|
HTML rendering created 2026-05-24 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. |
|
|
NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | STANDARDS | HISTORY | NOTES | SEE ALSO | COLOPHON |
|
|
|
aligned_alloc(3) Library Functions Manual aligned_alloc(3)
aligned_alloc - allocate aligned memory
Standard C library (libc, -lc)
#include <stdlib.h>
void *aligned_alloc(size_t alignment, size_t size);
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
aligned_alloc():
_ISOC11_SOURCE
aligned_alloc() 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. This address can later
be successfully passed to free(3).
The memory is not zeroed.
aligned_alloc() returns a pointer to the allocated memory on
success. On error, NULL is returned, and errno is set to indicate
the error.
EINVAL The alignment argument was not a power of two.
ENOMEM Out of memory.
For an explanation of the terms used in this section, see
attributes(7).
┌──────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├──────────────────────────────────────┼───────────────┼─────────┤
│ aligned_alloc() │ Thread safety │ MT-Safe │
└──────────────────────────────────────┴───────────────┴─────────┘
C23, POSIX.1-2024.
glibc 2.16. C11, POSIX.1-2024.
C11
In C11, the specification of this function had several issues
⟨https://port70.net/~nsz/c/c11/n1570.html#7.22.3.1p2⟩.
• size had to be a multiple of alignment. Otherwise, the
behavior was undefined.
• If alignment was not a power of two, the behavior was
undefined.
DR460
⟨https://www.open-std.org/jtc1/sc22/wg14/www/docs/summary.htm#dr_460⟩
reported both cases of UB as unnecessarily dangerous, and fixed
them with a Technical Corrigendum that transformed them into
errors.
N2072 ⟨https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2072.htm⟩
reported that the requirement that size is a multiple of alignment
is superfluous, and removed it with a Technical Corrigendum.
C17 incorporates both technical corrigenda. The API has been
stable since C17.
glibc initially implemented it as silently aligning as
stdc_bit_ceil(alignment) instead of alignment. Since glibc 2.38,
it implements the C17 specification.
Some implementations, such as FreeBSD/jemalloc, implement the C17
specification, even though their documentation claims having
undefined behavior.
Some implementations, such as OpenBSD, implement C11 amended with
DR460, even though their documentation claims having undefined
behavior.
No known implementations have exploited the undefined behavior in
a more dangerous way. This function should be safe to use.
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 aligned_alloc() to satisfy this
requirement.
The glibc malloc(3) always returns 8-byte aligned memory
addresses, so this function is 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-6.18.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2026-05-24. 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 6.18 2025-12-25 aligned_alloc(3)
Pages that refer to this page: memalign(3), posix_memalign(3)
|
HTML rendering created 2026-05-24 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. |
|