flag that tells whether the datum is null, in which case the value of the real host variable is ignored. Here is an example that handles the retrieval of null values correctly: EXEC SQL BEGIN DECLARE SECTION; VARCHAR val; int val_ind; EXEC SQL END DECLARE SECTION: ... EXEC SQL SELECT b INTO :val :val_ind FROM test1; The indicator variable val_ind will be zero if the value was not null, and it will be negative if the value was null. The indicator has another function: if the indicator value is positive, it means that the value is not null, but it was truncated when it was stored in the host variable. In many cases, the particular SQL statements that an application has to execute are known at the time the application is written. In some cases, however, the SQL statements are composed at run time or provided by an external source. In these cases you cannot embed the SQL statements directly into the C source code, but there is a facility that allows you to call arbitrary SQL statements that you provide in a string variable. The simplest way to execute an arbitrary SQL statement is to use the command EXECUTE IMMEDIATE. For example: EXEC SQL BEGIN DECLARE SECTION; const char *stmt = "CREATE TABLE test1 (...);"; EXEC SQL END DECLARE SECTION; EXEC SQL EXECUTE IMMEDIATE :stmt; You cannot execute statements that retrieve data (e.g., SELECT) this way. A more powerful way to execute arbitrary SQL statements is to prepare them once and execute the prepared statement as often as you like. It is also possible to prepare a generalized version of a statement and then execute specific versions of it by substituting parameters. When preparing the statement, write question marks where you want to substitute parameters later. For example: EXEC SQL BEGIN DECLARE SECTION; const char *stmt = "INSERT INTO test1 VALUES(?, ?);"; EXEC SQL END DECLARE SECTION; EXEC SQL PREPARE mystmt FROM :stmt; ... EXEC SQL EXECUTE mystmt USING 42, 'foobar'; If the statement you are executing returns values, then add an INTO clause: ?"; int v1, v2; VARCHAR v3; EXEC SQL END DECLARE SECTION; EXEC SQL PREPARE mystmt FROM :stmt; ... EXEC SQL EXECUTE mystmt INTO v1, v2, v3 USING 37; ]]> An EXECUTE command can have an INTO clause, a USING clause, both, or neither. When you don't need the prepared statement anymore, you should deallocate it: EXEC SQL DEALLOCATE PREPARE name; The pgtypes library maps PostgreSQL database types to C equivalents that can be used in C programs. It also offers functions to do basic calculations with those types within C, i.e., without the help of the PostgreSQL server. See the following example: The numeric type offers to do calculations with arbitrary precision. See for the equivalent type in the PostgreSQL server. Because of the arbitrary precision this variable needs to be able to expand and shrink dynamically. That's why you can only create numeric variables on the heap, by means of the PGTYPESnumeric_new and PGTYPESnumeric_free functions. The decimal type, which is similar but limited in precision, can be created on the stack as well as on the heap. The following functions can be used to work with the numeric type: PGTYPESnumeric_new Request a pointer to a newly allocated numeric variable. numeric *PGTYPESnumeric_new(void); PGTYPESnumeric_free Free a numeric type, release all of its memory. void PGTYPESnumeric_free(numeric *var); PGTYPESnumeric_from_asc Parse a numeric type from its string notation. numeric *PGTYPESnumeric_from_asc(char *str, char **endptr); Valid formats are for example: -2, .794, +3.44, 592.49E07 or -32.84e-4. If the value could be parsed successfully, a valid pointer is returned, else the NULL pointer. At the moment ECPG always parses the complete string and so it currently does not support to store the address of the first invalid character in *endptr. You can safely set endptr to NULL. PGTYPESnumeric_to_asc Returns a pointer to a string allocated by malloc that contains the string representation of the numeric type num. char *PGTYPESnumeric_to_asc(numeric *num, int dscale); The numeric value will be printed with dscale decimal digits, with rounding applied if necessary. PGTYPESnumeric_add Add two numeric variables into a third one. int PGTYPESnumeric_add(numeric *var1, numeric *var2, numeric *result); The function adds the variables var1 and var2 into the result variable result. The function returns 0 on success and -1 in case of error. PGTYPESnumeric_sub Subtract two numeric variables and return the result in a third one. int PGTYPESnumeric_sub(numeric *var1, numeric *var2, numeric *result); The function subtracts the variable var2 from the variable var1. The result of the operation is stored in the variable result. The function returns 0 on success and -1 in case of error. PGTYPESnumeric_mul Multiply two numeric variables and return the result in a third one. int PGTYPESnumeric_mul(numeric *var1, numeric *var2, numeric *result); The function multiplies the variables var1 and var2. The result of the operation is stored in the variable result. The function returns 0 on success and -1 in case of error. PGTYPESnumeric_div Divide two numeric variables and return the result in a third one. int PGTYPESnumeric_div(numeric *var1, numeric *var2, numeric *result); The function divides the variables var1 by var2. The result of the operation is stored in the variable result. The function returns 0 on success and -1 in case of error. PGTYPESnumeric_cmp Compare two numeric variables. int PGTYPESnumeric_cmp(numeric *var1, numeric *var2) This function compares two numeric variables. In case of error, INT_MAX is returned. On success, the function returns one of three possible results: 1, if var1 is bigger than var2 -1, if var1 is smaller than var2 0, if var1 and var2 are equal PGTYPESnumeric_from_int Convert an int variable to a numeric variable. int PGTYPESnumeric_from_int(signed int int_val, numeric *var); This function accepts a variable of type signed int and stores it in the numeric variable var. Upon success, 0 is returned and -1 in case of a failure. PGTYPESnumeric_from_long Convert a long int variable to a numeric variable. int PGTYPESnumeric_from_long(signed long int long_val, numeric *var); This function accepts a variable of type signed long int and stores it in the numeric variable var. Upon success, 0 is returned and -1 in case of a failure. PGTYPESnumeric_copy Copy over one numeric variable into another one. int PGTYPESnumeric_copy(numeric *src, numeric *dst); This function copies over the value of the variable that src points to into the variable that dst points to. It returns 0 on success and -1 if an error occurs. PGTYPESnumeric_from_double Convert a variable of type double to a numeric. int PGTYPESnumeric_from_double(double d, numeric *dst); This function accepts a variable of type double and stores the result in the variable that dst points to. It returns 0 on success and -1 if an error occurs. PGTYPESnumeric_to_double Convert a variable of type numeric to double. int PGTYPESnumeric_to_double(numeric *nv, double *dp) The function converts the numeric value from the variable that nv points to into the double variable that dp points to. It returns 0 on success and -1 if an error occurs, including overflow. On overflow, the global variable errno will be set to PGTYPES_NUM_OVERFLOW additionally. PGTYPESnumeric_to_int Convert a variable of type numeric to int. int PGTYPESnumeric_to_int(numeric *nv, int *ip); The function converts the numeric value from the variable that nv points to into the integer variable that ip points to. It returns 0 on success and -1 if an error occurs, including overflow. On overflow, the global variable errno will be set to PGTYPES_NUM_OVERFLOW additionally. PGTYPESnumeric_to_long Convert a variable of type numeric to long. int PGTYPESnumeric_to_long(numeric *nv, long *lp); The function converts the numeric value from the variable that nv points to into the long integer variable that lp points to. It returns 0 on success and -1 if an error occurs, including overflow. On overflow, the global variable errno will be set to PGTYPES_NUM_OVERFLOW additionally. PGTYPESnumeric_to_decimal Convert a variable of type numeric to decimal. int PGTYPESnumeric_to_decimal(numeric *src, decimal *dst); The function converts the numeric value from the variable that src points to into the decimal variable that dst points to. It returns 0 on success and -1 if an error occurs, including overflow. On overflow, the global variable errno will be set to PGTYPES_NUM_OVERFLOW additionally. PGTYPESnumeric_from_decimal Convert a variable of type decimal to numeric. int PGTYPESnumeric_from_decimal(decimal *src, numeric *dst); The function converts the decimal value from the variable that src points to into the numeric variable that dst points to. It returns 0 on success and -1 if an error occurs. Since the decimal type is implemented as a limited version of the numeric type, overflow cannot occur with this conversion. The date type in C enables your programs to deal with data of the SQL type date. See for the equivalent type in the PostgreSQL server. The following functions can be used to work with the date type: PGTYPESdate_from_timestamp Extract the date part from a timestamp. date PGTYPESdate_from_timestamp(timestamp dt); The function receives a timestamp as its only argument and returns the extracted date part from this timestamp. PGTYPESdate_from_asc Parse a date from its textual representation. date PGTYPESdate_from_asc(char *str, char **endptr); The function receives a C char* string str and a pointer to a C char* string endptr. At the moment ECPG always parses the complete string and so it currently does not support to store the address of the first invalid character in *endptr. You can safely set endptr to NULL. Note that the function always assumes MDY-formatted dates and there is currently no variable to change that within ECPG. shows the allowed input formats. Input Result January 8, 1999 January 8, 1999 1999-01-08 January 8, 1999 1/8/1999 January 8, 1999 1/18/1999 January 18, 1999 01/02/03 February 1, 2003 1999-Jan-08 January 8, 