Should a null character string be equal to a null-terminator in C?

Question

Is there a difference between initializing a character array with null characters and with a null terminator ? These two examples:

char * result = ""
char * result = '\0';

The first one gives me an error but the other one is passable.

Function prototype:

char * form(int n, ...);

The code is:

char * result = "";
char *next;
char val_int[12];
int i;
va_list args;
va_start(args, n);

for (i = 0; i < n; i++)
{
    int len = result ? strlen(result) : 0;
    sprintf(val_int, "%d", va_arg(args,int));
    next = val_int;
    char *tmp = (char *)malloc(len + strlen(next) + 1);

    strcpy(tmp, result ? result : "");
    strcat(tmp, next);

    free(result);
    result = tmp;
}
va_end(args);
return result;

Main function:

  char *s;
  s = form(3,123,456,789);
  printf("%s", s);
  free(s);
  return 0;

Answer 1

The first declaration

char * result = "";

declares a pointer to the string literal that contains one character: the terminating zero '\0'.

This declaration

char * result = '\0';

initialize the pointer by a null pointer constant and is equivalent to

char * result = NULL;

Here is a demonstrative program

#include <stdio.h>

int main(void) 
{
    char *s1 = "";
    char *s2 = '\0';

    printf( "s1 == NULL is %s\n", s1 == NULL ? "true" : "false" );
    printf( "s2 == NULL is %s\n", s2 == NULL ? "true" : "false" );

    return 0;
}

Its output is

s1 == NULL is false
s2 == NULL is true

As for the error you are saying about then its reason is this statement

free(result);

You may not free a memory occupied by a string literal. You may free a memory that was allocated dynamically.

When there is used a pointer initialized by the character literal '\0' then such a pointer is a null pointer and you may call free for a null pointer.

That is if you have a pointer initialized like

char * result = NULL;

that is the same as to write

char * result = '\0';

then you may call

free( result );

No action will be performed by such a call.

But if you will write

free( "" );

trying to free a string literal that has static storage duration you will get undefined behavior.

And this call

free( "" );

is equivalent to the following code snippet

char * result = "";
free( result );