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 mem-
       ory.  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().  Otherwise, 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 equiva-
       lent 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 realloc() 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).  Allo-
       cations 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 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 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 processes will be killed by the infamous OOM killer.  In case Linux is employed under circum-
       stances where it would be less desirable to suddenly 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)