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Passing NULL to printf in C

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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).

// 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:


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


This article is contributed by Aditya Chatterjee. If you like GeeksforGeeks and would like to contribute, you can also write an article and mail your article to See your article appearing on the GeeksforGeeks main page and help other Geeks.

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Last Updated : 02 Jun, 2017
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