man3/posix_memalign.3

.\" Copyright (c) 2001 by John Levon <moz@compsoc.man.ac.uk>
.\" Based in part on GNU libc documentation.
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.\" 2001-10-11, 2003-08-22, aeb, added some details
.\" 2012-03-23, Michael Kerrisk <mtk.manpages@mail.com>
.\"     Document pvalloc() and aligned_alloc()
.TH POSIX_MEMALIGN 3  2021-03-22 "GNU" "Linux Programmer's Manual"
.SH NAME
posix_memalign, aligned_alloc, memalign, valloc, pvalloc \- allocate aligned memory
.SH SYNOPSIS
.nf
.B #include <stdlib.h>
.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 );
.PP
.B #include <malloc.h>
.PP
.BI "void *memalign(size_t " alignment ", size_t " size );
.BI "void *pvalloc(size_t " size );
.fi
.PP
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.PP
.BR posix_memalign ():
.nf
    _POSIX_C_SOURCE >= 200112L
.fi
.PP
.BR aligned_alloc ():
.nf
    _ISOC11_SOURCE
.fi
.PP
.BR valloc ():
.nf
    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:
        _BSD_SOURCE || _XOPEN_SOURCE >= 500
.\"    || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED
.fi
.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\ *)" .
This address can later be successfully passed to
.BR free (3).
If
.I size
is 0, then
the value placed in
.IR "*memptr"
is either NULL
.\" glibc does this:
or a unique pointer value.
.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 on success.
On error, NULL is returned, and \fIerrno\fP is set
to indicate the error.
.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.1-2008 TC2.
.\" 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 since at least glibc 2.0.
.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).
.ad l
.nh
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.BR aligned_alloc (),
.BR memalign (),
.BR posix_memalign ()
T}	Thread safety	MT-Safe
T{
.BR valloc (),
.BR pvalloc ()
T}	Thread safety	MT-Unsafe init
.TE
.hy
.ad
.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<stdlib.h>\fP.
.PP
On some systems
.BR memalign ()
is declared in \fI<stdlib.h>\fP instead of \fI<malloc.h>\fP.
.PP
According to SUSv2,
.BR valloc ()
is declared in \fI<stdlib.h>\fP.
.\" Libc4,5 and
Glibc declares it in \fI<malloc.h>\fP, and also in
\fI<stdlib.h>\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)