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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(). Otherwise, or if free(ptr) has
already been called before, undefined behaviour occurs. If ptr is NULL, no opera-
tion 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 allo-
cated memory will be uninitialized. If ptr is NULL, the call is equivalent to mal-
loc(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 allocated 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 corrup-
tion, 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 mal-
loc 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 argu-
ment, 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 sud-
denly 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)
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