'\" t
.\" Copyright (c) 2006, Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.TH posix_fallocate 3 2024-06-16 "Linux man-pages (unreleased)"
.SH NAME
posix_fallocate \- allocate file space
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <fcntl.h>
.P
.BI "int posix_fallocate(int " fd ", off_t " offset ", off_t " len );
.fi
.P
.ad l
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.P
.BR posix_fallocate ():
.nf
    _POSIX_C_SOURCE >= 200112L
.fi
.SH DESCRIPTION
The function
.BR posix_fallocate ()
ensures that disk space is allocated for the file referred to by the
file descriptor
.I fd
for the bytes in the range starting at
.I offset
and continuing for
.I len
bytes.
After a successful call to
.BR posix_fallocate (),
subsequent writes to bytes in the specified range are
guaranteed not to fail because of lack of disk space.
.P
If the size of the file is less than
.IR offset + len ,
then the file is increased to this size;
otherwise the file size is left unchanged.
.SH RETURN VALUE
.BR posix_fallocate ()
returns zero on success, or an error number on failure.
Note that
.I errno
is not set.
.SH ERRORS
.TP
.B EBADF
.I fd
is not a valid file descriptor, or is not opened for writing.
.TP
.B EFBIG
.I offset+len
exceeds the maximum file size.
.TP
.B EINTR
A signal was caught during execution.
.TP
.B EINVAL
.I offset
was less than 0, or
.I len
was less than or equal to 0, or the underlying filesystem does not
support the operation.
.TP
.B ENODEV
.I fd
does not refer to a regular file.
.TP
.B ENOSPC
There is not enough space left on the device containing the file
referred to by
.IR fd .
.TP
.B EOPNOTSUPP
The filesystem containing the file referred to by
.I fd
does not support this operation.
This error code can be returned by C libraries that don't perform the
emulation shown in CAVEATS, such as musl libc.
.TP
.B ESPIPE
.I fd
refers to a pipe.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lb lb lbx
l l l.
Interface	Attribute	Value
T{
.na
.nh
.BR posix_fallocate ()
T}	Thread safety	T{
.na
.nh
MT-Safe (but see CAVEATS)
T}
.TE
.SH STANDARDS
POSIX.1-2008.
.SH HISTORY
glibc 2.1.94.
POSIX.1-2001
.P
POSIX.1-2008 says that an implementation
.I shall
give the
.B EINVAL
error if
.I len
was 0, or
.I offset
was less than 0.
POSIX.1-2001 says that an implementation
.I shall
give the
.B EINVAL
error if
.I len
is less than 0, or
.I offset
was less than 0, and
.I may
give the error if
.I len
equals zero.
.SH CAVEATS
In the glibc implementation,
.BR posix_fallocate ()
is implemented using the
.BR fallocate (2)
system call, which is MT-safe.
If the underlying filesystem does not support
.BR fallocate (2),
then the operation is emulated with the following caveats:
.IP \[bu] 3
The emulation is inefficient.
.IP \[bu]
There is a race condition where concurrent writes from another thread or
process could be overwritten with null bytes.
.IP \[bu]
There is a race condition where concurrent file size increases by
another thread or process could result in a file whose size is smaller
than expected.
.IP \[bu]
If
.I fd
has been opened with the
.B O_APPEND
or
.B O_WRONLY
flags, the function fails with the error
.BR EBADF .
.P
In general, the emulation is not MT-safe.
On Linux, applications may use
.BR fallocate (2)
if they cannot tolerate the emulation caveats.
In general, this is
only recommended if the application plans to terminate the operation if
.B EOPNOTSUPP
is returned, otherwise the application itself will need to implement a
fallback with all the same problems as the emulation provided by glibc.
.SH SEE ALSO
.BR fallocate (1),
.BR fallocate (2),
.BR lseek (2),
.BR posix_fadvise (2)