man-pages/man2/truncate.2

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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@gmx.net>
.\" Modified 2002-04-06 by Andries Brouwer <aeb@cwi.nl>
.\" Modified 2004-06-23 by Michael Kerrisk <mtk-manpages@gmx.net>
.\"
.TH TRUNCATE 2 2004-06-23 "Linux 2.6.7" "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 );
.SH DESCRIPTION
The
.B truncate
and
.B 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 zero bytes.
.LP
The file pointer is not changed.
.LP
If the size changed, then the ctime and mtime fields for the file
are updated, 
and 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 (2).)
.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 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 path name 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 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
.B 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
.IR fd :
.TP
.B EBADF
The
.I fd
is not a valid descriptor.
.TP
.BR EBADF " or " EINVAL
The
.I fd
is not open for writing.
.TP
.B EINVAL
The
.I fd
does not reference a regular file.
.SH "CONFORMING TO"
4.4BSD, SVr4 (these function calls first appeared in BSD 4.2).
POSIX 1003.1-1996 has
.BR ftruncate .
POSIX 1003.1-2001 also has
.IR truncate ,
as an XSI extension.
.LP
SVr4 documents additional
.B truncate
error conditions EMFILE, EMULTIHP, ENFILE, ENOLINK.  SVr4 documents for
.B 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 behaviours for
.B ftruncate
when
.I length
exceeds the file length
(note that
.B truncate
is not specified at all in such an environment):
either returning an error, or extending the file.
(Most Unices follow the XSI requirement.)
.\" At the very least: OSF/1, Solaris 7, and FreeBSD conform, mtk, Jan 2002
.SH "SEE ALSO"
.BR open (2),
.BR path_resolution (2)