Nested printf (printf inside printf) in C

Passing NULL to printf in C

Last Updated : 02 Jun, 2017

Consider the following C code snippet.

char* p = NULL;
printf("%s", p);

What should be the output of the above program? The print expects a '\0' terminated array of characters (or string literal) whereas it receives a null pointer. Passing NULL to printf is undefined behavior.

According to Section 7.1.4(of C99 or C11) : Use of library functions

If an argument to a function has an invalid value (such as a value outside the domain of the function, or a pointer outside the address space of the program, or a null pointer, or a pointer to non-modifiable storage when the corresponding parameter is not const-qualified) or a type (after promotion) not expected by a function with variable number of arguments, the behavior is undefined.
Some compilers may produce null while others Segmentation Fault. GCC prints (null).

C

// Effect of passing null pointers to ( %s ) 
// printf in C
#include <stdio.h>

int main()
{
   char* p = NULL;
   printf( "%s", p);
   return 0;
}

Output in GCC:

(null)

Note that the above program may cause undefined behavior as per C standard.

Nested printf (printf inside printf) in C

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Aditya Chatterjee

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