malloc(0) actually works? [duplicate]

Question

Possible Duplicate:
what’s the point in malloc(0)?

Why does malloc(0) actually return a valid pointer for writing ?

char *str = NULL;

str = (char*)malloc(0); // allocate 0 bytes ?

printf("Pointer of str: %p\n", str);

strcpy(str, "A very long string ...................");

printf("Value of str: %s", str);

free(str); // Causes crash if str is too long

Output:

Pointer of str: 0xa9d010
Aborted
Value of str: A very long string ...................

When str is shorter then it just works as it should.

BTW: For compiling I used GCC with "-D_FORTIY_SOURCE=0 -fno-stack-protector"

*** glibc detected *** ..: free(): invalid next size (fast): 0x0000000000a9d010 ***

Answer 1

malloc() is supposed to return a void* pointer. And it faithfully does that. But leads to UB when you dereference it.

Answer 2

It is undefined behavior to dereference the pointer returned by malloc(0).

From the C Standard:

(C99, 7.20.3p1) "If the size of the space requested is zero, the behavior is implementation defined: either a null pointer is returned, or the behavior is as if the size were some nonzero value, except that the returned pointer shall not be used to access an object."

Answer 3

Why does malloc(0) actually return a valid pointer for writing?

It doesn't return a valid pointer for writing. It returns a valid pointer for not using it. Or it may return NULL as well since the C standard specifies this case to be implementation defined.