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.TH POSIX_FALLOCATE 3  2017-09-15 "GNU" "Linux Programmer's Manual"

.SH NAME

posix_fallocate \- allocate file space

.SH SYNOPSIS

.nf

.B #include <fcntl.h>

.PP

.BI "int posix_fallocate(int " fd ", off_t " offset ", off_t " len );

.fi

.PP

.ad l

.in -4n

Feature Test Macro Requirements for glibc (see

.BR feature_test_macros (7)):

.in

.PP

.BR posix_fallocate ():

.RS 4

_POSIX_C_SOURCE\ >=\ 200112L

.RE

.ad

.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.

.PP

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 ESPIPE

.I fd

refers to a pipe.

.SH VERSIONS

.BR posix_fallocate ()

is available since glibc 2.1.94.

.SH ATTRIBUTES

For an explanation of the terms used in this section, see

.BR attributes (7).

.TS

allbox;

lb lb lb

l l l.

Interface	Attribute	Value

T{

.BR posix_fallocate ()

T}	Thread safety	MT-Safe (but see NOTES)

.TE

.SH CONFORMING TO

POSIX.1-2001.

.PP

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 NOTES

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 * 2

The emulation is inefficient.

.IP *

There is a race condition where concurrent writes from another thread or

process could be overwritten with null bytes.

.IP *

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 *

If

.I fd

has been opened with the

.B O_APPEND

or

.B O_WRONLY

flags, the function fails with the error

.BR EBADF .

.PP

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)

.SH COLOPHON

This page is part of release 4.15 of the Linux

.I 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/.