Memory release

[Classification]

Standard library

[Syntax]

#include <stdlib.h>

void free(void *ptr);

[Description]

This function releases the area pointed to by ptr so that this area is subsequently available for allocation. The area that was acquired by calloc, malloc, or realloc must be specified for ptr.

[Example]

#include    <stdlib.h>

typedef struct {

    double  d[3];

    int     i[2];

} s_data;

int func(void) {

    s_data   *buf;

    if((buf = calloc(40, sizeof(s_data))) == NULL)  /*allocate an area for 40 s_data*/

        return(1);

      :

    free(buf);                                      /*release the area*/

    return(0);

}

If one of the following operations is performed when using the library for the security facility, the __heap_chk_fail function is called.

The __heap_chk_fail function needs to be defined by the user and it describes the processing to be executed when an error occurs in management of dynamic memory.

Note the following points when defining the __heap_chk_fail function.

The calloc, malloc, and realloc functions for the security facility allocate four extra bytes before and after each allocated area for the purpose of detecting writing to addresses outside the allocated area. This consumes more heap memory area than with the usual functions.

#include <stdlib.h>

void sub(int *ip) {

    ...

    free(ip);

}

int func(void) {

    int *ip;

    if ((ip = malloc(40 * sizeof(int))) == NULL)

        if ((ip = malloc(10 * sizeof(int))) == NULL) return(1);

        else sub(ip); /* First appearance of free */

    else

        ...

    free(ip); /* Second appearance of free */

    return(0);

}

void __heap_chk_fail(void) {

    /* Processing when corruption of heap memory area is detected */

}