free

void free( void *ptr );

Deallocates the space previously allocated by malloc(), calloc(), aligned_alloc(),(since C11) or realloc().

If ptr is a null pointer, the function does nothing.

The behavior is undefined if the value of ptr does not equal a value returned earlier by malloc(), calloc(), realloc(), or aligned_alloc()(since C11).

The behavior is undefined if the memory area referred to by ptr has already been deallocated, that is, free(), free_sized(), free_aligned_sized()(since C23), or realloc() has already been called with ptr as the argument and no calls to malloc(), calloc(), realloc(), or aligned_alloc()(since C11) resulted in a pointer equal to ptr afterwards.

The behavior is undefined if after free() returns, an access is made through the pointer ptr (unless another allocation function happened to result in a pointer value equal to ptr).

Parameters

Return value

(none)

Notes

The function accepts (and does nothing with) the null pointer to reduce the amount of special-casing. Whether allocation succeeds or not, the pointer returned by an allocation function can be passed to free().

Example

#include <stdlib.h>
 
int main(void)
{
    int *p1 = malloc(10*sizeof *p1);
    free(p1); // every allocated pointer must be freed
 
    int *p2 = calloc(10, sizeof *p2);
    int *p3 = realloc(p2, 1000*sizeof *p3);
    if(p3) // p3 not null means p2 was freed by realloc
       free(p3);
    else // p3 null means p2 was not freed
       free(p2);
}

References

• C23 standard (ISO/IEC 9899:2023):
• 7.24.3.3 The free function
• C17 standard (ISO/IEC 9899:2018):
• 7.22.3.3 The free function (p: 254)
• C11 standard (ISO/IEC 9899:2011):
• 7.22.3.3 The free function (p: 348)
• C99 standard (ISO/IEC 9899:1999):
• 7.20.3.2 The free function (p: 313)
• C89/C90 standard (ISO/IEC 9899:1990):
• 4.10.3.2 The free function

See also