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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | STANDARDS | HISTORY | NOTES | SEE ALSO | COLOPHON |
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posix_memalign(3) Library Functions Manual posix_memalign(3)
posix_memalign, aligned_alloc, memalign, valloc, pvalloc -
allocate aligned memory
Standard C library (libc, -lc)
#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
posix_memalign() allocates size bytes and places the address of
the allocated 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
restriction 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_PAGESIZE),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.
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.
EINVAL The alignment argument was not a power of two, or was not a
multiple of sizeof(void *).
ENOMEM Out of memory.
For an explanation of the terms used in this section, see
attributes(7).
┌───────────────────────────────┬───────────────┬────────────────┐
│ Interface │ Attribute │ Value │
├───────────────────────────────┼───────────────┼────────────────┤
│ aligned_alloc(), memalign(), │ Thread safety │ MT-Safe │
│ posix_memalign() │ │ │
├───────────────────────────────┼───────────────┼────────────────┤
│ valloc(), pvalloc() │ Thread safety │ MT-Unsafe init │
└───────────────────────────────┴───────────────┴────────────────┘
aligned_alloc()
C11.
posix_memalign()
POSIX.1-2008.
memalign()
valloc()
None.
pvalloc()
GNU.
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
<malloc.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).
On many systems there are alignment restrictions, for example, on
buffers used for direct block device I/O. POSIX specifies the
pathconf(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
detailed 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 implementation allows memory obtained
from any of these functions to be reclaimed 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.
brk(2), getpagesize(2), free(3), malloc(3)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-05-17 posix_memalign(3)
Pages that refer to this page: io_uring_register(2), io_uring_register_buf_ring(3), malloc(3), malloc_hook(3), mallopt(3), mtrace(3), pthread_attr_setstack(3)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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