Manpage of MALLOC
MALLOCSection: Linux Programmer's Manual (3)
NAMEmalloc, 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);void *reallocarray(void *ptr, size_t nmemb, size_t size);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
The malloc() function allocates sizebytes and returns a pointer to the allocated memory. The memory is not initialized. If sizeis 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 acall to malloc(), calloc(), or realloc(). Otherwise, or if free(ptr)has already been called before, undefined behavior occurs. If ptris NULL, no operation is performed.
The calloc() function allocates memory for an array of nmembelements of sizebytes each and returns a pointer to the allocated memory. The memory is set to zero. If nmembor sizeis 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 ptrto sizebytes. Thewill 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 notbe initialized. If ptris NULL, then the call is equivalent to malloc(size), for all values of size; if sizeis equal to zero, and ptris not NULL, then the call is equivalent to free(ptr). Unless ptris 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 ptrto be large enough for an array of nmembelements, each of which is sizebytes. 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 errnoto ENOMEM, and leaves the original block of memory unchanged.
RETURN VALUEThe 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 sizeof zero, or by a successful call to calloc() with nmembor sizeequal 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 sizewas 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.
ERRORScalloc(), malloc(), realloc(), and reallocarray() can fail with the following error:
- Out of memory. Possibly, the application hit the RLIMIT_ASor RLIMIT_DATAlimit described in getrlimit(2).
ATTRIBUTESFor an explanation of the terms used in this section, see attributes(7).
CONFORMING TOmalloc(), free(), calloc(), realloc(): POSIX.1-2001, POSIX.1-2008, C89, C99.
NOTESBy 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/overcommit_memoryand /proc/sys/vm/oom_adjin 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_THRESHOLDbytes, the glibc malloc() implementation allocates the memory as a private anonymous mapping using mmap(2). MMAP_THRESHOLDis 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_DATAresource 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 arenasif 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 errnoto ENOMEMupon 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 ALSObrk(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)
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