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home | helpx minix x x minixx POSIX_MEMALIGN(3) Linux Programmer's Manual POSIX_MEMALIGN(3) NAME posix_memalign, aligned_alloc, memalign, valloc, pvalloc - allocate aligned memory SYNOPSIS #include <stdlib.h> int posix_memalign(void **memptr, size_t alignment, size_t size); void *aligned_alloc(size_t alignment, size_t size); void *valloc(size_t size); #include <malloc.h> void *memalign(size_t alignment, size_t size); 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 since 2.19: */ _DEFAULT_SOURCE || /* Glibc versions <= 2.19: */ _SVID_SOURCE || _BSD_SOURCE Before glibc 2.12: _BSD_SOURCE || _XOPEN_SOURCE >= 500 (The (nonstandard) header file _malloc.h_ also exposes the dec- laration of valloc(); no feature test macros are required.) DESCRIPTION The function posix_memalign() allocates size bytes and places the ad- dress 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 *). If size is 0, then the value placed in *memptr is either NULL, or a unique pointer value that can later be successfully passed to free(3). 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. The function aligned_alloc() is the same as memalign(), except for the added restriction that size should be a multiple of alignment. 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 cause of 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.1-2016. ERRORS EINVAL The alignment argument was not a power of two, or was not a mul- tiple of sizeof(void *). ENOMEM There was insufficient memory to fulfill the allocation request. VERSIONS The functions memalign(), valloc(), and pvalloc() have been available in all Linux libc libraries. The function aligned_alloc() was added to glibc in version 2.16. The function posix_memalign() is available since glibc 2.1.91. ATTRIBUTES For an explanation of the terms used in this section, see at- tributes(7). +-----------------+---------------+----------------+ |Interface | Attribute | Value | +-----------------+---------------+----------------+ |aligned_alloc(), | Thread safety | MT-Safe | |memalign(), | | | |posix_memalign() | | | +-----------------+---------------+----------------+ |valloc(), | Thread safety | MT-Unsafe init | |pvalloc() | | | +-----------------+---------------+----------------+ CONFORMING TO The function 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. The function pvalloc() is a GNU extension. The function memalign() appears in SunOS 4.1.3 but not in 4.4BSD. The function posix_memalign() comes from POSIX.1d and is specified in POSIX.1-2001 and POSIX.1-2008. The function aligned_alloc() is specified in the C11 standard. 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_. Libc4,5 and glibc declare it in _malloc.h_, and also in _stdlib.h_ if suitable fea- ture test macros are defined (see above). NOTES On many systems there are alignment restrictions, for example, on buf- fers 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) COLOPHON This page is part of release 5.05 of the Linux 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/. GNU 2019-05-09 POSIX_MEMALIGN(3)
NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | ATTRIBUTES | CONFORMING TO | NOTES | SEE ALSO | COLOPHON