MALLOC man page on Gentoo

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MALLOC(3)		   Linux Programmer's Manual		     MALLOC(3)

NAME
       malloc, free, calloc, realloc - allocate and free dynamic memory

SYNOPSIS
       #include <stdlib.h>

       void *malloc(size_t size);
       void free(void *ptr);
       void *calloc(size_t nmemb, size_t size);
       void *realloc(void *ptr, size_t size);

DESCRIPTION
       The malloc() function allocates size bytes and returns a pointer to the
       allocated memory.  The memory is not initialized.  If size is  0,  then
       malloc()	 returns either NULL, or a unique pointer value that can later
       be successfully passed to free().

       The free() function 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.

       The  calloc()  function 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().

       The  realloc() function changes the size of the memory block pointed to
       by ptr to size bytes.  The contents will be unchanged in the range from
       the start of the region up to the minimum of the old and new sizes.  If
       the new size is larger than the old size, the added memory will not  be
       initialized.   If  ptr  is  NULL,  then	the call is equivalent to mal‐
       loc(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(),  cal‐
       loc()  or  realloc().  If the area pointed to was moved, a free(ptr) is
       done.

RETURN VALUE
       The malloc() and calloc() functions return a pointer to	the  allocated
       memory,	which  is  suitably  aligned for any built-in type.  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 cal‐
       loc() with nmemb or size equal to zero.

       The free() function returns no value.

       The realloc() function returns a pointer to the newly allocated memory,
       which  is  suitably  aligned for any built-in type 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
       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.	 In case it turns out that the
       system is out of memory, one or more processes will be  killed  by  the
       OOM   killer.	For   more   information,   see	  the  description  of
       /proc/sys/vm/overcommit_memory and /proc/sys/vm/oom_adj in proc(5), and
       the Linux kernel source file Documentation/vm/overcommit-accounting.

       Normally, malloc() allocates memory from the heap, and adjusts the size
       of the heap as required, using sbrk(2).	When allocating blocks of mem‐
       ory 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 mal‐
       lopt(3).	 Allocations performed using mmap(2)  are  unaffected  by  the
       RLIMIT_DATA resource limit (see getrlimit(2)).

       To  avoid  corruption  in  multithreaded applications, mutexes are used
       internally to protect the memory-management data structures employed by
       these  functions.   In  a  multithreaded	 application  in which threads
       simultaneously allocate and free memory, there could be contention  for
       these  mutexes.	 To scalably handle memory allocation in multithreaded
       applications, glibc creates  additional	memory	allocation  arenas  if
       mutex  contention  is detected.	Each arena is a large region of memory
       that is internally allocated by the system (using brk(2)	 or  mmap(2)),
       and managed with its own mutexes.

       The  UNIX 98 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.  For details, see mallopt(3).

SEE ALSO
       brk(2), mmap(2), alloca(3), malloc_get_state(3), malloc_info(3),
       malloc_trim(3), malloc_usable_size(3), mallopt(3), mcheck(3),
       mtrace(3), posix_memalign(3)

COLOPHON
       This page is part of release 3.63 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				  2013-12-12			     MALLOC(3)
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