malloc - Online Linux Manual PageSection : 3
Updated : 2023-02-05
Source : Linux man-pages 6.03
NAMEmalloc, free, calloc, realloc, reallocarray − allocate and free dynamic memory
LIBRARYStandard C library (libc, −lc)
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(): Since glibc 2.29:
_DEFAULT_SOURCE
glibc 2.28 and earlier:
_GNU_SOURCE
DESCRIPTION
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 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 a unique pointer value that can later be successfully passed to free(). If the multiplication of nmemb 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(nmemb * 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 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 an error.
RETURN VALUEThe 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. 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. 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.
ERRORScalloc(), 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).
VERSIONSreallocarray() was added in glibc 2.26. malloc() and related functions rejected sizes greater than PTRDIFF_MAX starting in glibc 2.30. free() preserved errno starting in glibc 2.33.
ATTRIBUTESFor an explanation of the terms used in this section, see attributes(7). InterfaceAttributeValue malloc(), free(), calloc(), realloc() Thread safetyMT-Safe
STANDARDSmalloc(), free(), calloc(), realloc(): POSIX.1-2001, POSIX.1-2008, C99. reallocarray() is a nonstandard extension that first appeared in OpenBSD 5.6 and FreeBSD 11.0.
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_memory and /proc/sys/vm/oom_adj in proc(5), 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). 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. 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 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, then seemingly unrelated library routines may fail without having a valid reason in errno. 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) for details.
Nonportable behaviorThe 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 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().
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) For details of the GNU C library implementation, see https://sourceware.org/glibc/wiki/MallocInternals . 0
Johanes Gumabo
Data Size : 29,232 byte
man-malloc.3Build : 2024-12-05, 20:55 :
Visitor Screen : x
Visitor Counter ( page / site ) : 5 / 185,645
Visitor ID : :
Visitor IP : 3.141.19.115 :
Visitor Provider : AMAZON-02 :
Provider Position ( lat x lon ) : 39.962500 x -83.006100 : x
Provider Accuracy Radius ( km ) : 1000 :
Provider City : Columbus :
Provider Province : Ohio , : ,
Provider Country : United States :
Provider Continent : North America :
Visitor Recorder : Version :
Visitor Recorder : Library :
Online Linux Manual Page : Version : Online Linux Manual Page - Fedora.40 - march=x86-64 - mtune=generic - 24.12.05
Online Linux Manual Page : Library : lib_c - 24.10.03 - march=x86-64 - mtune=generic - Fedora.40
Online Linux Manual Page : Library : lib_m - 24.10.03 - march=x86-64 - mtune=generic - Fedora.40
Data Base : Version : Online Linux Manual Page Database - 24.04.13 - march=x86-64 - mtune=generic - fedora-38
Data Base : Library : lib_c - 23.02.07 - march=x86-64 - mtune=generic - fedora.36
Very long time ago, I have the best tutor, Wenzel Svojanovsky . If someone knows the email address of Wenzel Svojanovsky , please send an email to johanes_gumabo@yahoo.co.id .
If error, please print screen and send to johanes_gumabo@yahoo.co.id
Under development. Support me via PayPal.