truncate, ftruncate — truncate a file to a specified length
#include <unistd.h> #include <sys/types.h>
int
truncate( |
const char * | path, |
off_t | length) ; |
int
ftruncate( |
int | fd, |
off_t | length) ; |
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 (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.
On success, zero is returned. On error, −1 is
returned, and errno
is set
appropriately.
For truncate
():
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(2).)
Path
points outside the process's allocated address
space.
The argument length
is larger than the
maximum file size. (XSI)
A signal was caught during execution.
The argument length
is negative or
larger than the maximum file size.
An I/O error occurred updating the inode.
The named file is a directory.
Too many symbolic links were encountered in translating the pathname.
A component of a pathname exceeded 255 characters, or an entire pathname exceeded 1023 characters.
The named file does not exist.
A component of the path prefix is not a directory.
The underlying file system does not support extending a file beyond its current size.
The named file resides on a read-only file system.
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 fd
:
The fd
is
not a valid descriptor.
The fd
is
not open for writing.
The fd
does
not reference a regular file.
The above description is for XSI-compliant systems. For
non-XSI-compliant systems, the POSIX standard allows two
behaviours 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 file systems. However,
some non-native 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.
open(2), path_resolution(2), stat(2)
|