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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 2021-03-22 "Linux" "Linux Programmer's Manual"
.SH NAME
truncate, ftruncate \- truncate a file to a specified length
.SH SYNOPSIS
.nf
.B #include <unistd.h>
.PP
.BI "int truncate(const char *" path ", off_t " length );
.BI "int ftruncate(int " fd ", off_t " length );
.fi
.PP
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.PP
.BR truncate ():
.nf
    _XOPEN_SOURCE >= 500
.\"    || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED
        || /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L
        || /* Glibc <= 2.19: */ _BSD_SOURCE
.fi
.PP
.BR ftruncate ():
.nf
    _XOPEN_SOURCE >= 500
.\"    || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED
        || /* Since glibc 2.3.5: */ _POSIX_C_SOURCE >= 200112L
        || /* Glibc <= 2.19: */ _BSD_SOURCE
.fi
.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.
.PP
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\e0\(aq).
.PP
The file offset is not changed.
.PP
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 inode (7))
for the file are updated,
and the set-user-ID and set-group-ID mode bits may be cleared.
.PP
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 to indicate the error.
.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
The argument
.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
While blocked waiting to complete,
the call was interrupted by a signal handler; see
.BR fcntl (2)
and
.BR signal (7).
.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 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 filesystems
.\" on kernel 2.6.13
The underlying filesystem does not support extending
a file beyond its current size.
.TP
.B EPERM
The operation was prevented by a file seal; see
.BR fcntl (2).
.TP
.B EROFS
The named file resides on a read-only filesystem.
.TP
.B ETXTBSY
The file is an executable 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 file descriptor.
.TP
.BR EBADF " or " EINVAL
.I fd
is not open for writing.
.TP
.B EINVAL
.I fd
does not reference a regular file or a POSIX shared memory object.
.TP
.BR EINVAL " or " EBADF
The file descriptor
.I fd
is not open for writing.
POSIX permits, and portable applications should handle,
either error for this case.
(Linux produces
.BR EINVAL .)
.SH CONFORMING TO
POSIX.1-2001, POSIX.1-2008,
4.4BSD, SVr4 (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
.BR ftruncate ()
can also be used to set the size of a POSIX shared memory object; see
.BR shm_open (3).
.PP
The details in DESCRIPTION are 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 filesystems.
However, some nonnative filesystems 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
.PP
The original Linux
.BR truncate ()
and
.BR ftruncate ()
system calls were not designed to handle large file offsets.
Consequently, Linux 2.4 added
.BR truncate64 ()
and
.BR ftruncate64 ()
system calls that handle large files.
However, these details can be ignored by applications using glibc, whose
wrapper functions transparently employ the more recent system calls
where they are available.
.PP
On some 32-bit architectures,
the calling signature for these system calls differ,
for the reasons described in
.BR syscall (2).
.SH BUGS
A header file bug in glibc 2.12 meant that the minimum value of
.\" http://sourceware.org/bugzilla/show_bug.cgi?id=12037
.BR _POSIX_C_SOURCE
required to expose the declaration of
.BR ftruncate ()
was 200809L instead of 200112L.
This has been fixed in later glibc versions.
.SH SEE ALSO
.BR truncate (1),
.BR open (2),
.BR stat (2),
.BR path_resolution (7)