#include <stdlib.h> void *calloc(size_t nmemb, size_t size);
void *malloc(size_t size);
void free(void *ptr);
void *realloc(void *ptr, size_t size);
malloc() allocates size bytes and returns a pointer to the allocated memory. The memory is not cleared. If size is 0, then malloc() returns either NULL, or a unique pointer value that can later be successfully passed to free().
free() frees the memory space pointed to by ptr, which must have been returned by a previous call to malloc(), calloc() or realloc(). Otherwise, or if free(ptr) has already been called before, undefined behavior occurs. If ptr is NULL, no operation is performed.
realloc() changes the size of the memory block pointed to by ptr to size bytes. The contents will be unchanged to the minimum of the old and new sizes; newly allocated memory will be uninitialized. 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). Unless ptr is NULL, it must have been returned by an earlier call to malloc(), calloc() or realloc(). If the area pointed to was moved, a free(ptr) is done.
free() returns no value.
realloc() returns a pointer to the newly allocated memory, which is suitably aligned for any kind of variable and may be different from ptr, or NULL if the request fails. If size was equal to 0, either NULL or a pointer suitable to be passed to free() is returned. If realloc() fails the original block is left untouched; it is not freed or moved.
The Unix98 standard requires malloc(), calloc(), and realloc() to set errno to ENOMEM upon failure. Glibc assumes that this is done (and the glibc versions of these routines do this); if you use a private malloc implementation that does not set errno, then certain library routines may fail without having a reason in errno.
Crashes in malloc(), calloc(), realloc(), or free() are almost always related to heap corruption, such as overflowing an allocated chunk or freeing the same pointer twice.
Recent versions of Linux libc (later than 5.4.23) and glibc (2.x) include a malloc() implementation which is tunable via environment variables. When MALLOC_CHECK_ is set, a special (less efficient) implementation is used which is designed to be tolerant against simple errors, such as double calls of free() with the same argument, or overruns of a single byte (off-by-one bugs). Not all such errors can be protected against, however, and memory leaks can result. If MALLOC_CHECK_ is set to 0, any detected heap corruption is silently ignored; if set to 1, a diagnostic message is printed on stderr; if set to 2, abort(3) is called immediately; if set to 3, a diagnostic message is printed on stderr and the program is aborted. Using a nonzero MALLOC_CHECK_ value can be useful because otherwise a crash may happen much later, and the true cause for the problem is then very hard to track down.
# echo 2 > /proc/sys/vm/overcommit_memorySee also the kernel Documentation directory, files vm/overcommit-accounting and sysctl/vm.txt.