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POSIX_MEMALIGN(3) Linux Programmer's Manual POSIX_MEMALIGN(3)


posix_memalign, aligned_alloc, memalign, valloc, pvalloc - allocate aligned memory


#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_C_SOURCE >= 200112L




Since glibc 2.12:
(_XOPEN_SOURCE >= 500) && !(_POSIX_C_SOURCE >= 200112L)
|| /* Glibc since 2.19: */ _DEFAULT_SOURCE
|| /* Glibc <= 2.19: */ _SVID_SOURCE || _BSD_SOURCE
Before glibc 2.12:


The function 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.

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_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.


The alignment argument was not a power of two, or was not a multiple of sizeof(void *).
There was insufficient memory to fulfill the allocation request.


The functions memalign(), valloc(), and pvalloc() have been available since at least glibc 2.0.

The function aligned_alloc() was added to glibc in version 2.16.

The function posix_memalign() is available since glibc 2.1.91.


For an explanation of the terms used in this section, see attributes(7).

Interface Attribute Value
aligned_alloc (), memalign (), posix_memalign () Thread safety MT-Safe
valloc (), pvalloc () Thread safety MT-Unsafe init


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.


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)


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