malloc - allocate memory dynamically

malloc, free, calloc, realloc, reallocarray - allocate and free dynamic memory

Header File

#include <stdlib.h>

Prototype

void *malloc(size_t size);

Think of void * as meaning the address of any type of value in memory. Think of size_t as a long.

#include <stdlib.h>
void *malloc(size_t size);
void free(void *_Nullable ptr);
void *calloc(size_t nmemb, size_t size);
void *realloc(void *_Nullable ptr, size_t size);
void *reallocarray(void *_Nullable ptr, size_t nmemb, size_t size);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

    Since glibc 2.29:
        _DEFAULT_SOURCE
    glibc 2.28 and earlier:
        _GNU_SOURCE

This function dynamically allocates size contiguous bytes of memory (on the heap) that can be used to store any type of values.

malloc()

The malloc() function allocates size bytes and returns a pointer to the allocated memory. The memory is not initialized. If size is 0, then malloc() returns a unique pointer value that can later be successfully passed to free(). (See "Nonportable behavior" for portability issues.)

free()

The free() function frees the memory space pointed to by ptr, which must have been returned by a previous call to malloc() or related functions. Otherwise, or if ptr has already been freed, undefined behavior occurs. If ptr is NULL, no operation is performed.

calloc()

The calloc() function allocates memory for an array of nmemb elements of size bytes each and returns a pointer to the allocated memory. The memory is set to zero. If nmemb or size is 0, then calloc() returns a unique pointer value that can later be successfully passed to free().

If the multiplication of nmemb and size would result in integer overflow, then calloc() returns an error. By contrast, an integer overflow would not be detected in the following call to malloc(), with the result that an incorrectly sized block of memory would be allocated:

malloc(nmemb * size);

realloc()

The realloc() function changes the size of the memory block pointed to by ptr to size bytes. The contents of the memory will be unchanged in the range from the start of the region up to the minimum of the old and new sizes. If the new size is larger than the old size, the added memory will not be initialized.

If ptr is NULL, then the call is equivalent to malloc(size), for all values of size.

If size is equal to zero, and ptr is not NULL, then the call is equivalent to free(ptr) (but see "Nonportable behavior" for portability issues).

Unless ptr is NULL, it must have been returned by an earlier call to malloc or related functions. If the area pointed to was moved, a free(ptr) is done.

reallocarray()

The reallocarray() function changes the size of (and possibly moves) the memory block pointed to by ptr to be large enough for an array of nmemb elements, each of which is size bytes. It is equivalent to the call

realloc(ptr, nmemb * size);

However, unlike that realloc() call, reallocarray() fails safely in the case where the multiplication would overflow. If such an overflow occurs, reallocarray() returns an error.

This function returns the address of the first byte of memory allocated or NULL in cases of error (as when insufficient memory is available).

The malloc(), calloc(), realloc(), and reallocarray() functions return a pointer to the allocated memory, which is suitably aligned for any type that fits into the requested size or less. On error, these functions return NULL and set errno. Attempting to allocate more than PTRDIFF_MAX bytes is considered an error, as an object that large could cause later pointer subtraction to overflow.

The free() function returns no value, and preserves errno.

The realloc() and reallocarray() functions return NULL if ptr is not NULL and the requested size is zero; this is not considered an error. (See "Nonportable behavior" for portability issues.) Otherwise, the returned pointer may be the same as ptr if the allocation was not moved (e.g., there was room to expand the allocation in-place), or different from ptr if the allocation was moved to a new address. If these functions fail, the original block is left untouched; it is not freed or moved.

#include <err.h>
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MALLOCARRAY(n, type)  ((type *) my_mallocarray(n, sizeof(type)))
#define MALLOC(type)          MALLOCARRAY(1, type)
static inline void *my_mallocarray(size_t nmemb, size_t size);
int
main(void)
{
    char  *p;
    p = MALLOCARRAY(32, char);
    if (p == NULL)
        err(EXIT_FAILURE, "malloc");
    strlcpy(p, "foo", 32);
    puts(p);
}
static inline void *
my_mallocarray(size_t nmemb, size_t size)
{
    return reallocarray(NULL, nmemb, size);
}

valgrind(1), brk(2), mmap(2), alloca(3), malloc_get_state(3), malloc_info(3), malloc_trim(3), malloc_usable_size(3), mallopt(3), mcheck(3), mtrace(3), posix_memalign(3)

For details of the GNU C library implementation, see https://sourceware.org/glibc/wiki/MallocInternals">.