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

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

       #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);

       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(),  calloc()  or realloc().  If the area pointed to was moved, a
       free(ptr) is done.

       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 calloc() 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

       C89, C99.

       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/overcom‐
       mit_memory  and /proc/sys/vm/oom_adj in proc(5), and the Linux kernel source file Documen‐

       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  map‐
       ping  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 con‐
       tention for these mutexes.  To scalably handle memory allocation in multithreaded applica‐
       tions,  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 cor‐
       ruption, such as overflowing an allocated chunk or freeing the same pointer twice.

       The  malloc()  implementation  is  tunable  via  environment variables; see mallopt(3) for

       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)

       This page is part of release 3.74 of the Linux man-pages project.  A description of the
       project, information about reporting bugs, and the latest version of this page, can be
       found at http://www.kernel.org/doc/man-pages/.

GNU                                         2014-05-21                                  MALLOC(3)

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