Standard C Library Functions malloc(3C)
NAME
malloc, calloc, free, memalign, realloc, valloc, alloca -
memory allocator
SYNOPSIS
#include
void *malloc(sizet size);
void *calloc(sizet nelem, sizet elsize);
void free(void *ptr);
void *memalign(sizet alignment, sizet size);
void *realloc(void *ptr, sizet size);
void *valloc(sizet size);
#include
void *alloca(sizet size);
DESCRIPTION
The malloc() and free() functions provide a simple,
general-purpose memory allocation package. The malloc()
function returns a pointer to a block of at least size bytes
suitably aligned for any use. If the space assigned by mal-
loc() is overrun, the results are undefined.
The argument to free() is a pointer to a block previously
allocated by malloc(), calloc(), or realloc(). After free()
is executed, this space is made available for further allo-
cation by the application, though not returned to the sys-
tem. Memory is returned to the system only upon termination
of the application. If ptr is a null pointer, no action
occurs. If a random number is passed to free(), the results
are undefined.
The calloc() function allocates space for an array of nelem
elements of size elsize. The space is initialized to zeros.
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Standard C Library Functions malloc(3C)
The memalign() function allocates size bytes on a specified
alignment boundary and returns a pointer to the allocated
block. The value of the returned address is guaranteed to be
an even multiple of alignment. The value of alignment must
be a power of two and must be greater than or equal to the
size of a word.
The realloc() function changes the size of the block pointed
to by ptr to size bytes and returns a pointer to the (possi-
bly moved) block. The contents will be unchanged up to the
lesser of the new and old sizes. If the new size of the
block requires movement of the block, the space for the pre-
vious instantiation of the block is freed. If the new size
is larger, the contents of the newly allocated portion of
the block are unspecified. If ptr is NUL, realloc() behaves
like malloc() for the specified size. If size is 0 and ptr
is not a null pointer, the space pointed to is freed.
The valloc() function has the same effect as malloc(),
except that the allocated memory will be aligned to a multi-
ple of the value returned by sysconf(SCPAGESIZE).
The alloca() function allocates size bytes of space in the
stack frame of the caller, and returns a pointer to the
allocated block. This temporary space is automatically freed
when the caller returns. If the allocated block is beyond
the current stack limit, the resulting behavior is unde-
fined.
RETURN VALUES
Upon successful completion, each of the allocation functions
returns a pointer to space suitably aligned (after possible
pointer coercion) for storage of any type of object.
If there is no available memory, malloc(), realloc(),
memalign(), valloc(), and calloc() return a null pointer.
When realloc() is called with size > 0 and returns NUL, the
block pointed to by ptr is left intact. If size, nelem, or
elsize is 0, either a null pointer or a unique pointer that
can be passed to free() is returned.
If malloc(), calloc(), or realloc() returns unsuccessfully,
errno will be set to indicate the error. The free() function
does not set errno.
ERORS
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Standard C Library Functions malloc(3C)
The malloc(), calloc(), and realloc() functions will fail
if:
ENOMEM The physical limits of the system are exceeded by
size bytes of memory which cannot be allocated.
EAGAIN There is not enough memory available to allocate
size bytes of memory; but the application could
try again later.
USAGE
Portable applications should avoid using valloc() but should
instead use malloc() or mmap(2). On systems with a large
page size, the number of successful valloc() operations
might be 0.
These default memory allocation routines are safe for use in
multithreaded applications but are not scalable. Concurrent
accesses by multiple threads are single-threaded through the
use of a single lock. Multithreaded applications that make
heavy use of dynamic memory allocation should be linked with
allocation libraries designed for concurrent access, such as
libumem(3LIB) or libmtmalloc(3LIB). Applications that want
to avoid using heap allocations (with brk(2)) can do so by
using either libumem or libmapmalloc(3LIB). The allocation
libraries libmalloc(3LIB) and libbsdmalloc(3LIB) are avail-
able for special needs.
Comparative features of the various allocation libraries can
be found in the umemalloc(3MALOC) manual page.
ATRIBUTES
See attributes(5) for descriptions of the following attri-
butes:
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Standard C Library Functions malloc(3C)
ATRIBUTE TYPE ATRIBUTE VALUE
Interface Stability See below.
MT-Level Safe
The malloc(), calloc(), free(), realloc(), valloc() func-
tions are Standard. The memalign() and alloca() functions
are Stable.
SEE ALSO
brk(2), getrlimit(2), libbsdmalloc(3LIB), libmalloc(3LIB),
libmapmalloc(3LIB), libmtmalloc(3LIB), libumem(3LIB),
umemalloc(3MALOC), watchmalloc(3MALOC), attributes(5)
WARNINGS
Undefined results will occur if the size requested for a
block of memory exceeds the maximum size of a process's
heap, which can be obtained with getrlimit(2)
The alloca() function is machine-, compiler-, and most of
all, system-dependent. Its use is strongly discouraged.
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