1999 Jan-08-1999 January 8, 1999 08-Jan-1999 January 8, 1999 99-Jan-08 January 8, 1999 08-Jan-99 January 8, 1999 08-Jan-06 January 8, 2006 Jan-08-99 January 8, 1999 19990108 ISO 8601; January 8, 1999 990108 ISO 8601; January 8, 1999 1999.008 year and day of year J2451187 Julian day January 8, 99 BC year 99 before the Common Era PGTYPESdate_to_asc Return the textual representation of a date variable. char *PGTYPESdate_to_asc(date dDate); The function receives the date dDate as its only parameter. It will output the date in the form 1999-01-18, i.e., in the YYYY-MM-DD format. PGTYPESdate_julmdy Extract the values for the day, the month and the year from a variable of type date. void PGTYPESdate_julmdy(date d, int *mdy); The function receives the date d and a pointer to an array of 3 integer values mdy. The variable name indicates the sequential order: mdy[0] will be set to contain the number of the month, mdy[1] will be set to the value of the day and mdy[2] will contain the year. PGTYPESdate_mdyjul Create a date value from an array of 3 integers that specify the day, the month and the year of the date. void PGTYPESdate_mdyjul(int *mdy, date *jdate); The function receives the array of the 3 integers (mdy) as its first argument and as its second argument a pointer to a variable of type date that should hold the result of the operation. PGTYPESdate_dayofweek Return a number representing the day of the week for a date value. int PGTYPESdate_dayofweek(date d); The function receives the date variable d as its only argument and returns an integer that indicates the day of the week for this date. 0 - Sunday 1 - Monday 2 - Tuesday 3 - Wednesday 4 - Thursday 5 - Friday 6 - Saturday PGTYPESdate_today Get the current date. void PGTYPESdate_today(date *d); The function receives a pointer to a date variable (d) that it sets to the current date. PGTYPESdate_fmt_asc Convert a variable of type date to its textual representation using a format mask. int PGTYPESdate_fmt_asc(date dDate, char *fmtstring, char *outbuf); The function receives the date to convert (dDate), the format mask (fmtstring) and the string that will hold the textual representation of the date (outbuf). On success, 0 is returned and a negative value if an error occurred. The following literals are the field specifiers you can use: dd - The number of the day of the month. mm - The number of the month of the year. yy - The number of the year as a two digit number. yyyy - The number of the year as a four digit number. ddd - The name of the day (abbreviated). mmm - The name of the month (abbreviated). All other characters are copied 1:1 to the output string. indicates a few possible formats. This will give you an idea of how to use this function. All output lines are based on the same date: November 23, 1959. Format Result mmddyy 112359 ddmmyy 231159 yymmdd 591123 yy/mm/dd 59/11/23 yy mm dd 59 11 23 yy.mm.dd 59.11.23 .mm.yyyy.dd. .11.1959.23. mmm. dd, yyyy Nov. 23, 1959 mmm dd yyyy Nov 23 1959 yyyy dd mm 1959 23 11 ddd, mmm. dd, yyyy Mon, Nov. 23, 1959 (ddd) mmm. dd, yyyy (Mon) Nov. 23, 1959 PGTYPESdate_defmt_asc Use a format mask to convert a C char* string to a value of type date. int PGTYPESdate_defmt_asc(date *d, char *fmt, char *str); The function receives a pointer to the date value that should hold the result of the operation (d), the format mask to use for parsing the date (fmt) and the C char* string containing the textual representation of the date (str). The textual representation is expected to match the format mask. However you do not need to have a 1:1 mapping of the string to the format mask. The function only analyzes the sequential order and looks for the literals yy or yyyy that indicate the position of the year, mm to indicate the position of the month and dd to indicate the position of the day. indicates a few possible formats. This will give you an idea of how to use this function. Format String Result ddmmyy 21-2-54 1954-02-21 ddmmyy 2-12-54 1954-12-02 ddmmyy 20111954 1954-11-20 ddmmyy 130464 1964-04-13 mmm.dd.yyyy MAR-12-1967 1967-03-12 yy/mm/dd 1954, February 3rd 1954-02-03 mmm.dd.yyyy 041269 1969-04-12 yy/mm/dd In the year 2525, in the month of July, mankind will be alive on the 28th day 2525-07-28 dd-mm-yy I said on the 28th of July in the year 2525 2525-07-28 mmm.dd.yyyy 9/14/58 1958-09-14 yy/mm/dd 47/03/29 1947-03-29 mmm.dd.yyyy oct 28 1975 1975-10-28 mmddyy Nov 14th, 1985 1985-11-14 The timestamp type in C enables your programs to deal with data of the SQL type timestamp. See for the equivalent type in the PostgreSQL server. The following functions can be used to work with the timestamp type: PGTYPEStimestamp_from_asc Parse a timestamp from its textual representation into a timestamp variable. timestamp PGTYPEStimestamp_from_asc(char *str, char **endptr); The function receives the string to parse (str) and a pointer to a C char* (endptr). At the moment ECPG always parses the complete string and so it currently does not support to store the address of the first invalid character in *endptr. You can safely set endptr to NULL. The function returns the parsed timestamp on success. On error, PGTYPESInvalidTimestamp is returned and errno is set to PGTYPES_TS_BAD_TIMESTAMP. See for important notes on this value. In general, the input string can contain any combination of an allowed date specification, a whitespace character and an allowed time specification. Note that timezones are not supported by ECPG. It can parse them but does not apply any calculation as the PostgreSQL server does for example. Timezone specifiers are silently discarded. contains a few examples for input strings. Input Result 1999-01-08 04:05:06 1999-01-08 04:05:06 January 8 04:05:06 1999 PST 1999-01-08 04:05:06 1999-Jan-08 04:05:06.789-8 1999-01-08 04:05:06.789 (time zone specifier ignored) J2451187 04:05-08:00 1999-01-08 04:05:00 (time zone specifier ignored) PGTYPEStimestamp_to_asc Converts a date to a C char* string. char *PGTYPEStimestamp_to_asc(timestamp tstamp); The function receives the timestamp tstamp as its only argument and returns an allocated string that contains the textual representation of the timestamp. PGTYPEStimestamp_current Retrieve the current timestamp. void PGTYPEStimestamp_current(timestamp *ts); The function retrieves the current timestamp and saves it into the timestamp variable that ts points to. PGTYPEStimestamp_fmt_asc Convert a timestamp variable to a C char* using a format mask. int PGTYPEStimestamp_fmt_asc(timestamp *ts, char *output, int str_len, char *fmtstr); The function receives a pointer to the timestamp to convert as its first argument (ts), a pointer to the output buffer (output), the maximal length that has been allocated for the output buffer (str_len) and the format mask to use for the conversion (fmtstr). Upon success, the function returns 0 and a negative value if an error occurred. You can use the following format specifiers for the format mask. The format specifiers are the same ones that are used in the strftime function in libc. Any non-format specifier will be copied into the output buffer. %A - is replaced by national representation of the full weekday name. %a - is replaced by national representation of the abbreviated weekday name. %B - is replaced by national representation of the full month name. %b - is replaced by national representation of the abbreviated month name. %C - is replaced by (year / 100) as decimal number; single digits are preceded by a zero. %c - is replaced by national representation of time and date. %D - is equivalent to %m/%d/%y. %d - is replaced by the day of the month as a decimal number (01-31). %E* %O* - POSIX locale extensions. The sequences %Ec %EC %Ex %EX %Ey %EY %Od %Oe %OH %OI %Om %OM %OS %Ou %OU %OV %Ow %OW %Oy are supposed to provide alternative representations. Additionally %OB implemented to represent alternative months names (used standalone, without day mentioned). %e - is replaced by the day of month as a decimal number (1-31); single digits are preceded by a blank. %F - is equivalent to %Y-%m-%d. %G - is replaced by a year as a decimal number with century. This year is the one that contains the greater part of the week (Monday as the first day of the week). %g - is replaced by the same year as in %G, but as a decimal number without century (00-99). %H - is replaced by the hour (24-hour clock) as a decimal number (00-23). %h - the same as %b. %I - is replaced by the hour (12-hour clock) as a decimal number (01-12). %j - is replaced by the day of the year as a decimal number (001-366). %k - is replaced by the hour (24-hour clock) as a decimal number (0-23); single digits are preceded by a blank. %l - is replaced by the hour (12-hour clock) as a decimal number (1-12); single digits are preceded by a blank. %M - is replaced by the minute as a decimal number (00-59). %m - is replaced by the month as a decimal number (01-12). %n - is replaced by a newline. %O* - the same as %E*. %p - is replaced by national representation of either "ante meridiem" or "post meridiem" as appropriate. %R - is equivalent to %H:%M. %r - is equivalent to %I:%M:%S %p. %S - is replaced by the second as a decimal number (00-60). %s - is replaced by the number of seconds since the Epoch, UTC. %T - is equivalent to %H:%M:%S %t - is replaced by a tab. %U - is replaced by the week number of the year (Sunday as the first day of the week) as a decimal number (00-53). %u - is replaced by the weekday (Monday as the first day of the week) as a decimal number (1-7). %V - is replaced by the week number of the year (Monday as the first day of the week) as a decimal number (01-53). If the week containing January 1 has four or more days in the new year, then it is week 1; otherwise it is the last week of the previous year, and the next week is week 1. %v - is equivalent to %e-%b-%Y. %W - is replaced by the week number of the year (Monday as the first day of the week) as a decimal number (00-53). %w - is replaced by the weekday (Sunday as the first day of the week) as a decimal number (0-6). %X - is replaced by national representation of the time. %x - is replaced by national representation of the date. %Y - is replaced by the year with century as a decimal number. %y - is replaced by the year without century as a decimal number (00-99). %Z - is replaced by the time zone name. %z - is replaced by the time zone offset from UTC; a leading plus sign stands for east of UTC, a minus sign for west of UTC, hours and minutes follow with two digits each and no delimiter between them (common form for RFC 822 date headers). %+ - is replaced by national representation of the date and time. %-* - GNU libc extension. Do not do any padding when performing numerical outputs. $_* - GNU libc extension. Explicitly specify space for padding. %0* - GNU libc extension. Explicitly specify zero for padding. %% - is replaced by %. PGTYPEStimestamp_sub Subtract one timestamp from another one and save the result in a variable of type interval. int PGTYPEStimestamp_sub(timestamp *ts1, timestamp *ts2, interval *iv); The function will subtract the timestamp variable that ts2 points to from the timestamp variable that ts1 points to and will store the result in the interval variable that iv points to. Upon success, the function returns 0 and a negative value if an error occurred. PGTYPEStimestamp_defmt_asc Parse a timestamp value from its textual representation using a formatting mask. int PGTYPEStimestamp_defmt_asc(char *str, char *fmt, timestamp *d); The function receives the textual representation of a timestamp in the variable str as well as the formatting mask to use in the variable fmt. The result will be stored in the variable that d points to. If the formatting mask fmt is NULL, the function will fall back to the default formatting mask which is %Y-%m-%d %H:%M:%S. This is the reverse function to . See the documentation there in order to find out about the possible formatting mask entries. PGTYPEStimestamp_add_interval Add an interval variable to a timestamp variable. int PGTYPEStimestamp_add_interval(timestamp *tin, interval *span, timestamp *tout); The function receives a pointer to a timestamp variable tin and a pointer to an interval variable span. It adds the interval to the timestamp and saves the resulting timestamp in the variable that tout points to. Upon success, the function returns 0 and a negative value if an error occurred. PGTYPEStimestamp_sub_interval Subtract an interval variable from a timestamp variable. int PGTYPEStimestamp_sub_interval(timestamp *tin, interval *span, timestamp *tout); The function subtracts the interval variable that span points to from the timestamp variable that tin points to and saves the result into the variable that tout points to. Upon success, the function returns 0 and a negative value if an error occurred. The interval type in C enables your programs to deal with data of the SQL type interval. See for the equivalent type in the PostgreSQL server. The following functions can be used to work with the interval type: PGTYPESinterval_new Return a pointer to a newly allocated interval variable. interval *PGTYPESinterval_new(void); PGTYPESinterval_free Release the memory of a previously allocated interval variable. void PGTYPESinterval_new(interval *intvl); PGTYPESinterval_from_asc Parse an interval from its textual representation. interval *PGTYPESinterval_from_asc(char *str, char **endptr); The function parses the input string str and returns a pointer to an allocated interval variable. At the moment ECPG always parses the complete string and so it currently does not support to store the address of the first invalid character in *endptr. You can safely set endptr to NULL. PGTYPESinterval_to_asc Convert a variable of type interval to its textual representation. char *PGTYPESinterval_to_asc(interval *span); The function converts the interval variable that span points to into a C char*. The output looks like this example: @ 1 day 12 hours 59 mins 10 secs. PGTYPESinterval_copy Copy a variable of type interval. int PGTYPESinterval_copy(interval *intvlsrc, interval *intvldest); The function copies the interval variable that intvlsrc points to into the variable that intvldest points to. Note that you need to allocate the memory for the destination variable before. The decimal type is similar to the numeric type. However it is limited to a maximum precision of 30 significant digits. In contrast to the numeric type which can be created on the heap only, the decimal type can be created either on the stack or on the heap (by means of the functions PGTYPESdecimal_new and PGTYPESdecimal_free). There are a lot of other functions that deal with the decimal type in the Informix compatibility mode described in . The following functions can be used to work with the decimal type and are not only contained in the libcompat library. PGTYPESdecimal_new Request a pointer to a newly allocated decimal variable. decimal *PGTYPESdecimal_new(void); PGTYPESdecimal_free Free a decimal type, release all of its memory. void PGTYPESdecimal_free(decimal *var); PGTYPES_NUM_BAD_NUMERIC An argument should contain a numeric variable (or point to a numeric variable) but in fact its in-memory representation was invalid. PGTYPES_NUM_OVERFLOW An overflow occurred. Since the numeric type can deal with almost arbitrary precision, converting a numeric variable into other types might cause overflow. PGTYPES_NUM_UNDERFLOW An underflow occurred. Since the numeric type can deal with almost arbitrary precision, converting a numeric variable into other types might cause underflow. PGTYPES_NUM_DIVIDE_ZERO A division by zero has been attempted. PGTYPES_DATE_BAD_DATE PGTYPES_DATE_ERR_EARGS PGTYPES_DATE_ERR_ENOSHORTDATE PGTYPES_INTVL_BAD_INTERVAL PGTYPES_DATE_ERR_ENOTDMY PGTYPES_DATE_BAD_DAY PGTYPES_DATE_BAD_MONTH PGTYPES_TS_BAD_TIMESTAMP PGTYPESInvalidTimestamp A value of type timestamp representing an invalid time stamp. This is returned by the function PGTYPEStimestamp_from_asc on parse error. Note that due to the internal representation of the timestamp data type, PGTYPESInvalidTimestamp is also a valid timestamp at the same time. It is set to 1899-12-31 23:59:59. In order to detect errors, make sure that your application does not only test for PGTYPESInvalidTimestamp but also for errno != 0 after each call to PGTYPEStimestamp_from_asc. An SQL descriptor area is a more sophisticated method for processing the result of a SELECT, FETCH or a DESCRIBE statement. An SQL descriptor area groups the data of one row of data together with metadata items into one data structure. The metadata is particularly useful when executing dynamic SQL statements, where the nature of the result columns might not be known ahead of time. PostgreSQL provides two ways to use Descriptor Areas: the named SQL Descriptor Areas and the C-structure SQLDAs. A named SQL descriptor area consists of a header, which contains information concerning the entire descriptor, and one or more item descriptor areas, which basically each describe one column in the result row. Before you can use an SQL descriptor area, you need to allocate one: EXEC SQL ALLOCATE DESCRIPTOR identifier; The identifier serves as the variable name of the descriptor area. The scope of the allocated descriptor is WHAT?. When you don't need the descriptor anymore, you should deallocate it: EXEC SQL DEALLOCATE DESCRIPTOR identifier; To use a descriptor area, specify it as the storage target in an INTO clause, instead of listing host variables: EXEC SQL FETCH NEXT FROM mycursor INTO SQL DESCRIPTOR mydesc; If the result set is empty, the Descriptor Area will still contain the metadata from the query, i.e. the field names. For not yet executed prepared queries, the DESCRIBE statement can be used to get the metadata of the result set: EXEC SQL BEGIN DECLARE SECTION; char *sql_stmt = "SELECT * FROM table1"; EXEC SQL END DECLARE SECTION; EXEC SQL PREPARE stmt1 FROM :sql_stmt; EXEC SQL DESCRIBE stmt1 INTO SQL DESCRIPTOR mydesc; Before PostgreSQL 9.0, the SQL keyword was optional, so using DESCRIPTOR and SQL DESCRIPTOR produced named SQL Descriptor Areas. Now it is mandatory, omitting the SQL keyword produces SQLDA Descriptor Areas, see . In DESCRIBE and FETCH statements, the INTO and USING keywords can be used to similarly: they produce the result set and the metadata in a Descriptor Area. Now how do you get the data out of the descriptor area? You can think of the descriptor area as a structure with named fields. To retrieve the value of a field from the header and store it into a host variable, use the following command: EXEC SQL GET DESCRIPTOR name :hostvar = field; Currently, there is only one header field defined: COUNT, which tells how many item descriptor areas exist (that is, how many columns are contained in the result). The host variable needs to be of an integer type. To get a field from the item descriptor area, use the following command: EXEC SQL GET DESCRIPTOR name VALUE num :hostvar = field; num can be a literal integer or a host variable containing an integer. Possible fields are: CARDINALITY (integer) number of rows in the result set DATA actual data item (therefore, the data type of this field depends on the query) DATETIME_INTERVAL_CODE (integer) ? DATETIME_INTERVAL_PRECISION (integer) not implemented INDICATOR (integer) the indicator (indicating a null value or a value truncation) KEY_MEMBER (integer) not implemented LENGTH (integer) length of the datum in characters NAME (string) name of the column NULLABLE (integer) not implemented OCTET_LENGTH (integer) length of the character representation of the datum in bytes PRECISION (integer) precision (for type numeric) RETURNED_LENGTH (integer) length of the datum in characters RETURNED_OCTET_LENGTH (integer) length of the character representation of the datum in bytes SCALE (integer) scale (for type numeric) TYPE (integer) numeric code of the data type of the column In EXECUTE, DECLARE and OPEN statements, the effect of the INTO and USING keywords are different. A Descriptor Area can also be manually built to provide the input parameters for a query or a cursor and USING SQL DESCRIPTOR name is the way to pass the input parameters