posix_memalign(3) Library Functions Manual posix_memalign(3)
NAME
posix_memalign, aligned_alloc, memalign, valloc, pvalloc - allocate
aligned memory
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <stdlib.h>
int posix_memalign(void **memptr, size_t alignment, size_t size);
void *aligned_alloc(size_t alignment, size_t size);
[[deprecated]] void *valloc(size_t size);
#include <malloc.h>
[[deprecated]] void *memalign(size_t alignment, size_t size);
[[deprecated]] void *pvalloc(size_t size);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
posix_memalign():
_POSIX_C_SOURCE >= 200112L
aligned_alloc():
_ISOC11_SOURCE
valloc():
Since glibc 2.12:
(_XOPEN_SOURCE >= 500) && !(_POSIX_C_SOURCE >= 200112L)
|| /* glibc >= 2.19: */ _DEFAULT_SOURCE
|| /* glibc <= 2.19: */ _SVID_SOURCE || _BSD_SOURCE
Before glibc 2.12:
_BSD_SOURCE || _XOPEN_SOURCE >= 500
DESCRIPTION
posix_memalign() allocates size bytes and places the address of the al-
located memory in *memptr. The address of the allocated memory will be
a multiple of alignment, which must be a power of two and a multiple of
sizeof(void *). This address can later be successfully passed to
free(3). If size is 0, then the value placed in *memptr is either NULL
or a unique pointer value.
The obsolete function memalign() allocates size bytes and returns a
pointer to the allocated memory. The memory address will be a multiple
of alignment, which must be a power of two.
aligned_alloc() is the same as memalign(), except for the added re-
striction that alignment must be a power of two.
The obsolete function valloc() allocates size bytes and returns a
pointer to the allocated memory. The memory address will be a multiple
of the page size. It is equivalent to memalign(sysconf(_SC_PAGE-
SIZE),size).
The obsolete function pvalloc() is similar to valloc(), but rounds the
size of the allocation up to the next multiple of the system page size.
For all of these functions, the memory is not zeroed.
RETURN VALUE
aligned_alloc(), memalign(), valloc(), and pvalloc() return a pointer
to the allocated memory on success. On error, NULL is returned, and
errno is set to indicate the error.
posix_memalign() returns zero on success, or one of the error values
listed in the next section on failure. The value of errno is not set.
On Linux (and other systems), posix_memalign() does not modify memptr
on failure. A requirement standardizing this behavior was added in
POSIX.1-2008 TC2.
ERRORS
EINVAL The alignment argument was not a power of two, or was not a mul-
tiple of sizeof(void *).
ENOMEM Out of memory.
ATTRIBUTES
For an explanation of the terms used in this section, see attrib-
utes(7).
┌────────────────────────────────────┬───────────────┬────────────────┐
│ Interface │ Attribute │ Value │
├────────────────────────────────────┼───────────────┼────────────────┤
│ aligned_alloc(), memalign(), │ Thread safety │ MT-Safe │
│ posix_memalign() │ │ │
├────────────────────────────────────┼───────────────┼────────────────┤
│ valloc(), pvalloc() │ Thread safety │ MT-Unsafe init │
└────────────────────────────────────┴───────────────┴────────────────┘
STANDARDS
aligned_alloc()
C11.
posix_memalign()
POSIX.1-2008.
memalign()
valloc()
None.
pvalloc()
GNU.
HISTORY
aligned_alloc()
glibc 2.16. C11.
posix_memalign()
glibc 2.1.91. POSIX.1d, POSIX.1-2001.
memalign()
glibc 2.0. SunOS 4.1.3.
valloc()
glibc 2.0. 3.0BSD. Documented as obsolete in 4.3BSD, and as
legacy in SUSv2.
pvalloc()
glibc 2.0.
Headers
Everybody agrees that posix_memalign() is declared in <stdlib.h>.
On some systems memalign() is declared in <stdlib.h> instead of <mal-
loc.h>.
According to SUSv2, valloc() is declared in <stdlib.h>. glibc declares
it in <malloc.h>, and also in <stdlib.h> if suitable feature test
macros are defined (see above).
NOTES
On many systems there are alignment restrictions, for example, on
buffers used for direct block device I/O. POSIX specifies the path-
conf(path,_PC_REC_XFER_ALIGN) call that tells what alignment is needed.
Now one can use posix_memalign() to satisfy this requirement.
posix_memalign() verifies that alignment matches the requirements de-
tailed above. memalign() may not check that the alignment argument is
correct.
POSIX requires that memory obtained from posix_memalign() can be freed
using free(3). Some systems provide no way to reclaim memory allocated
with memalign() or valloc() (because one can pass to free(3) only a
pointer obtained from malloc(3), while, for example, memalign() would
call malloc(3) and then align the obtained value). The glibc implemen-
tation allows memory obtained from any of these functions to be re-
claimed with free(3).
The glibc malloc(3) always returns 8-byte aligned memory addresses, so
these functions are needed only if you require larger alignment values.
SEE ALSO
brk(2), getpagesize(2), free(3), malloc(3)
Linux man-pages 6.7 2023-11-24 posix_memalign(3)
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