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cpointersdynamic-memory-allocationfreec-strings

ANSI C - Why malloc and free dont work for char pointers?

When I try to run this code:

char *s;
s = (char *) malloc (15);
s = "hello world";
free(s);

using gcc ts.c -ansi -Wall the result is:

free(): invalid pointer
Aborted (core dumped)

and the warning is:

‘free’ called on a pointer to an unallocated object

I don't understand why char pointers are different from other pointers.

Solution

  • This code snippet

    char *s;
s = (char *) malloc (15);
s = "hello world";

    produces a memory leak.

    At first a memory was dynamically allocated and its address was assigned to the pointer s

    s = (char *) malloc (15);

    and then the pointer s was reassigned with the address of the first character of a string literal

    s = "hello world";

    In fact the above statement is equivalent to

    s = &"hello world"[0];

    String literals have static storage duration. So you may not apply the function free for string literals.

    Instead of this assignment

    s = "hello world";

    you need to use the standard string function strcpy declared in the header <string.h>

    #include <string.h>

//...

strcpy( s, "hello world" );