malloc(3) - Linux manual page (original) (raw)

malloc(3) Library Functions Manual malloc(3)

NAME top

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

LIBRARY top

   Standard C library (_libc_, _-lc_)

SYNOPSIS top

   **#include <stdlib.h>**

   **void *malloc(size_t** _size_**);**
   **void free(void *_Nullable** _ptr_**);**
   **void *calloc(size_t** _n_**, size_t** _size_**);**
   **void *realloc(void *_Nullable** _ptr_**, size_t** _size_**);**
   **void *reallocarray(void *_Nullable** _ptr_**, size_t** _n_**, size_t** _size_**);**

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

   **reallocarray**():
       Since glibc 2.29:
           _DEFAULT_SOURCE
       glibc 2.28 and earlier:
           _GNU_SOURCE

DESCRIPTION top

malloc() 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 a unique pointer value that can later be successfully passed to free(). (See "Nonportable behavior" for portability issues.)

free() The free() function frees the memory space pointed to by ptr, which must have been returned by a previous call to malloc() or related functions. Otherwise, or if ptr has already been freed, undefined behavior occurs. If ptr is NULL, no operation is performed.

calloc() The calloc() function allocates memory for an array of n elements of size bytes each and returns a pointer to the allocated memory. The memory is set to zero. If n or size is 0, then calloc() returns a unique pointer value that can later be successfully passed to free().

   If the multiplication of _n_ and _size_ would result in integer
   overflow, then **calloc**() returns an error.  By contrast, an integer
   overflow would not be detected in the following call to **malloc**(),
   with the result that an incorrectly sized block of memory would be
   allocated:

       malloc(n * size);

realloc() The realloc() function changes the size of the memory block pointed to by ptr to size bytes. The contents of the memory 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)_ (but see "Nonportable behavior" for
   portability issues).

   Unless _ptr_ is NULL, it must have been returned by an earlier call
   to **malloc** or related functions.  If the area pointed to was moved,
   a _free(ptr)_ is done.

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

       realloc(ptr, n * size);

   However, unlike that **realloc**() call, **reallocarray**() fails safely
   in the case where the multiplication would overflow.  If such an
   overflow occurs, **reallocarray**() returns an error.

RETURN VALUE top

   The **malloc**(), **calloc**(), **realloc**(), and **reallocarray**() functions
   return a pointer to the allocated memory, which is suitably
   aligned for any type that fits into the requested size or less.
   On error, these functions return NULL and set _[errno](../man3/errno.3.html)_.  Attempting
   to allocate more than **PTRDIFF_MAX** bytes is considered an error, as
   an object that large could cause later pointer subtraction to
   overflow.

   The **free**() function returns no value, and preserves _[errno](../man3/errno.3.html)_.

   The **realloc**() and **reallocarray**() functions return NULL if _ptr_ is
   not NULL and the requested size is zero; this is not considered an
   error.  (See "Nonportable behavior" for portability issues.)
   Otherwise, the returned pointer may be the same as _ptr_ if the
   allocation was not moved (e.g., there was room to expand the
   allocation in-place), or different from _ptr_ if the allocation was
   moved to a new address.  If these functions fail, the original
   block is left untouched; it is not freed or moved.

ERRORS top

   **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)](../man2/getrlimit.2.html).  Another
          reason could be that the number of mappings created by the
          caller process exceeded the limit specified by
          _/proc/sys/vm/maxmapcount_.

ATTRIBUTES top

   For an explanation of the terms used in this section, see
   [attributes(7)](../man7/attributes.7.html).
   ┌──────────────────────────────────────┬───────────────┬─────────┐
   │ **Interface** │ **Attribute** │ **Value** │
   ├──────────────────────────────────────┼───────────────┼─────────┤
   │ **malloc**(), **free**(), **calloc**(),          │ Thread safety │ MT-Safe │
   │ **realloc**()                            │               │         │
   └──────────────────────────────────────┴───────────────┴─────────┘

STANDARDS top

   **malloc**()
   **free**()
   **calloc**()
   **realloc**()
          C11, POSIX.1-2008.

   **reallocarray**()
          None.

HISTORY top

   **malloc**()
   **free**()
   **calloc**()
   **realloc**()
          POSIX.1-2001, C89.

   **reallocarray**()
          glibc 2.26.  OpenBSD 5.6, FreeBSD 11.0.

   **malloc**() and related functions rejected sizes greater than
   **PTRDIFF_MAX** starting in glibc 2.30.

   **free**() preserved _[errno](../man3/errno.3.html)_ starting in glibc 2.33.

NOTES top

   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/overcommitmemory_ and
   _/proc/sys/vm/oomadj_ in [proc(5)](../man5/proc.5.html), and the Linux kernel source file
   _Documentation/vm/overcommit-accounting.rst_.

   Normally, **malloc**() allocates memory from the heap, and adjusts the
   size of the heap as required, using [sbrk(2)](../man2/sbrk.2.html).  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)](../man2/mmap.2.html).  **MMAP_THRESHOLD** is 128 kB by
   default, but is adjustable using [mallopt(3)](../man3/mallopt.3.html).  Prior to Linux 4.7
   allocations performed using [mmap(2)](../man2/mmap.2.html) were unaffected by the
   **RLIMIT_DATA** resource limit; since Linux 4.7, this limit is also
   enforced for allocations performed using [mmap(2)](../man2/mmap.2.html).

   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)](../man2/brk.2.html) or [mmap(2)](../man2/mmap.2.html)), and managed with its own
   mutexes.

   If your program uses a private memory allocator, it should do so
   by replacing **malloc**(), **free**(), **calloc**(), and **realloc**().  The
   replacement functions must implement the documented glibc
   behaviors, including _[errno](../man3/errno.3.html)_ handling, size-zero allocations, and
   overflow checking; otherwise, other library routines may crash or
   operate incorrectly.  For example, if the replacement _free_() does
   not preserve _[errno](../man3/errno.3.html)_, then seemingly unrelated library routines may
   fail without having a valid reason in _[errno](../man3/errno.3.html)_.  Private memory
   allocators may also need to replace other glibc functions; see
   "Replacing malloc" in the glibc manual for details.

   Crashes in memory allocators 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)](../man3/mallopt.3.html) for details.

Nonportable behavior The behavior of these functions when the requested size is zero is glibc specific; other implementations may return NULL without setting errno, and portable POSIX programs should tolerate such behavior. See realloc(3p).

   POSIX requires memory allocators to set _[errno](../man3/errno.3.html)_ upon failure.
   However, the C standard does not require this, and applications
   portable to non-POSIX platforms should not assume this.

   Portable programs should not use private memory allocators, as
   POSIX and the C standard do not allow replacement of **malloc**(),
   **free**(), **calloc**(), and **realloc**().

EXAMPLES top

   #include <err.h>
   #include <stddef.h>
   #include <stdio.h>
   #include <stdlib.h>
   #include <string.h>

   #define MALLOCARRAY(n, type)  ((type *) my_mallocarray(n, sizeof(type)))
   #define MALLOC(type)          MALLOCARRAY(1, type)

   static inline void *my_mallocarray(size_t n, size_t size);

   int
   main(void)
   {
       char  *p;

       p = MALLOCARRAY(32, char);
       if (p == NULL)
           err(EXIT_FAILURE, "malloc");

       strlcpy(p, "foo", 32);
       puts(p);
   }

   static inline void *
   my_mallocarray(size_t n, size_t size)
   {
       return reallocarray(NULL, n, size);
   }

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Linux man-pages 6.10 2024-11-17 malloc(3)

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