into a parametrized query. The statement to build a named SQL Descriptor Area is below: EXEC SQL SET DESCRIPTOR name VALUE num field = :hostvar; PostgreSQL supports retrieving more that one record in one FETCH statement and storing the data in host variables in this case assumes that the variable is an array. E.g.: EXEC SQL BEGIN DECLARE SECTION; int id[5]; EXEC SQL END DECLARE SECTION; EXEC SQL FETCH 5 FROM mycursor INTO SQL DESCRIPTOR mydesc; EXEC SQL GET DESCRIPTOR mydesc VALUE 1 :id = DATA; An SQLDA Descriptor Area is a C language structure which can be also used to get the result set and the metadata of a query. One structure stores one record from the result set. EXEC SQL include sqlda.h; sqlda_t *mysqlda; EXEC SQL FETCH 3 FROM mycursor INTO DESCRIPTOR mysqlda; Note that the SQL keyword is omitted. The paragraphs about the use cases of the INTO and USING keywords in also apply here with an addition. In a DESCRIBE statement the DESCRIPTOR keyword can be completely omitted if the INTO keyword is used: EXEC SQL DESCRIBE prepared_statement INTO mysqlda; The structure of SQLDA is: #define NAMEDATALEN 64 struct sqlname { short length; char data[NAMEDATALEN]; }; struct sqlvar_struct { short sqltype; short sqllen; char *sqldata; short *sqlind; struct sqlname sqlname; }; struct sqlda_struct { char sqldaid[8]; long sqldabc; short sqln; short sqld; struct sqlda_struct *desc_next; struct sqlvar_struct sqlvar[1]; }; typedef struct sqlvar_struct sqlvar_t; typedef struct sqlda_struct sqlda_t; The allocated data for an SQLDA structure is variable as it depends on the number of fields in a result set and also depends on the length of the string data values in a record. The individual fields of the SQLDA structure are: sqldaid It contains the "SQLDA " literal string. sqldabc It contains the size of the allocated space in bytes. sqln It contains the number of input parameters for a parametrized query case it's passed into OPEN, DECLARE or EXECUTE statements using the USING keyword. In case it's used as output of SELECT, EXECUTE or FETCH statements, its value is the same as sqld statement sqld It contains the number of fields in a result set. desc_next If the query returns more than one records, multiple linked SQLDA structures are returned, the first record is stored in the SQLDA returned in the sqlvar This is the array of the fields in the result set. The fields are: sqltype It contains the type identifier of the field. For values, see enum ECPGttype in ecpgtype.h. sqllen It contains the binary length of the field. E.g. 4 bytes for ECPGt_int. sqldata (char *)sqldata points to the data. sqlind (char *)sqlind points to the NULL indicator for data. 0 means NOT NULL, -1 means NULL. sqlname struct sqlname sqlname contains the name of the field in a structure: struct sqlname { short length; char data[NAMEDATALEN]; }; length sqlname.length contains the length of the field name. data sqlname.data contains the actual field name. ecpg can be run in a so-called Informix compatibility mode. If this mode is active, it tries to behave as if it were the Informix precompiler for Informix E/SQL. Generally spoken this will allow you to use the dollar sign instead of the EXEC SQL primitive to introduce embedded SQL commands.: $int j = 3; $CONNECT TO :dbname; $CREATE TABLE test(i INT PRIMARY KEY, j INT); $INSERT INTO test(i, j) VALUES (7, :j); $COMMIT; There are two compatibility modes: INFORMIX, INFORMIX_SE When linking programs that use this compatibility mode, remember to link against libcompat that is shipped with ECPG. Besides the previously explained syntactic sugar, the Informix compatibility mode ports some functions for input, output and transformation of data as well as embedded SQL statements known from E/SQL to ECPG. Informix compatibility mode is closely connected to the pgtypeslib library of ECPG. pgtypeslib maps SQL data types to data types within the C host program and most of the additional functions of the Informix compatibility mode allow you to operate on those C host program types. Note however that the extent of the compatibility is limited. It does not try to copy Informix behavior; it allows you to do more or less the same operations and gives you functions that have the same name and the same basic behavior but it is no drop-in replacement if you are using Informix at the moment. Moreover, some of the data types are different. For example, PostgreSQL's datetime and interval types do not know about ranges like for example YEAR TO MINUTE so you won't find support in ECPG for that either. The Informix-special "string" pseudo-type for storing right-trimmed character string data is now supported in Informix-mode without using typedef. In fact, in Informix-mode, ECPG refuses to process source files that contain typedef sometype string; EXEC SQL BEGIN DECLARE SECTION; string userid; /* this variable will contain trimmed data */ EXEC SQL END DECLARE SECTION; EXEC SQL FETCH MYCUR INTO :userid; CLOSE DATABASE This statement closes the current connection. In fact, this is a synonym for ECPG's DISCONNECT CURRENT.: $CLOSE DATABASE; /* close the current connection */ EXEC SQL CLOSE DATABASE; FREE cursor_name Due to the differences how ECPG works compared to Informix's ESQL/C (i.e. which steps are purely grammar transformations and which steps rely on the underlying run-time library) there is no FREE cursor_name statement in ECPG. This is because in ECPG, DECLARE CURSOR doesn't translate to a function call into the run-time library that uses to the cursor name. This means that there's no run-time bookkeeping of SQL cursors in the ECPG run-time library, only in the PostgreSQL server. FREE statement_name FREE statement_name is a synonym for DEALLOCATE PREPARE statement_name. Informix-compatible mode supports a different structure than the one described in . See below: struct sqlvar_compat { short sqltype; int sqllen; char *sqldata; short *sqlind; char *sqlname; char *sqlformat; short sqlitype; short sqlilen; char *sqlidata; int sqlxid; char *sqltypename; short sqltypelen; short sqlownerlen; short sqlsourcetype; char *sqlownername; int sqlsourceid; char *sqlilongdata; int sqlflags; void *sqlreserved; }; struct sqlda_compat { short sqld; struct sqlvar_compat *sqlvar; char desc_name[19]; short desc_occ; struct sqlda_compat *desc_next; void *reserved; }; typedef struct sqlvar_compat sqlvar_t; typedef struct sqlda_compat sqlda_t; The global properties are: sqld The number of fields in the SQLDA descriptor. sqlvar Pointer to the per-field properties. desc_name Unused, filled with zero-bytes. desc_occ Size of the allocated structure. desc_next Pointer to the next SQLDA structure if the result set contains more than one records. reserved Unused pointer, contains NULL. Kept for Informix-compatibility. The per-field properties are below, they are stored in the sqlvar array: sqltype Type of the field. Constants are in sqltypes.h sqllen Length of the field data. sqldata Pointer to the field data. The pointer is of char * type, the data pointed by it is in a binary format. Example: int intval; switch (sqldata->sqlvar[i].sqltype) { case SQLINTEGER: intval = *(int *)sqldata->sqlvar[i].sqldata; break; ... } sqlind Pointer to the NULL indicator. If returned by DESCRIBE or FETCH then it's always a valid pointer. If used as input for EXECUTE ... USING sqlda; then NULL-pointer value means that the value for this field is non-NULL. Otherwise a valid pointer and sqlitype has to be properly set. Example: if (*(int2 *)sqldata->sqlvar[i].sqlind != 0) printf("value is NULL\n"); sqlname Name of the field. 0-terminated string. sqlformat Reserved in Informix, value of PQfformat() for the field. sqlitype Type of the NULL indicator data. It's always SQLSMINT when returning data from the server. When the SQLDA is used for a parametrized query, the data is treated according to the set type. sqlilen Length of the NULL indicator data. sqlxid Extended type of the field, result of PQftype(). sqltypename sqltypelen sqlownerlen sqlsourcetype sqlownername sqlsourceid sqlflags sqlreserved Unused. sqlilongdata It equals to sqldata if sqllen is larger than 32KB. Example: EXEC SQL INCLUDE sqlda.h; sqlda_t *sqlda; /* This doesn't need to be under embedded DECLARE SECTION */ EXEC SQL BEGIN DECLARE SECTION; char *prep_stmt = "select * from table1"; int i; EXEC SQL END DECLARE SECTION; ... EXEC SQL PREPARE mystmt FROM :prep_stmt; EXEC SQL DESCRIBE mystmt INTO sqlda; printf("# of fields: %d\n", sqlda->sqld); for (i = 0; i < sqlda->sqld; i++) printf("field %d: \"%s\"\n", sqlda->sqlvar[i]->sqlname); EXEC SQL DECLARE mycursor CURSOR FOR mystmt; EXEC SQL OPEN mycursor; EXEC SQL WHENEVER NOT FOUND GOTO out; while (1) { EXEC SQL FETCH mycursor USING sqlda; } EXEC SQL CLOSE mycursor; free(sqlda); /* The main structure is all to be free(), * sqlda and sqlda->sqlvar is in one allocated area */ For more information, see the sqlda.h header and the src/interfaces/ecpg/test/compat_informix/sqlda.pgc regression test. decadd Add two decimal type values. int decadd(decimal *arg1, decimal *arg2, decimal *sum); The function receives a pointer to the first operand of type decimal (arg1), a pointer to the second operand of type decimal (arg2) and a pointer to a value of type decimal that will contain the sum (sum). On success, the function returns 0. ECPG_INFORMIX_NUM_OVERFLOW is returned in case of overflow and ECPG_INFORMIX_NUM_UNDERFLOW in case of underflow. -1 is returned for other failures and errno is set to the respective errno number of the pgtypeslib. deccmp Compare two variables of type decimal. int deccmp(decimal *arg1, decimal *arg2); The function receives a pointer to the first decimal value (arg1), a pointer to the second decimal value (arg2) and returns an integer value that indicates which is the bigger value. 1, if the value that arg1 points to is bigger than the value that var2 points to -1, if the value that arg1 points to is smaller than the value that arg2 points to 0, if the value that arg1 points to and the value that arg2 points to are equal deccopy Copy a decimal value. void deccopy(decimal *src, decimal *target); The function receives a pointer to the decimal value that should be copied as the first argument (src) and a pointer to the target structure of type decimal (target) as the second argument. deccvasc Convert a value from its ASCII representation into a decimal type. int deccvasc(char *cp, int len, decimal *np); The function receives a pointer to string that contains the string representation of the number to be converted (cp) as well as its length len. np is a pointer to the decimal value that saves the result of the operation. Valid formats are for example: -2, .794, +3.44, 592.49E07 or -32.84e-4. The function returns 0 on success. If overflow or underflow occurred, ECPG_INFORMIX_NUM_OVERFLOW or ECPG_INFORMIX_NUM_UNDERFLOW is returned. If the ASCII representation could not be parsed, ECPG_INFORMIX_BAD_NUMERIC is returned or ECPG_INFORMIX_BAD_EXPONENT if this problem occurred while parsing the exponent. deccvdbl Convert a value of type double to a value of type decimal. int deccvdbl(double dbl, decimal *np); The function receives the variable of type double that should be converted as its first argument (dbl). As the second argument (np), the function receives a pointer to the decimal variable that should hold the result of the operation. The function returns 0 on success and a negative value if the conversion failed. deccvint Convert a value of type int to a value of type decimal. int deccvint(int in, decimal *np); The function receives the variable of type int that should be converted as its first argument (in). As the second argument (np), the function receives a pointer to the decimal variable that should hold the result of the operation. The function returns 0 on success and a negative value if the conversion failed. deccvlong Convert a value of type long to a value of type decimal. int deccvlong(long lng, decimal *np); The function receives the variable of type long that should be converted as its first argument (lng). As the second argument (np), the function receives a pointer to the decimal variable that should hold the result of the operation. The function returns 0 on success and a negative value if the conversion failed. decdiv Divide two variables of type decimal. int decdiv(decimal *n1, decimal *n2, decimal *result); The function receives pointers to the variables that are the first (n1) and the second (n2) operands and calculates n1/n2. result is a pointer to the variable that should hold the result of the operation. On success, 0 is returned and a negative value if the division fails. If overflow or underflow occurred, the function returns ECPG_INFORMIX_NUM_OVERFLOW or ECPG_INFORMIX_NUM_UNDERFLOW respectively. If an attempt to divide by zero is observed, the function returns ECPG_INFORMIX_DIVIDE_ZERO. decmul Multiply two decimal values. int decmul(decimal *n1, decimal *n2, decimal *result); The function receives pointers to the variables that are the first (n1) and the second (n2) operands and calculates n1*n2. result is a pointer to the variable that should hold the result of the operation. On success, 0 is returned and a negative value if the multiplication fails. If overflow or underflow occurred, the function returns ECPG_INFORMIX_NUM_OVERFLOW or ECPG_INFORMIX_NUM_UNDERFLOW respectively. decsub Subtract one decimal value from another. int decsub(decimal *n1, decimal *n2, decimal *result); The function receives pointers to the variables that are the first (n1) and the second (n2) operands and calculates n1-n2. result is a pointer to the variable that should hold the result of the operation. On success, 0 is returned and a negative value if the subtraction fails. If overflow or underflow occurred, the function returns ECPG_INFORMIX_NUM_OVERFLOW or ECPG_INFORMIX_NUM_UNDERFLOW respectively. dectoasc Convert a variable of type decimal to its ASCII representation in a C char* string. int dectoasc(decimal *np, char *cp, int len, int right) The function receives a pointer to a variable of type decimal (np) that it converts to its textual representation. cp is the buffer that should hold the result of the operation. The parameter right specifies, how many digits right of the decimal point should be included in the output. The result will be rounded to this number of decimal digits. Setting right to -1 indicates that all available decimal digits should be included in the output. If the length of the output buffer, which is indicated by len is not sufficient to hold the textual representation including the trailing NUL character, only a single * character is stored in the result and -1 is returned. The function returns either -1 if the buffer cp was too small or ECPG_INFORMIX_OUT_OF_MEMORY if memory was exhausted. dectodbl Convert a variable of type decimal to a double. int dectodbl(decimal *np, double *dblp); The function receives a pointer to the decimal value to convert (np) and a pointer to the double variable that should hold the result of the operation (dblp). On success, 0 is returned and a negative value if the conversion failed. dectoint Convert a variable to type decimal to an integer. int dectoint(decimal *np, int *ip); The function receives a pointer to the decimal value to convert (np) and a pointer to the integer variable that should hold the result of the operation (ip). On success, 0 is returned and a negative value if the conversion failed. If an overflow occurred, ECPG_INFORMIX_NUM_OVERFLOW is returned. Note that the ECPG implementation differs from the Informix implementation. Informix limits an integer to the range from -32767 to 32767, while the limits in the ECPG implementation depend on the architecture (-INT_MAX .. INT_MAX). dectolong Convert a variable to type decimal to a long integer. int dectolong(decimal *np, long *lngp); The function receives a pointer to the decimal value to convert (np) and a pointer to the long variable that should hold the result of the operation (lngp). On success, 0 is returned and a negative value if the conversion failed. If an overflow occurred, ECPG_INFORMIX_NUM_OVERFLOW is returned. Note that the ECPG implementation differs from the Informix implementation. Informix limits a long integer to the range from -2,147,483,647 to 2,147,483,647, while the limits in the ECPG implementation depend on the architecture (-LONG_MAX .. LONG_MAX). rdatestr Converts a date to a C char* string. int rdatestr(date d, char *str); The function receives two arguments, the first one is the date to convert (d and the second one is a pointer to the target string. The output format is always yyyy-mm-dd, so you need to allocate at least 11 bytes (including the NUL-terminator) for the string. The function returns 0 on success and a negative value in case of error. Note that ECPG's implementation differs from the Informix implementation. In Informix the format can be influenced by setting environment variables. In ECPG however, you cannot change the output format. rstrdate Parse the textual representation of a date. int rstrdate(char *str, date *d); The function receives the textual representation of the date to convert (str) and a pointer to a variable of type date (d). This function does not allow you to specify a format mask. It uses the default format mask of Informix which is mm/dd/yyyy. Internally, this function is implemented by means of rdefmtdate. Therefore, rstrdate is not faster and if you have the choice you should opt for rdefmtdate which allows you to specify the format mask explicitly. The function returns the same values as rdefmtdate. rtoday Get the current date. void rtoday(date *d); The function receives a pointer to a date variable (d) that it sets to the current date. Internally this function uses the function. rjulmdy Extract the values for the day, the month and the year from a variable of type date. int rjulmdy(date d, short mdy[3]); The function receives the date d and a pointer to an array of 3 short integer values mdy. The variable name indicates the sequential order: mdy[0] will be set to contain the number of the month, mdy[1] will be set to the value of the day and mdy[2] will contain the year. The function always returns 0 at the moment. Internally the function uses the function. rdefmtdate Use a format mask to convert a character string to a value of type date. int rdefmtdate(date *d, char *fmt, char *str); The function receives a pointer to the date value that should hold the result of the operation (d), the format mask to use for parsing the date (fmt) and the C char* string containing the textual representation of the date (str). The textual representation is expected to match the format mask. However you do not need to have a 1:1 mapping of the string to the format mask. The function only analyzes the sequential order and looks for the literals yy or yyyy that indicate the position of the year, mm to indicate the position of the month and dd to indicate the position of the day. The function returns the following values: 0 - The function terminated successfully. ECPG_INFORMIX_ENOSHORTDATE - The date does not contain delimiters between day, month and year. In this case the input string must be exactly 6 or 8 bytes long but isn't. ECPG_INFORMIX_ENOTDMY - The format string did not correctly indicate the sequential order of year, month and day. ECPG_INFORMIX_BAD_DAY - The input string does not contain a valid day. ECPG_INFORMIX_BAD_MONTH - The input string does not contain a valid month. ECPG_INFORMIX_BAD_YEAR - The input string does not contain a valid year. Internally this function is implemented to use the function. See the reference there for a table of example input. rfmtdate Convert a variable of type date to its textual representation using a format mask. int rfmtdate(date d, char *fmt, char *str); The function receives the date to convert (d), the format mask (fmt) and the string that will hold the textual representation of the date (str). On success, 0 is returned and a negative value if an error occurred. Internally this function uses the function, see the reference there for examples. rmdyjul Create a date value from an array of 3 short integers that