truncate(2) System Calls Manual truncate(2)
NAME
truncate, ftruncate - truncate a file to a specified length
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <unistd.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():
_XOPEN_SOURCE >= 500
|| /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L
|| /* glibc <= 2.19: */ _BSD_SOURCE
ftruncate():
_XOPEN_SOURCE >= 500
|| /* Since glibc 2.3.5: */ _POSIX_C_SOURCE >= 200112L
|| /* glibc <= 2.19: */ _BSD_SOURCE
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 ex-
tended 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
inode(7)) for the file are updated, and the set-user-ID and set-group-ID
mode 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 to indicate the error.
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 The argument 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 path-
name.
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 be-
yond its current size.
EPERM The operation was prevented by a file seal; see fcntl(2).
EROFS The named file resides on a read-only filesystem.
ETXTBSY
The file is an executable 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 de-
scriptor, fd:
EBADF fd is not a valid file descriptor.
EBADF or EINVAL
fd is not open for writing.
EINVAL fd does not reference a regular file or a POSIX shared memory ob-
ject.
EINVAL or EBADF
The file descriptor fd is not open for writing. POSIX permits,
and portable applications should handle, either error for this
case. (Linux produces EINVAL.)
VERSIONS
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 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.
On some 32-bit architectures, the calling signature for these system
calls differ, for the reasons described in syscall(2).
STANDARDS
POSIX.1-2008.
HISTORY
POSIX.1-2001, 4.4BSD, SVr4 (first appeared in 4.2BSD).
The original Linux truncate() and ftruncate() system calls were not de-
signed 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.
NOTES
ftruncate() can also be used to set the size of a POSIX shared memory
object; see shm_open(3).
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 ALSO
truncate(1), open(2), stat(2), path_resolution(7)
Linux man-pages 6.9.1 2024-06-15 truncate(2)
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