.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved 
.TH "FTRUNCATE" 3P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\" ftruncate 
.SH PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
.SH NAME
ftruncate \- truncate a file to a specified length
.SH SYNOPSIS
.LP
\fB#include <unistd.h>
.br
.sp
int ftruncate(int\fP \fIfildes\fP\fB, off_t\fP \fIlength\fP\fB);
.br
\fP
.SH DESCRIPTION
.LP
If \fIfildes\fP is not a valid file descriptor open for writing, the
\fIftruncate\fP() function shall fail.
.LP
If \fIfildes\fP refers to a regular file, the \fIftruncate\fP() function
shall cause the size of the file to be truncated to
\fIlength\fP. If the size of the file previously exceeded \fIlength\fP,
the extra data shall no longer be available to reads on
the file. If the file previously was smaller than this size, \fIftruncate\fP()
shall either increase the size of the file or fail.
\ XSI-conformant systems shall increase the size of the file. 
If the file size is increased, the extended area shall appear as if
it were zero-filled. The value of the seek pointer shall not be
modified by a call to \fIftruncate\fP().
.LP
Upon successful completion, if \fIfildes\fP refers to a regular file,
the \fIftruncate\fP() function shall mark for update the
\fIst_ctime\fP and \fIst_mtime\fP fields of the file and the S_ISUID
and S_ISGID bits of the file mode may be cleared. If the
\fIftruncate\fP() function is unsuccessful, the file is unaffected.
.LP
If the request would cause the file size to exceed the soft file size
limit for the process, the request shall fail and the
implementation shall generate the SIGXFSZ signal for the thread. 
.LP
If \fIfildes\fP refers to a directory, \fIftruncate\fP() shall fail.
.LP
If \fIfildes\fP refers to any other file type, except a shared memory
object, the result is unspecified.
.LP
If \fIfildes\fP refers to a shared memory object, \fIftruncate\fP()
shall set the size of the shared memory object to
\fIlength\fP. 
.LP
If the effect of \fIftruncate\fP() is to decrease the size of a shared
memory object or memory mapped file and whole pages beyond
the new end were previously mapped, then the whole pages beyond the
new end shall be discarded. 
.LP
If the Memory Protection option is supported, references to discarded
pages shall result in the generation of a SIGBUS signal;
otherwise, the result of such references is undefined. 
.LP
If the effect of \fIftruncate\fP() is to increase the size of a shared
memory object, it is unspecified whether the contents of
any mapped pages between the old end-of-file and the new are flushed
to the underlying object. 
.SH RETURN VALUE
.LP
Upon successful completion, \fIftruncate\fP() shall return 0; otherwise,
-1 shall be returned and \fIerrno\fP set to indicate
the error.
.SH ERRORS
.LP
The \fIftruncate\fP() function shall fail if:
.TP 7
.B EINTR
A signal was caught during execution.
.TP 7
.B EINVAL
The \fIlength\fP argument was less than 0.
.TP 7
.B EFBIG \fRor\fP EINVAL
.sp
The \fIlength\fP argument was greater than the maximum file size.
.TP 7
.B EFBIG
The file is a regular file and \fIlength\fP is greater than the offset
maximum established in the open file description associated
with \fIfildes\fP. 
.TP 7
.B EIO
An I/O error occurred while reading from or writing to a file system.
.TP 7
.B EBADF \fRor\fP EINVAL
.sp
The \fIfildes\fP argument is not a file descriptor open for writing.
.TP 7
.B EINVAL
The \fIfildes\fP argument references a file that was opened without
write permission.
.TP 7
.B EROFS
The named file resides on a read-only file system.
.sp
.LP
\fIThe following sections are informative.\fP
.SH EXAMPLES
.LP
None.
.SH APPLICATION USAGE
.LP
None.
.SH RATIONALE
.LP
The \fIftruncate\fP() function is part of IEEE\ Std\ 1003.1-2001 as
it was deemed to be more useful than \fItruncate\fP(). The \fItruncate\fP()
function is
provided as an XSI extension.
.SH FUTURE DIRECTIONS
.LP
None.
.SH SEE ALSO
.LP
\fIopen\fP(), \fItruncate\fP(), the Base Definitions volume of
IEEE\ Std\ 1003.1-2001, \fI<unistd.h>\fP
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .