path: root/man3/posix_memalign.3

.\" (c) 2001 by John Levon <moz@compsoc.man.ac.uk>
.\" Based in part on GNU libc documentation.
.\"
.\" 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.\" manual page may be incorrect or out-of-date.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.
.\"
.\" 2001-10-11, 2003-08-22, aeb, added some details
.TH POSIX_MEMALIGN 3  2007-07-26 "GNU" "Linux Programmer's Manual"
.SH NAME
posix_memalign, memalign, valloc \- Allocate aligned memory
.SH SYNOPSIS
.nf
.B #include <stdlib.h>
.sp
.BI "int posix_memalign(void **" memptr ", size_t " alignment ", size_t " size );
.sp
.B #include <malloc.h>
.sp
.BI "void *valloc(size_t " size );
.BI "void *memalign(size_t " boundary ", size_t " size );
.fi
.sp
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.in
.sp
.ad l
.BR posix_memalign ():
_POSIX_C_SOURCE\ >=\ 200112L || _XOPEN_SOURCE\ >=\ 600
.ad b
.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 *)".

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 "boundary" ,
which must be a power of two.

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)" .

For all three routines, the memory is not zeroed.
.SH "RETURN VALUE"
.BR memalign ()
and
.BR valloc ()
return the pointer to the allocated memory, or NULL if the request fails.

.BR posix_memalign ()
returns zero on success, or one of the error values listed in the
next section on failure.
Note that
.I errno
is not set.
.SH "ERRORS"
.TP
.B EINVAL
The
.I alignment
parameter 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 ()
and
.BR valloc ()
have been available in all Linux libc libraries.
The function
.BR posix_memalign ()
is available since glibc 2.1.91.
.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.1-2001.
The function
.BR memalign ()
appears in SunOS 4.1.3 but not in 4.4BSD.
The function
.BR posix_memalign ()
comes from POSIX.1d.
.SS Headers
Everybody agrees that
.BR posix_memalign ()
is declared in \fI<stdlib.h>\fP.

On some systems
.BR memalign ()
is declared in \fI<stdlib.h>\fP instead of \fI<malloc.h>\fP.

According to SUSv2,
.BR valloc ()
is declared in \fI<stdlib.h>\fP.
Libc4,5 and glibc declare it in \fI<malloc.h>\fP and perhaps also in <stdlib.h>
(namely, if
.B _GNU_SOURCE
is defined, or
.B _BSD_SOURCE
is defined, or,
for glibc, if
.B _XOPEN_SOURCE_EXTENDED
is defined, or, equivalently,
.B _XOPEN_SOURCE
is defined to a value not less than 500).
.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.

.BR posix_memalign ()
verifies that
.I alignment
matches the requirements detailed above.
.BR memalign ()
may not check that the
.I boundary
parameter is correct.

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 only pass to
.BR free (3)
a pointer gotten 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 three routines to be
reclaimed with
.BR free (3).

The glibc
.BR malloc (3)
always returns 8-byte aligned memory addresses, so these routines are only
needed if you require larger alignment values.
.SH "SEE ALSO"
.BR brk (2),
.BR getpagesize (2),
.BR free (3),
.BR malloc (3)