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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);
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(), calloc() 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, 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.
CONFORMING TO
C89, C99.
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 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‐
tation/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 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
details.
SEE ALSO
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)
COLOPHON
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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