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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(). 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
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) 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; if set to 1, a
diagnostic is printed on stderr; if set to 2, abort() is called immedi-
ately. 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)
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This page was generated on Tue Feb 13 02:20:56 GMT 2007
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