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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 refer‐
enced 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 (respectively, 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 filesystem does not support extending a file beyond its current
size.
EROFS The named file resides on a read-only filesystem.
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 ftruncate() 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 filesystems. However, some nonnative
filesystems 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 versions.
SEE ALSO
open(2), stat(2), path_resolution(7)
COLOPHON
This page is part of release 3.74 of the Linux man-pages project. A description of the
project, information about reporting bugs, and the latest version of this page, can be
found at http://www.kernel.org/doc/man-pages/.
Linux 2013-04-01 TRUNCATE(2)
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