MALLOC(3) Linux Programmer s Manual MALLOC(3)
NAME calloc, malloc, free, realloc - Allocate and free dynamic memory
SYNOPSIS #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);
DESCRIPTION calloc() 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.
malloc() allocates size bytes and returns a pointer to the allocated memory. The memory is not cleared.
free() frees the memory space pointed to by ptr, which must have been returned by a previous call to malloc(), calloc() or realloc(). Other- wise, or if free(ptr) has already been called before, undefined behaviour 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, the call is equivalent to malloc(size); if size is equal to zero, 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.
RETURN VALUE For calloc() and malloc(), the value returned is a pointer to the allo- cated memory, which is suitably aligned for any kind of variable, or NULL if the request fails.
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.
CONFORMING TO C89, C99.
SEE ALSO brk(2), posix_memalign(3)
NOTES 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(), free() or realloc() 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 GNU libc (2.x) include a malloc implementation which is tunable via environment vari- ables. When MALLOC_CHECK_ is set, a special (less efficient) implemen- tation 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 and an error message is not generated; if set to 1, the error message is printed on stderr, but the program is not aborted; if set to 2, abort() is called immediately, but the error message is not generated; if set to 3, the error message is printed on stderr and program is aborted. This 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.
BUGS By default, Linux follows an optimistic memory allocation strategy. This means that when malloc() returns non-NULL there is no guarantee that the memory really is available. This is a really bad bug. In case it turns out that the system is out of memory, one or more processes will be killed by the infamous OOM killer. In case Linux is employed under circumstances where it would be less desirable to suddenly lose some randomly picked processes, and moreover the kernel version is suf- ficiently recent, one can switch off this overcommitting behavior using a command like # echo 2 > /proc/sys/vm/overcommit_memory See also the kernel Documentation directory, files vm/overcommit- accounting and sysctl/vm.txt.
GNU 1993-04-04 MALLOC(3)