-.\" (c) 2006, Michael Kerrisk <mtk-manpages@gmx.net>
+.\" Copyright (c) 2006, Michael Kerrisk <mtk.manpages@gmail.com>
.\"
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.\" have taken the same level of care in the production of this manual,
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.\"
-.TH POSIX_FALLOCATE 3 2006-03-01 "GNU" "Linux Programmer's Manual"
+.TH POSIX_FALLOCATE 3 2017-09-15 "GNU" "Linux Programmer's Manual"
.SH NAME
posix_fallocate \- allocate file space
.SH SYNOPSIS
.nf
-.B #define _XOPEN_SOURCE 600
-.B #include <stdlib.h>
-.sp
+.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
-descriptor
+file descriptor
.I fd
for the bytes in the range starting at
-.IR offset
-and continuing for
+.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"
+.SH RETURN VALUE
.BR posix_fallocate ()
returns zero on success, or an error number on failure.
Note that
-.IR errno
+.I errno
is not set.
-.SH "ERRORS"
+.SH ERRORS
.TP
.B EBADF
.I fd
.I offset+len
exceeds the maximum file size.
.TP
+.B EINTR
+A signal was caught during execution.
+.TP
.B EINVAL
.I offset
-or
+was less than 0, or
.I len
-was less than 0.
+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
+There is not enough space left on the device containing the file
referred to by
-.IR fd.
+.IR fd .
.TP
.B ESPIPE
.I fd
-refers to a pipe of file descriptor.
-.SH AVAILABILITY
+refers to a pipe.
+.SH VERSIONS
.BR posix_fallocate ()
is available since glibc 2.1.94.
-.SH "CONFORMING TO"
+.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.
-.SH "SEE ALSO"
+.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)
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