Question

I came across an issue that I am not sure if it’s an issue at all. I have a simple C funcion that gets a char* passed from string like so:

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

int main(int argc, char** argv) {

    passString("hello");

    return (EXIT_SUCCESS);
}

void passString(char * string) {
    // .... some code ....
    free(string); // ???
}

and I was taught to free every memory block that I’m not working with anymore (mainly arrays). So my though was to free string as well but the program freezes or crashes even with this simple example. I’m not sure whether I really need to free string here or not and if so how do I achieve that?

Answer 1

You do not need to free memory which you did not allocate with malloc.

In your example string is not allocated by malloc in your program so you do not need to free it. string is a string literal and is allocated somewhere in the read-only memory(implementation defined) and is automatically freed.

For standerdese fans:

References:
c99 Standard: 7.20.3.2 The free function

Synopsis
#include
void free(void *ptr);

Description:
The free function causes the space pointed to by ptr to be deallocated, that is, made
available for further allocation. If ptr is a null pointer, no action occurs. Otherwise, if
the argument does not match a pointer earlier returned by the calloc, malloc,or
realloc function, or if the space has been deallocated by a call to free or realloc,
the behavior is undefined.

Returns
The free function returns no value.