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.  If nmemb or size is 0, then calloc() returns either NULL, or a unique pointer value that can later be successfully passed	to
       free().

       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.

RETURN VALUE

       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.

CONFORMING TO

       C89, C99.

NOTES

       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.

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.

SEE ALSO

       brk(2), mmap(2), alloca(3), posix_memalign(3)

COLOPHON

       This page is part of release 3.25 of the Linux man-pages project.  A description of the project, and information about reporting bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

GNU
								    2009-01-13								 MALLOC(3)