Read/Write Structure From/to a File in C

For writing in the file, it is easy to write a string or an int to a file using fprintf and putc, but you might have faced difficulty when writing the contents of the struct. fwrite and fread make tasks easier when you want to write and read blocks of data.

In C language, structures are written/read in binary format, which ensures exact byte-by-byte storage and avoids issues like text formatting.

For writing/reading structures, the file should be opened in the binary mode ("rb", "wb" or "ab" ).

rb: for binary reading
wb: for binary writing
ab: for appending binary

Writing Structures to a File using fwrite

The first step for writing structures in a file is to open the file in "wb" or "ab" mode. Then, we can use the fwrite() function to easily write a structure to a file. The fwrite() function writes to the file stream in the form of a binary data block.

Syntax of fwrite()

size_t fwrite(const void *ptr, size_t size, size_t nmemb, FILE *stream)

Parameters:

ptr: pointer to the block of memory to be written.
size: the size of each element to be written (in bytes).
nmemb: number of elements.
stream: FILE pointer to the output file stream.

Return Value: Number of objects written.

Example

#include <stdio.h>
#include <stdlib.h>

// a struct to be read and written
struct person {
    int id;
    char fname[20];
    char lname[20];
};

int main()
{
    FILE* outfile;

    // open file for writing
    outfile = fopen("person.bin", "wb");
    if (outfile == NULL) {
        fprintf(stderr, "\nError opened file\n");
        exit(1);
    }

    struct person input1 = { 1, "rohan", "sharma" };

    // write struct to file
    int flag = 0;
    flag = fwrite(&input1, sizeof(struct person), 1,
                  outfile);
    if (flag) {
        printf("Contents of the structure written "
               "successfully");
    }
    else
        printf("Error Writing to File!");

    // close file
    fclose(outfile);

    return 0;
}

Output

Contents of the structure written successfully

Reading Structure from a File using fread

To read structure from a file we need to open the file in "rb" binary read mode and then we can simply read structures using fread() function. This function reads a block of memory from the given stream.

Syntax of fread()

size_t fread(void *ptr, size_t size, size_t nmemb, FILE *stream)