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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
       C89, C99.

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	and  an	 error
       message	is not generated; if set to 1, the error message is printed on
       stderr, but the program is not aborted; if set to 2, abort() is	called
       immediately,  but  the error message is not generated; if set to 3, the
       error message is printed on stderr and program is aborted.  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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