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.\"     @(#)truncate.2	6.9 (Berkeley) 3/10/91
.\"
.\" Modified 1993-07-24 by Rik Faith <faith@cs.unc.edu>
.\" Modified 1996-10-22 by Eric S. Raymond <esr@thyrsus.com>
.\" Modified 1998-12-21 by Andries Brouwer <aeb@cwi.nl>
.\" Modified 2002-01-07 by Michael Kerrisk <mtk.manpages@gmail.com>
.\" Modified 2002-04-06 by Andries Brouwer <aeb@cwi.nl>
.\" Modified 2004-06-23 by Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.TH TRUNCATE 2 2010-09-20 "Linux" "Linux Programmer's Manual"
.SH NAME
truncate, ftruncate \- truncate a file to a specified length
.SH SYNOPSIS
.B #include <unistd.h>
.br
.B #include <sys/types.h>
.sp
.BI "int truncate(const char *" path ", off_t " length );
.br
.BI "int ftruncate(int " fd ", off_t " length );
.sp
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.in
.ad l
.PD 0
.sp
.BR truncate ():
.RS 4
_BSD_SOURCE || _XOPEN_SOURCE\ >=\ 500 ||
_XOPEN_SOURCE\ &&\ _XOPEN_SOURCE_EXTENDED
.br
|| /* Since glibc 2.12: */ _POSIX_C_SOURCE\ >=\ 200809L
.RE
.sp
.BR ftruncate ():
.RS 4
.TP 4
.TP
Since glibc 2.12:
_BSD_SOURCE || _XOPEN_SOURCE\ >=\ 500 ||
_XOPEN_SOURCE\ &&\ _XOPEN_SOURCE_EXTENDED ||
.\" FIXME The next piece seems incorrect; see
.\" http://sourceware.org/bugzilla/show_bug.cgi?id=12037
_POSIX_C_SOURCE\ >=\ 200809L
.TP
Between glibc 2.3.5 and 2.11
_BSD_SOURCE || _XOPEN_SOURCE\ >=\ 500 ||
_XOPEN_SOURCE\ &&\ _XOPEN_SOURCE_EXTENDED ||
_POSIX_C_SOURCE\ >=\ 200112L
.TP
Before glibc 2.3.5:
_BSD_SOURCE || _XOPEN_SOURCE\ >=\ 500 ||
_XOPEN_SOURCE\ &&\ _XOPEN_SOURCE_EXTENDED
.RE
.PD
.ad b
.SH DESCRIPTION
The
.BR truncate ()
and
.BR ftruncate ()
functions cause the regular file named by
.I path
or referenced by
.I fd
to be truncated to a size of precisely
.I length
bytes.
.LP
If the file previously was larger than this size, the extra data is lost.
If the file previously was shorter, it is extended, and
the extended part reads as null bytes (\(aq\\0\(aq).
.LP
The file offset is not changed.
.LP
If the size changed, then the st_ctime and st_mtime fields
(respectively, time of last status change and
time of last modification; see
.BR stat (2))
for the file are updated,
and the set-user-ID and set-group-ID permission bits may be cleared.
.LP
With
.BR ftruncate (),
the file must be open for writing; with
.BR truncate (),
the file must be writable.
.SH "RETURN VALUE"
On success, zero is returned.
On error, \-1 is returned, and
.I errno
is set appropriately.
.SH ERRORS
For
.BR truncate ():
.TP
.B EACCES
Search permission is denied for a component of the path prefix,
or the named file is not writable by the user.
(See also
.BR path_resolution (7).)
.TP
.B EFAULT
.I Path
points outside the process's allocated address space.
.TP
.B EFBIG
The argument
.I length
is larger than the maximum file size. (XSI)
.TP
.B EINTR
A signal was caught during execution.
.TP
.B EINVAL
The argument
.I length
is negative or larger than the maximum file size.
.TP
.B EIO
An I/O error occurred updating the inode.
.TP
.B EINTR
While blocked waiting to complete,
the call was interrupted by a signal handler; see
.BR fcntl (2)
and
.BR signal (7).
.TP
.B EISDIR
The named file is a directory.
.TP
.B ELOOP
Too many symbolic links were encountered in translating the pathname.
.TP
.B ENAMETOOLONG
A component of a pathname exceeded 255 characters,
or an entire pathname exceeded 1023 characters.
.TP
.B ENOENT
The named file does not exist.
.TP
.B ENOTDIR
A component of the path prefix is not a directory.
.TP
.B EPERM
.\" This happens for at least MSDOS and VFAT file systems
.\" on kernel 2.6.13
The underlying file system does not support extending
a file beyond its current size.
.TP
.B EROFS
The named file resides on a read-only file system.
.TP
.B ETXTBSY
The file is a pure procedure (shared text) file that is being executed.
.PP
For
.BR ftruncate ()
the same errors apply, but instead of things that can be wrong with
.IR path ,
we now have things that can be wrong with the file descriptor,
.IR fd :
.TP
.B EBADF
.I fd
is not a valid descriptor.
.TP
.BR EBADF " or " EINVAL
.I fd
is not open for writing.
.TP
.B EINVAL
.I fd
does not reference a regular file.
.SH "CONFORMING TO"
4.4BSD, SVr4, POSIX.1-2001 (these calls first appeared in 4.2BSD).
.\" POSIX.1-1996 has
.\" .BR ftruncate ().
.\" POSIX.1-2001 also has
.\" .BR truncate (),
.\" as an XSI extension.
.\" .LP
.\" SVr4 documents additional
.\" .BR truncate ()
.\" error conditions EMFILE, EMULTIHP, ENFILE, ENOLINK.  SVr4 documents for
.\" .BR ftruncate ()
.\" an additional EAGAIN error condition.
.SH NOTES
The above description is for XSI-compliant systems.
For non-XSI-compliant systems, the POSIX standard allows
two behaviors for
.BR ftruncate ()
when
.I length
exceeds the file length
(note that
.BR truncate ()
is not specified at all in such an environment):
either returning an error, or extending the file.
Like most Unix implementations, Linux follows the XSI requirement
when dealing with native file systems.
However, some nonnative file systems do not permit
.BR truncate ()
and
.BR ftruncate ()
to be used to extend a file beyond its current length:
a notable example on Linux is VFAT.
.\" At the very least: OSF/1, Solaris 7, and FreeBSD conform, mtk, Jan 2002
.SH "SEE ALSO"
.BR open (2),
.BR stat (2),
.BR path_resolution (7)