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# free

From cppreference.com
< c‎ | memory

Defined in header `<stdlib.h>` |  |
void free( void* ptr ); |  |

Deallocates the space previously allocated by malloc(), calloc() or realloc(). If `ptr` is null-pointer, the function does nothing.

The behavior is undefined if `ptr` does not match a pointer returned earlier by malloc(), calloc(), realloc(), or aligned_alloc(). Also, the behavior is undefined if the memory area referred to by `ptr` has already been deallocated, that is, `free()` or realloc() has already been called with `ptr` as the argument and no calls to malloc(), calloc() or realloc() resulted in a pointer equal to `ptr` afterwards.

`free` is thread-safe: it behaves as though only accessing the memory locations visible through its argument, and not any static storage. A call to free that deallocates a region of memory synchronizes-with a call to any subsequent allocation function that allocates the same or a part of the same region of memory. This synchronization occurs after any access to the memory by the deallocating function and before any access to the memory by the allocation function. | (since C11)

### [edit] Parameters

ptr | - | pointer to the memory to deallocate

### [edit] Return value

(none)

### [edit] Example

| This section is incomplete Reason: simplify or make more practical

Run this code

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

    int main(void)
    {
        /* Allocate an array with 100 integers. */
        int *pa = malloc(100*sizeof(int));
        if (pa == NULL) {        /* check that memory allocation occurred */
           printf("malloc() failed in file %s at line # %d", __FILE__,__LINE__);
           printf("***  PROGRAM TERMINATED  ***");
           exit(1);
        }
        printf("starting address of pa:   %p\n", (void*)pa);

        /* Deallocate array pa. */
        free(pa), pa = NULL;

        /* Allocate an array with 100 doubles. */
        double *pb = malloc(100*sizeof(double));
        if (pb == NULL) {        /* check that memory allocation occurred */
           printf("malloc() failed in file %s at line # %d", __FILE__,__LINE__);
           printf("***  PROGRAM TERMINATED  ***");
           exit(1);
        }

        /* Array pb may have the same starting address that array pa had. */
        printf("starting address of pb:   %p\n", (void*)pb);

        free(pb);
        return 0;
    }

Possible output:

    starting address of pa:   0x2113010
    starting address of pb:   0x2113010

### [edit] See also

C++ documentation for free

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