.\" Copyright (c) 2001 by John Levon .\" Based in part on GNU libc documentation. .\" .\" %%%LICENSE_START(VERBATIM) .\" Permission is granted to make and distribute verbatim copies of this .\" manual provided the copyright notice and this permission notice are .\" preserved on all copies. .\" .\" Permission is granted to copy and distribute modified versions of this .\" manual under the conditions for verbatim copying, provided that the .\" entire resulting derived work is distributed under the terms of a .\" permission notice identical to this one. .\" .\" Since the Linux kernel and libraries are constantly changing, this .\" manual page may be incorrect or out-of-date. The author(s) assume no .\" responsibility for errors or omissions, or for damages resulting from .\" the use of the information contained herein. The author(s) may not .\" have taken the same level of care in the production of this manual, .\" which is licensed free of charge, as they might when working .\" professionally. .\" .\" Formatted or processed versions of this manual, if unaccompanied by .\" the source, must acknowledge the copyright and authors of this work. .\" %%%LICENSE_END .\" .\" 2001-10-11, 2003-08-22, aeb, added some details .\" 2012-03-23, Michael Kerrisk .\" Document pvalloc() and aligned_alloc() .TH POSIX_MEMALIGN 3 2017-09-15 "GNU" "Linux Programmer's Manual" .SH NAME posix_memalign, aligned_alloc, memalign, valloc, pvalloc \- allocate aligned memory .SH SYNOPSIS .nf .B #include .PP .BI "int posix_memalign(void **" memptr ", size_t " alignment ", size_t " size ); .BI "void *aligned_alloc(size_t " alignment ", size_t " size ); .BI "void *valloc(size_t " size ); .B #include .PP .BI "void *memalign(size_t " alignment ", size_t " size ); .BI "void *pvalloc(size_t " size ); .fi .PP .in -4n Feature Test Macro Requirements for glibc (see .BR feature_test_macros (7)): .in .PP .ad l .BR posix_memalign (): _POSIX_C_SOURCE\ >=\ 200112L .PP .BR aligned_alloc (): _ISOC11_SOURCE .PP .BR valloc (): .br .PD 0 .RS 4 .TP 4 Since glibc 2.12: .nf (_XOPEN_SOURCE\ >=\ 500) && !(_POSIX_C_SOURCE\ >=\ 200112L) || /* Glibc since 2.19: */ _DEFAULT_SOURCE || /* Glibc versions <= 2.19: */ _SVID_SOURCE || _BSD_SOURCE .br .fi .TP Before glibc 2.12: _BSD_SOURCE || _XOPEN_SOURCE\ >=\ 500 .\" || _XOPEN_SOURCE\ &&\ _XOPEN_SOURCE_EXTENDED .ad b .br (The (nonstandard) header file .I also exposes the declaration of .BR valloc (); no feature test macros are required.) .RE .PD .SH DESCRIPTION The function .BR posix_memalign () allocates .I size bytes and places the address of the allocated memory in .IR "*memptr" . The address of the allocated memory will be a multiple of .IR "alignment" , which must be a power of two and a multiple of .IR "sizeof(void\ *)" . If .I size is 0, then the value placed in .IR "*memptr" is either NULL, .\" glibc does this: or a unique pointer value that can later be successfully passed to .BR free (3). .PP The obsolete function .BR memalign () allocates .I size bytes and returns a pointer to the allocated memory. The memory address will be a multiple of .IR alignment , which must be a power of two. .\" The behavior of memalign() for size==0 is as for posix_memalign() .\" but no standards govern this. .PP The function .BR aligned_alloc () is the same as .BR memalign (), except for the added restriction that .I size should be a multiple of .IR alignment . .PP The obsolete function .BR valloc () allocates .I 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 .IR "memalign(sysconf(_SC_PAGESIZE),size)" . .PP The obsolete function .BR pvalloc () is similar to .BR valloc (), but rounds the size of the allocation up to the next multiple of the system page size. .PP For all of these functions, the memory is not zeroed. .SH RETURN VALUE .BR aligned_alloc (), .BR memalign (), .BR valloc (), and .BR pvalloc () return a pointer to the allocated memory, or NULL if the request fails. .PP .BR posix_memalign () returns zero on success, or one of the error values listed in the next section on failure. The value of .I errno is not set. On Linux (and other systems), .BR posix_memalign () does not modify .I memptr on failure. A requirement standardizing this behavior was added in POSIX.1-2016. .\" http://austingroupbugs.net/view.php?id=520 .SH ERRORS .TP .B EINVAL The .I alignment argument was not a power of two, or was not a multiple of .IR "sizeof(void\ *)" . .TP .B ENOMEM There was insufficient memory to fulfill the allocation request. .SH VERSIONS The functions .BR memalign (), .BR valloc (), and .BR pvalloc () have been available in all Linux libc libraries. .PP The function .BR aligned_alloc () was added to glibc in version 2.16. .PP The function .BR posix_memalign () is available since glibc 2.1.91. .SH ATTRIBUTES For an explanation of the terms used in this section, see .BR attributes (7). .TS allbox; lb lb lb l l l. Interface Attribute Value T{ .BR aligned_alloc (), .br .BR memalign (), .br .BR posix_memalign () T} Thread safety MT-Safe T{ .BR valloc (), .br .BR pvalloc () T} Thread safety MT-Unsafe init .TE .sp 1 .SH CONFORMING TO The function .BR valloc () appeared in 3.0BSD. It is documented as being obsolete in 4.3BSD, and as legacy in SUSv2. It does not appear in POSIX.1. .PP The function .BR pvalloc () is a GNU extension. .PP The function .BR memalign () appears in SunOS 4.1.3 but not in 4.4BSD. .PP The function .BR posix_memalign () comes from POSIX.1d and is specified in POSIX.1-2001 and POSIX.1-2008. .PP The function .BR aligned_alloc () is specified in the C11 standard. .\" .SS Headers Everybody agrees that .BR posix_memalign () is declared in \fI\fP. .PP On some systems .BR memalign () is declared in \fI\fP instead of \fI\fP. .PP According to SUSv2, .BR valloc () is declared in \fI\fP. Libc4,5 and glibc declare it in \fI\fP, and also in \fI\fP if suitable feature test macros are defined (see above). .SH NOTES On many systems there are alignment restrictions, for example, on buffers used for direct block device I/O. POSIX specifies the .I "pathconf(path,_PC_REC_XFER_ALIGN)" call that tells what alignment is needed. Now one can use .BR posix_memalign () to satisfy this requirement. .PP .BR posix_memalign () verifies that .I alignment matches the requirements detailed above. .BR memalign () may not check that the .I alignment argument is correct. .PP POSIX requires that memory obtained from .BR posix_memalign () can be freed using .BR free (3). Some systems provide no way to reclaim memory allocated with .BR memalign () or .BR valloc () (because one can pass to .BR free (3) only a pointer obtained from .BR malloc (3), while, for example, .BR memalign () would call .BR malloc (3) and then align the obtained value). .\" Other systems allow passing the result of .\" .IR valloc () .\" to .\" .IR free (3), .\" but not to .\" .IR realloc (3). The glibc implementation allows memory obtained from any of these functions to be reclaimed with .BR free (3). .PP The glibc .BR malloc (3) always returns 8-byte aligned memory addresses, so these functions are needed only if you require larger alignment values. .SH SEE ALSO .BR brk (2), .BR getpagesize (2), .BR free (3), .BR malloc (3) .SH COLOPHON This page is part of release 4.16 of the Linux .I man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at \%https://www.kernel.org/doc/man\-pages/.