Appendix

Copyright (c) 1993 by Thomas Koenig (ig25@rz.uni-karlsruhe.de)
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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);
       void *reallocarray(void *ptr, size_t nmemb, size_t size);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       reallocarray():
           _GNU_SOURCE

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 malloc(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 reallocarray() function changes the size of the memory block
       pointed to by ptr to be large enough for an array of nmemb
       elements, each of which is size bytes. It is equivalent to
       the call

               realloc(ptr, nmemb * size);

       However, unlike that realloc() call, reallocarray() fails
       safely in the case where the multiplication would overflow.
       If such an overflow occurs, reallocarray() returns NULL,
       sets errno to ENOMEM, and leaves the original block of memory
       unchanged.

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 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 moved.

       On success, the reallocarray() function returns a pointer to the
       newly allocated memory. On failure, it returns NULL and the
       original block of memory is left untouched.

ERRORS
       calloc(), malloc(), realloc(), and reallocarray() can fail with
       the following error:

       ENOMEM Out of memory. Possibly, the application hit the
       RLIMIT_AS or RLIMIT_DATA limit described in getrlimit(2).

ATTRIBUTES
       For an explanation of the terms used in this section, see
       attributes(7).

       +---------------------+---------------+---------+
       |Interface            | Attribute     | Value   |
       |-----------------------------------------------|
       |malloc(), free(),    | Thread safety | MT-Safe |
       |calloc(), realloc()  |               |         |
       +---------------------+---------------+---------+

CONFORMING TO
       malloc(), free(), calloc(), realloc(): POSIX.1-2001,
       POSIX.1-2008, C89, C99.

       reallocarray() is a nonstandard extension that first appeared in
       OpenBSD 5.6 and FreeBSD 11.0.

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 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/over-    
       commit_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 memory 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 mallopt(3). Prior to
       Linux 4.7 allocations performed using mmap(2) were unaffected
       by the RLIMIT_DATA resource limit; since Linux 4.7, this limit
       is also enforced for allocations performed using mmap(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. 

       SUSv2 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. 

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

SEE ALSO
       valgrind(1), 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)