malloc


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  that  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 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 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
       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)).

       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.   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 over-
       runs of a single byte (off-by-one bugs).  Not all such  errors  can  be
       protected  against,  however,  and  memory  leaks  can result.  If MAL-
       LOC_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 MAL-
       LOC_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.

SEE ALSO
       brk(2), mallopt(3), mmap(2), alloca(3), posix_memalign(3)

COLOPHON
       This page is part of release 3.35 of the Linux  man-pages  project.   A
       description  of  the project, and information about reporting bugs, can
       be found at http://man7.org/linux/man-pages/.



GNU                               2011-09-08                         MALLOC(3)
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