specify the day, the month and the year of the date. int rmdyjul(short mdy[3], date *d); The function receives the array of the 3 short integers (mdy) and a pointer to a variable of type date that should hold the result of the operation. Currently the function returns always 0. Internally the function is implemented to use the function . rdayofweek Return a number representing the day of the week for a date value. int rdayofweek(date d); The function receives the date variable d as its only argument and returns an integer that indicates the day of the week for this date. 0 - Sunday 1 - Monday 2 - Tuesday 3 - Wednesday 4 - Thursday 5 - Friday 6 - Saturday Internally the function is implemented to use the function . dtcurrent Retrieve the current timestamp. void dtcurrent(timestamp *ts); The function retrieves the current timestamp and saves it into the timestamp variable that ts points to. dtcvasc Parses a timestamp from its textual representation into a timestamp variable. int dtcvasc(char *str, timestamp *ts); The function receives the string to parse (str) and a pointer to the timestamp variable that should hold the result of the operation (ts). The function returns 0 on success and a negative value in case of error. Internally this function uses the function. See the reference there for a table with example inputs. dtcvfmtasc Parses a timestamp from its textual representation using a format mask into a timestamp variable. dtcvfmtasc(char *inbuf, char *fmtstr, timestamp *dtvalue) The function receives the string to parse (inbuf), the format mask to use (fmtstr) and a pointer to the timestamp variable that should hold the result of the operation (dtvalue). This function is implemented by means of the function. See the documentation there for a list of format specifiers that can be used. The function returns 0 on success and a negative value in case of error. dtsub Subtract one timestamp from another and return a variable of type interval. int dtsub(timestamp *ts1, timestamp *ts2, interval *iv); The function will subtract the timestamp variable that ts2 points to from the timestamp variable that ts1 points to and will store the result in the interval variable that iv points to. Upon success, the function returns 0 and a negative value if an error occurred. dttoasc Convert a timestamp variable to a C char* string. int dttoasc(timestamp *ts, char *output); The function receives a pointer to the timestamp variable to convert (ts) and the string that should hold the result of the operation output). It converts ts to its textual representation according to the SQL standard, which is be YYYY-MM-DD HH:MM:SS. Upon success, the function returns 0 and a negative value if an error occurred. dttofmtasc Convert a timestamp variable to a C char* using a format mask. int dttofmtasc(timestamp *ts, char *output, int str_len, char *fmtstr); The function receives a pointer to the timestamp to convert as its first argument (ts), a pointer to the output buffer (output), the maximal length that has been allocated for the output buffer (str_len) and the format mask to use for the conversion (fmtstr). Upon success, the function returns 0 and a negative value if an error occurred. Internally, this function uses the function. See the reference there for information on what format mask specifiers can be used. intoasc Convert an interval variable to a C char* string. int intoasc(interval *i, char *str); The function receives a pointer to the interval variable to convert (i) and the string that should hold the result of the operation str). It converts i to its textual representation according to the SQL standard, which is be YYYY-MM-DD HH:MM:SS. Upon success, the function returns 0 and a negative value if an error occurred. rfmtlong Convert a long integer value to its textual representation using a format mask. int rfmtlong(long lng_val, char *fmt, char *outbuf); The function receives the long value lng_val, the format mask fmt and a pointer to the output buffer outbuf. It converts the long value according to the format mask to its textual representation. The format mask can be composed of the following format specifying characters: * (asterisk) - if this position would be blank otherwise, fill it with an asterisk. & (ampersand) - if this position would be blank otherwise, fill it with a zero. # - turn leading zeroes into blanks. < - left-justify the number in the string. , (comma) - group numbers of four or more digits into groups of three digits separated by a comma. . (period) - this character separates the whole-number part of the number from the fractional part. - (minus) - the minus sign appears if the number is a negative value. + (plus) - the plus sign appears if the number is a positive value. ( - this replaces the minus sign in front of the negative number. The minus sign will not appear. ) - this character replaces the minus and is printed behind the negative value. $ - the currency symbol. rupshift Convert a string to upper case. void rupshift(char *str); The function receives a pointer to the string and transforms every lower case character to upper case. byleng Return the number of characters in a string without counting trailing blanks. int byleng(char *str, int len); The function expects a fixed-length string as its first argument (str) and its length as its second argument (len). It returns the number of significant characters, that is the length of the string without trailing blanks. ldchar Copy a fixed-length string into a null-terminated string. void ldchar(char *src, int len, char *dest); The function receives the fixed-length string to copy (src), its length (len) and a pointer to the destination memory (dest). Note that you need to reserve at least len+1 bytes for the string that dest points to. The function copies at most len bytes to the new location (less if the source string has trailing blanks) and adds the null-terminator. rgetmsg int rgetmsg(int msgnum, char *s, int maxsize); This function exists but is not implemented at the moment! rtypalign int rtypalign(int offset, int type); This function exists but is not implemented at the moment! rtypmsize int rtypmsize(int type, int len); This function exists but is not implemented at the moment! rtypwidth int rtypwidth(int sqltype, int sqllen); This function exists but is not implemented at the moment! rsetnull Set a variable to NULL. int rsetnull(int t, char *ptr); The function receives an integer that indicates the type of the variable and a pointer to the variable itself that is casted to a C char* pointer. The following types exist: CCHARTYPE - For a variable of type char or char* CSHORTTYPE - For a variable of type short int CINTTYPE - For a variable of type int CBOOLTYPE - For a variable of type boolean CFLOATTYPE - For a variable of type float CLONGTYPE - For a variable of type long CDOUBLETYPE - For a variable of type double CDECIMALTYPE - For a variable of type decimal CDATETYPE - For a variable of type date CDTIMETYPE - For a variable of type timestamp Here is an example of a call to this function: risnull Test if a variable is NULL. int risnull(int t, char *ptr); The function receives the type of the variable to test (t) as well a pointer to this variable (ptr). Note that the latter needs to be casted to a char*. See the function for a list of possible variable types. Here is an example of how to use this function: Note that all constants here describe errors and all of them are defined to represent negative values. In the descriptions of the different constants you can also find the value that the constants represent in the current implementation. However you should not rely on this number. You can however rely on the fact all of them are defined to represent negative values. ECPG_INFORMIX_NUM_OVERFLOW Functions return this value if an overflow occurred in a calculation. Internally it is defined to -1200 (the Informix definition). ECPG_INFORMIX_NUM_UNDERFLOW Functions return this value if an underflow occurred in a calculation. Internally it is defined to -1201 (the Informix definition). ECPG_INFORMIX_DIVIDE_ZERO Functions return this value if an attempt to divide by zero is observed. Internally it is defined to -1202 (the Informix definition). ECPG_INFORMIX_BAD_YEAR Functions return this value if a bad value for a year was found while parsing a date. Internally it is defined to -1204 (the Informix definition). ECPG_INFORMIX_BAD_MONTH Functions return this value if a bad value for a month was found while parsing a date. Internally it is defined to -1205 (the Informix definition). ECPG_INFORMIX_BAD_DAY Functions return this value if a bad value for a day was found while parsing a date. Internally it is defined to -1206 (the Informix definition). ECPG_INFORMIX_ENOSHORTDATE Functions return this value if a parsing routine needs a short date representation but did not get the date string in the right length. Internally it is defined to -1209 (the Informix definition). ECPG_INFORMIX_DATE_CONVERT Functions return this value if Internally it is defined to -1210 (the Informix definition). ECPG_INFORMIX_OUT_OF_MEMORY Functions return this value if Internally it is defined to -1211 (the Informix definition). ECPG_INFORMIX_ENOTDMY Functions return this value if a parsing routine was supposed to get a format mask (like mmddyy) but not all fields were listed correctly. Internally it is defined to -1212 (the Informix definition). ECPG_INFORMIX_BAD_NUMERIC Functions return this value either if a parsing routine cannot parse the textual representation for a numeric value because it contains errors or if a routine cannot complete a calculation involving numeric variables because at least one of the numeric variables is invalid. Internally it is defined to -1213 (the Informix definition). ECPG_INFORMIX_BAD_EXPONENT Functions return this value if Internally it is defined to -1216 (the Informix definition). ECPG_INFORMIX_BAD_DATE Functions return this value if Internally it is defined to -1218 (the Informix definition). ECPG_INFORMIX_EXTRA_CHARS Functions return this value if Internally it is defined to -1264 (the Informix definition). This section describes how you can handle exceptional conditions and warnings in an embedded SQL program. There are several nonexclusive facilities for this. One simple method to catch errors and warnings is to set a specific action to be executed whenever a particular condition occurs. In general: EXEC SQL WHENEVER condition action; condition can be one of the following: SQLERROR The specified action is called whenever an error occurs during the execution of an SQL statement. SQLWARNING The specified action is called whenever a warning occurs during the execution of an SQL statement. NOT FOUND The specified action is called whenever an SQL statement retrieves or affects zero rows. (This condition is not an error, but you might be interested in handling it specially.) action can be one of the following: CONTINUE This effectively means that the condition is ignored. This is the default. GOTO label GO TO label Jump to the specified label (using a C goto statement). SQLPRINT Print a message to standard error. This is useful for simple programs or during prototyping. The details of the message cannot be configured. STOP Call exit(1), which will terminate the program. DO BREAK Execute the C statement break. This should only be used in loops or switch statements. CALL name (args) DO name (args) Call the specified C functions with the specified arguments. The SQL standard only provides for the actions CONTINUE and GOTO (and GO TO). Here is an example that you might want to use in a simple program. It prints a simple message when a warning occurs and aborts the program when an error happens: EXEC SQL WHENEVER SQLWARNING SQLPRINT; EXEC SQL WHENEVER SQLERROR STOP; The statement EXEC SQL WHENEVER is a directive of the SQL preprocessor, not a C statement. The error or warning actions that it sets apply to all embedded SQL statements that appear below the point where the handler is set, unless a different action was set for the same condition between the first EXEC SQL WHENEVER and the SQL statement causing the condition, regardless of the flow of control in the C program. So neither of the two following C program excerpts will have the desired effect: /* * WRONG */ int main(int argc, char *argv[]) { ... if (verbose) { EXEC SQL WHENEVER SQLWARNING SQLPRINT; } ... EXEC SQL SELECT ...; ... } /* * WRONG */ int main(int argc, char *argv[]) { ... set_error_handler(); ... EXEC SQL SELECT ...; ... } static void set_error_handler(void) { EXEC SQL WHENEVER SQLERROR STOP; } For more powerful error handling, the embedded SQL interface provides a global variable with the name sqlca that has the following structure: struct { char sqlcaid[8]; long sqlabc; long sqlcode; struct { int sqlerrml; char sqlerrmc[SQLERRMC_LEN]; } sqlerrm; char sqlerrp[8]; long sqlerrd[6]; char sqlwarn[8]; char sqlstate[5]; } sqlca; (In a multithreaded program, every thread automatically gets its own copy of sqlca. This works similarly to the handling of the standard C global variable errno.) sqlca covers both warnings and errors. If multiple warnings or errors occur during the execution of a statement, then sqlca will only contain information about the last one. If no error occurred in the last SQL statement, sqlca.sqlcode will be 0 and sqlca.sqlstate will be "00000". If a warning or error occurred, then sqlca.sqlcode will be negative and sqlca.sqlstate will be different from "00000". A positive sqlca.sqlcode indicates a harmless condition, such as that the last query returned zero rows. sqlcode and sqlstate are two different error code schemes; details appear below. If the last SQL statement was successful, then sqlca.sqlerrd[1] contains the OID of the processed row, if applicable, and sqlca.sqlerrd[2] contains the number of processed or returned rows, if applicable to the command. In case of an error or warning, sqlca.sqlerrm.sqlerrmc will contain a string that describes the error. The field sqlca.sqlerrm.sqlerrml contains the length of the error message that is stored in sqlca.sqlerrm.sqlerrmc (the result of strlen(), not really interesting for a C programmer). Note that some messages are too long to fit in the fixed-size sqlerrmc array; they will be truncated. In case of a warning, sqlca.sqlwarn[2] is set to W. (In all other cases, it is set to something different from W.) If sqlca.sqlwarn[1] is set to W, then a value was truncated when it was stored in a host variable. sqlca.sqlwarn[0] is set to W if any of the other elements are set to indicate a warning. The fields sqlcaid, sqlcabc, sqlerrp, and the remaining elements of sqlerrd and sqlwarn currently contain no useful information. The structure sqlca is not defined in the SQL standard, but is implemented in several other SQL database systems. The definitions are similar at the core, but if you want to write portable applications, then you should investigate the different implementations carefully. The fields sqlca.sqlstate and sqlca.sqlcode are two different schemes that provide error codes. Both are derived from the SQL standard, but SQLCODE has been marked deprecated in the SQL-92 edition of the standard and has been dropped in later editions. Therefore, new applications are strongly encouraged to use SQLSTATE. SQLSTATE is a five-character array. The five characters contain digits or upper-case letters that represent codes of various error and warning conditions. SQLSTATE has a hierarchical scheme: the first two characters indicate the general class of the condition, the last three characters indicate a subclass of the general condition. A successful state is indicated by the code 00000. The SQLSTATE codes are for the most part defined in the SQL standard. The PostgreSQL server natively supports SQLSTATE error codes; therefore a high degree of consistency can be achieved by using this error code scheme throughout all applications. For further information see . SQLCODE, the deprecated error code scheme, is a simple integer. A value of 0 indicates success, a positive value indicates success with additional information, a negative value indicates an error. The SQL standard only defines the positive value +100, which indicates that the last command returned or affected zero rows, and no specific negative values. Therefore, this scheme can only achieve poor portability and does not have a hierarchical code assignment. Historically, the embedded SQL processor for PostgreSQL has assigned some specific SQLCODE values for its use, which are listed below with their numeric value and their symbolic name. Remember that these are not portable to other SQL implementations. To simplify the porting of applications to the SQLSTATE scheme, the corresponding SQLSTATE is also listed. There is, however, no one-to-one or one-to-many mapping between the two schemes (indeed it is many-to-many), so you should consult the global SQLSTATE listing in in each case. These are the assigned SQLCODE values: -12 (ECPG_OUT_OF_MEMORY) Indicates that your virtual memory is exhausted. (SQLSTATE YE001) -200 (ECPG_UNSUPPORTED) Indicates the preprocessor has generated something that the library does not know about. Perhaps you are running incompatible versions of the preprocessor and the library. (SQLSTATE YE002) -201 (ECPG_TOO_MANY_ARGUMENTS) This means that the command specified more host variables than the command expected. (SQLSTATE 07001 or 07002) -202 (ECPG_TOO_FEW_ARGUMENTS) This means that the command specified fewer host variables than the command expected. (SQLSTATE 07001 or 07002) -203 (ECPG_TOO_MANY_MATCHES) This means a query has returned multiple rows but the statement was only prepared to store one result row (for example, because the specified variables are not arrays). (SQLSTATE 21000) -204 (ECPG_INT_FORMAT) The host variable is of type int and the datum in the database is of a different type and contains a value that cannot be interpreted as an int. The library uses strtol() for this conversion. (SQLSTATE 42804) -205 (ECPG_UINT_FORMAT) The host variable is of type unsigned int and the datum in the database is of a different type and contains a value that cannot be interpreted as an unsigned int. The library uses strtoul() for this conversion. (SQLSTATE 42804) -206 (ECPG_FLOAT_FORMAT) The host variable is of type float and the datum in the database is of another type and contains a value that cannot be interpreted as a float. The library uses strtod() for this conversion. (SQLSTATE 42804) -211 (ECPG_CONVERT_BOOL) This means the host variable is of type bool and the datum in the database is neither 't' nor 'f'. (SQLSTATE 42804) -212 (ECPG_EMPTY) The statement sent to the PostgreSQL server was empty. (This cannot normally happen in an embedded SQL program, so it might point to an internal error.) (SQLSTATE YE002) -213 (ECPG_MISSING_INDICATOR) A null value was returned and no null indicator variable was supplied. (SQLSTATE 22002) -214 (ECPG_NO_ARRAY) An ordinary variable was used in a place that requires an array. (SQLSTATE 42804) -215 (ECPG_DATA_NOT_ARRAY) The database returned an ordinary variable in a place that requires array value. (SQLSTATE 42804) -220 (ECPG_NO_CONN) The program tried to access a connection that does not exist. (SQLSTATE 08003) -221 (ECPG_NOT_CONN) The program tried to access a connection that does exist but is not open. (This is an internal error.) (SQLSTATE YE002) -230 (ECPG_INVALID_STMT) The statement you are trying to use has not been prepared. (SQLSTATE 26000) -240 (ECPG_UNKNOWN_DESCRIPTOR) The descriptor specified was not found. The statement you are trying to use has not been prepared. (SQLSTATE 33000) -241 (ECPG_INVALID_DESCRIPTOR_INDEX) The descriptor index specified was out of range. (SQLSTATE 07009) -242 (ECPG_UNKNOWN_DESCRIPTOR_ITEM) An invalid descriptor item was requested. (This is an internal error.) (SQLSTATE YE002) -243 (ECPG_VAR_NOT_NUMERIC) During the execution of a dynamic statement, the database returned a numeric value and the host variable was not numeric. (SQLSTATE 07006) -244 (ECPG_VAR_NOT_CHAR) During the execution of a dynamic statement, the database returned a non-numeric