TRUNCATE(2) Linux Programmer's Manual TRUNCATE(2)NAME
truncate, ftruncate - truncate a file to a specified length
SYNOPSIS
#include <unistd.h>
#include <sys/types.h>
int truncate(const char *path, off_t length);
int ftruncate(int fd, off_t length);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
truncate():
_BSD_SOURCE || _XOPEN_SOURCE >= 500 ||
_XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED
|| /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L
ftruncate():
_BSD_SOURCE || _XOPEN_SOURCE >= 500 ||
_XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED
|| /* Since glibc 2.3.5: */ _POSIX_C_SOURCE >= 200112L
DESCRIPTION
The truncate() and ftruncate() functions cause the regular file named
by path or referenced by fd to be truncated to a size of precisely
length bytes.
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 ('\0').
The file offset is not changed.
If the size changed, then the st_ctime and st_mtime fields (respec‐
tively, time of last status change and time of last modification; see
stat(2)) for the file are updated, and the set-user-ID and set-group-ID
permission bits may be cleared.
With ftruncate(), the file must be open for writing; with truncate(),
the file must be writable.
RETURN VALUE
On success, zero is returned. On error, -1 is returned, and errno is
set appropriately.
ERRORS
For truncate():
EACCES Search permission is denied for a component of the path prefix,
or the named file is not writable by the user. (See also
path_resolution(7).)
EFAULT Path points outside the process's allocated address space.
EFBIG The argument length is larger than the maximum file size. (XSI)
EINTR While blocked waiting to complete, the call was interrupted by a
signal handler; see fcntl(2) and signal(7).
EINVAL The argument length is negative or larger than the maximum file
size.
EIO An I/O error occurred updating the inode.
EISDIR The named file is a directory.
ELOOP Too many symbolic links were encountered in translating the
pathname.
ENAMETOOLONG
A component of a pathname exceeded 255 characters, or an entire
pathname exceeded 1023 characters.
ENOENT The named file does not exist.
ENOTDIR
A component of the path prefix is not a directory.
EPERM The underlying file system does not support extending a file
beyond its current size.
EROFS The named file resides on a read-only file system.
ETXTBSY
The file is a pure procedure (shared text) file that is being
executed.
For ftruncate() the same errors apply, but instead of things that can
be wrong with path, we now have things that can be wrong with the file
descriptor, fd:
EBADF fd is not a valid descriptor.
EBADF or EINVAL
fd is not open for writing.
EINVAL fd does not reference a regular file.
CONFORMING TO
4.4BSD, SVr4, POSIX.1-2001 (these calls first appeared in 4.2BSD).
NOTES
The details in DESCRIPTION are for XSI-compliant systems. For non-XSI-
compliant systems, the POSIX standard allows two behaviors for ftrun‐
cate() when length exceeds the file length (note that 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 truncate() and ftruncate() to be
used to extend a file beyond its current length: a notable example on
Linux is VFAT.
The original Linux truncate() and ftruncate() system calls were not
designed to handle large file offsets. Consequently, Linux 2.4 added
truncate64() and 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.
On some 32-bit architectures, the calling signature for these system
calls differ, for the reasons described in syscall(2).
BUGS
A header file bug in glibc 2.12 meant that the minimum value of
_POSIX_C_SOURCE required to expose the declaration of ftruncate() was
200809L instead of 200112L. This has been fixed in later glibc ver‐
sions.
SEE ALSOopen(2), stat(2), path_resolution(7)COLOPHON
This page is part of release 3.53 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
Linux 2013-04-01 TRUNCATE(2)
