Why can't one print NULL in c program?

Question

In the following program, i am getting a runtime error instead of my program printing NULL. May be this is very silly or simple understanding but i do not have right answer. Thanks in advance.

#include<stdio.h>

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

The runtime error is because you are dereferencing a null pointer — Ardent Coder, May 15 '20 at 14:57
You mean `printf("%s", p);`? (its behavior [depends on environment](https://stackoverflow.com/questions/11589342/what-is-the-behavior-of-printing-null-with-printfs-s-specifier)) — MikeCAT, May 15 '20 at 14:59
Does this answer your question? [What EXACTLY is meant by "de-referencing a NULL pointer"?](https://stackoverflow.com/questions/4007268/what-exactly-is-meant-by-de-referencing-a-null-pointer) — Hitokiri, May 15 '20 at 15:08

score 4 · Answer 1 · answered May 15 '20 at 15:00

4

C doesn't catch you trying to de-reference a null pointer.

If you want to see the value of the pointer (and see whether the pointer is null), use this printf syntax:

   printf("%p\n", p);

answered May 15 '20 at 15:00

rtx13

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`printf("%p\n", (void*)p);` is better to avoid invoking undefined behavior. – MikeCAT May 15 '20 at 15:01
1

@MikeCAT can you provide a reference to why using `char*` instead `void*` results in undefined behaviour? – rtx13 May 15 '20 at 15:03
[c - printf("%p") and casting to (void *) - Stack Overflow](https://stackoverflow.com/questions/24867814/printfp-and-casting-to-void) – MikeCAT May 15 '20 at 15:07
1

@MikeCAT please see [C11 § 6.2.5 ¶ 28](http://port70.net/~nsz/c/c11/n1570.html#6.2.5p28): _A pointer to void shall have the same representation and alignment requirements as a pointer to a character type_, and its footnote _The same representation and alignment requirements are meant to imply interchangeability as arguments to functions, return values from functions, and members of unions_. – rtx13 May 15 '20 at 15:19

score 2 · Accepted Answer · edited May 15 '20 at 15:03

2

printf("%c", *p); does not ask printf to print the value NULL. It attempts to use the value pointed to by p. Since p contains a null pointer, it does not point to a valid object.

To print the value of p, use printf("%p", (void *) p);.

edited May 15 '20 at 15:03

MikeCAT

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answered May 15 '20 at 15:02

Eric Postpischil

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Why can't one print NULL in c program?

2 Answers2