value and the host variable was numeric. (SQLSTATE 07006) -400 (ECPG_PGSQL) Some error caused by the PostgreSQL server. The message contains the error message from the PostgreSQL server. -401 (ECPG_TRANS) The PostgreSQL server signaled that we cannot start, commit, or rollback the transaction. (SQLSTATE 08007) -402 (ECPG_CONNECT) The connection attempt to the database did not succeed. (SQLSTATE 08001) 100 (ECPG_NOT_FOUND) This is a harmless condition indicating that the last command retrieved or processed zero rows, or that you are at the end of the cursor. (SQLSTATE 02000) To include an external file into your embedded SQL program, use: EXEC SQL INCLUDE filename; The embedded SQL preprocessor will look for a file named filename.h, preprocess it, and include it in the resulting C output. Thus, embedded SQL statements in the included file are handled correctly. Note that this is not the same as: #include <filename.h> because this file would not be subject to SQL command preprocessing. Naturally, you can continue to use the C #include directive to include other header files. The include file name is case-sensitive, even though the rest of the EXEC SQL INCLUDE command follows the normal SQL case-sensitivity rules. Similar to the directive #define that is known from C, embedded SQL has a similar concept: EXEC SQL DEFINE name; EXEC SQL DEFINE name value; So you can define a name: EXEC SQL DEFINE HAVE_FEATURE; And you can also define constants: EXEC SQL DEFINE MYNUMBER 12; EXEC SQL DEFINE MYSTRING 'abc'; Use undef to remove a previous definition: EXEC SQL UNDEF MYNUMBER; Of course you can continue to use the C versions #define and #undef in your embedded SQL program. The difference is where your defined values get evaluated. If you use EXEC SQL DEFINE then the ecpg preprocessor evaluates the defines and substitutes the values. For example if you write: EXEC SQL DEFINE MYNUMBER 12; ... EXEC SQL UPDATE Tbl SET col = MYNUMBER; then ecpg will already do the substitution and your C compiler will never see any name or identifier MYNUMBER. Note that you cannot use #define for a constant that you are going to use in an embedded SQL query because in this case the embedded SQL precompiler is not able to see this declaration. You can use the following directives to compile code sections conditionally: EXEC SQL ifdef name; Checks a name and processes subsequent lines if name has been created with EXEC SQL define name. EXEC SQL ifndef name; Checks a name and processes subsequent lines if name has not been created with EXEC SQL define name. EXEC SQL else; Starts processing an alternative section to a section introduced by either EXEC SQL ifdef name or EXEC SQL ifndef name. EXEC SQL elif name; Checks name and starts an alternative section if name has been created with EXEC SQL define name. EXEC SQL endif; Ends an alternative section. Example: EXEC SQL ifndef TZVAR; EXEC SQL SET TIMEZONE TO 'GMT'; EXEC SQL elif TZNAME; EXEC SQL SET TIMEZONE TO TZNAME; EXEC SQL else; EXEC SQL SET TIMEZONE TO TZVAR; EXEC SQL endif; Now that you have an idea how to form embedded SQL C programs, you probably want to know how to compile them. Before compiling you run the file through the embedded SQL C preprocessor, which converts the SQL statements you used to special function calls. After compiling, you must link with a special library that contains the needed functions. These functions fetch information from the arguments, perform the SQL command using the libpq interface, and put the result in the arguments specified for output. The preprocessor program is called ecpg and is included in a normal PostgreSQL installation. Embedded SQL programs are typically named with an extension .pgc. If you have a program file called prog1.pgc, you can preprocess it by simply calling: ecpg prog1.pgc This will create a file called prog1.c. If your input files do not follow the suggested naming pattern, you can specify the output file explicitly using the -o option. The preprocessed file can be compiled normally, for example: cc -c prog1.c The generated C source files include header files from the PostgreSQL installation, so if you installed PostgreSQL in a location that is not searched by default, you have to add an option such as -I/usr/local/pgsql/include to the compilation command line. To link an embedded SQL program, you need to include the libecpg library, like so: cc -o myprog prog1.o prog2.o ... -lecpg Again, you might have to add an option like -L/usr/local/pgsql/lib to that command line. If you manage the build process of a larger project using make, it might be convenient to include the following implicit rule to your makefiles: ECPG = ecpg %.c: %.pgc $(ECPG) $< The complete syntax of the ecpg command is detailed in . The ecpg library is thread-safe by default. However, you might need to use some threading command-line options to compile your client code. The libecpg library primarily contains hidden functions that are used to implement the functionality expressed by the embedded SQL commands. But there are some functions that can usefully be called directly. Note that this makes your code unportable. ECPGdebug(int on, FILE *stream) turns on debug logging if called with the first argument non-zero. Debug logging is done on stream. The log contains all SQL statements with all the input variables inserted, and the results from the PostgreSQL server. This can be very useful when searching for errors in your SQL statements. On Windows, if the ecpg libraries and an application are compiled with different flags, this function call will crash the application because the internal representation of the FILE pointers differ. Specifically, multithreaded/single-threaded, release/debug, and static/dynamic flags should be the same for the library and all applications using that library. ECPGget_PGconn(const char *connection_name) returns the library database connection handle identified by the given name. If connection_name is set to NULL, the current connection handle is returned. If no connection handle can be identified, the function returns NULL. The returned connection handle can be used to call any other functions from libpq, if necessary. It is a bad idea to manipulate database connection handles made from ecpg directly with libpq routines. ECPGtransactionStatus(const char *connection_name) returns the current transaction status of the given connection identified by connection_name. See and libpq's PQtransactionStatus() for details about the returned status codes. ECPGstatus(int lineno, const char* connection_name) returns true if you are connected to a database and false if not. connection_name can be NULL if a single connection is being used. This section explains how ECPG works internally. This information can occasionally be useful to help users understand how to use ECPG. The first four lines written by ecpg to the output are fixed lines. Two are comments and two are include lines necessary to interface to the library. Then the preprocessor reads through the file and writes output. Normally it just echoes everything to the output. When it sees an EXEC SQL statement, it intervenes and changes it. The command starts with EXEC SQL and ends with ;. Everything in between is treated as an SQL statement and parsed for variable substitution. Variable substitution occurs when a symbol starts with a colon (:). The variable with that name is looked up among the variables that were previously declared within a EXEC SQL DECLARE section. The most important function in the library is ECPGdo, which takes care of executing most commands. It takes a variable number of arguments. This can easily add up to 50 or so arguments, and we hope this will not be a problem on any platform. The arguments are: A line number This is the line number of the original line; used in error messages only. A string This is the SQL command that is to be issued. It is modified by the input variables, i.e., the variables that where not known at compile time but are to be entered in the command. Where the variables should go the string contains ?. Input variables Every input variable causes ten arguments to be created. (See below.) ECPGt_EOIT An enum telling that there are no more input variables. Output variables Every output variable causes ten arguments to be created. (See below.) These variables are filled by the function. ECPGt_EORT An enum telling that there are no more variables. For every variable that is part of the SQL command, the function gets ten arguments: The type as a special symbol. A pointer to the value or a pointer to the pointer. The size of the variable if it is a char or varchar. The number of elements in the array (for array fetches). The offset to the next element in the array (for array fetches). The type of the indicator variable as a special symbol. A pointer to the indicator variable. 0 The number of elements in the indicator array (for array fetches). The offset to the next element in the indicator array (for array fetches). Note that not all SQL commands are treated in this way. For instance, an open cursor statement like: EXEC SQL OPEN cursor; is not copied to the output. Instead, the cursor's DECLARE command is used at the position of the OPEN command because it indeed opens the cursor. Here is a complete example describing the output of the preprocessor of a file foo.pgc (details might change with each particular version of the preprocessor): EXEC SQL BEGIN DECLARE SECTION; int index; int result; EXEC SQL END DECLARE SECTION; ... EXEC SQL SELECT res INTO :result FROM mytable WHERE index = :index; is translated into: ; #include ; /* exec sql begin declare section */ #line 1 "foo.pgc" int index; int result; /* exec sql end declare section */ ... ECPGdo(__LINE__, NULL, "SELECT res FROM mytable WHERE index = ? ", ECPGt_int,&(index),1L,1L,sizeof(int), ECPGt_NO_INDICATOR, NULL , 0L, 0L, 0L, ECPGt_EOIT, ECPGt_int,&(result),1L,1L,sizeof(int), ECPGt_NO_INDICATOR, NULL , 0L, 0L, 0L, ECPGt_EORT); #line 147 "foo.pgc" ]]> (The indentation here is added for readability and not something the preprocessor does.)