MALLOC(3) Linux Programmer's Manual MALLOC(3)
calloc, malloc, free, realloc - Allocate and free dynamic memory
void *calloc(size_t nmemb, size_t size);
void *malloc(size_t size);
void free(void *ptr);
void *realloc(void *ptr, size_t size);
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. If nmemb or size is 0, then calloc() returns either NULL, or a unique pointer value that can later be successfully passed to
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(). Oth-
erwise, 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.
For calloc() and malloc(), return a pointer to the allocated memory, which is suitably aligned for any kind of variable. On error, these
functions return NULL. NULL may also be returned by a successful call to malloc() with a size of zero, or by a successful call to calloc()
with nmemb or size equal to zero.
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 real-
loc() fails the original block is left untouched; it is not freed or moved.
Normally, malloc() allocates memory from the heap, and adjusts the size of the heap as required, using sbrk(2). When allocating blocks of
memory larger than MMAP_THRESHOLD bytes, the glibc malloc() implementation allocates the memory as a private anonymous mapping using
mmap(2). MMAP_THRESHOLD is 128 kB by default, but is adjustable using mallopt(3). Allocations performed using mmap(2) are unaffected by
the RLIMIT_DATA resource limit (see getrlimit(2)).
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 vari-
ables. 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.
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 pro-
cesses will be killed by the infamous OOM killer. In case Linux is employed under circumstances where it would be less desirable to sud-
denly lose some randomly picked processes, and moreover the kernel version is sufficiently 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.
brk(2), mallopt(3), mmap(2), alloca(3), posix_memalign(3)
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