calloc
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(). Oth-
erwise, 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().
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 real-
loc() fails the original block is left untouched - it is not freed or
moved.
CONFORMING TO
ANSI-C
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) imple-
mentation 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 MAL-
LOC_CHECK_ is set to 0, any detected heap corruption is silently
ignored; if set to 1, a diagnostic is printed on stderr; if set to 2,
abort() is called immediately. 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 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.
GNU 1993-04-04 MALLOC(3)
