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malloc(3) - calloc, free, malloc, realloc, calloc, free, malloc, realloc - Allocate and free dynamic memory - man 3 malloc

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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(7,n,1 builtins)
       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(3,n) free(ptr) has already been called before, undefined  behav-
       iour 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(3,n) 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(3,n) 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(3,n) 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(7,n,1 builtins)
       errno  to ENOMEM upon failure. Glibc assumes that this is done (and the
       glibc versions of these routines do this); if(3,n) you use a private  malloc
       implementation  that  does not set(7,n,1 builtins) errno, then certain library routines
       may fail without having a reason in(1,8) errno.

       Crashes in(1,8) 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(7,n,1 builtins), a special (less(1,3) 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(7,n,1 builtins)
       to  0, any detected heap corruption is silently ignored; if(3,n) set(7,n,1 builtins) to 1, a
       diagnostic is printed on stderr; if(3,n) set(7,n,1 builtins) to 2, abort(3,7)() 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(1,3) desirable to  suddenly  lose
       some randomly picked processes, and moreover the kernel version(1,3,5) is suf-
       ficiently recent, one can switch(1,n) off this overcommitting behavior using
       a command like
              # echo(1,3x,1 builtins) 2 > /proc(5,n)/sys/vm/overcommit_memory
       See  also  the  kernel  Documentation  directory,  files vm/overcommit-
       accounting and sysctl(2,5,8)/vm.txt.



GNU                               1993-04-04                         MALLOC(3)

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