Originální popis anglicky:
truncate, ftruncate - truncate a file to a specified length
Návod, kniha: Linux Programmer's Manual
#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
zero bytes.
The file pointer is not changed.
If the size changed, then the ctime and mtime fields for the file are updated,
and suid and sgid mode 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:
- 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(2).)
- EFAULT
- Path points outside the process's allocated address
space.
- EFBIG
- The argument length is larger than the maximum file
size. (XSI)
- EINTR
- A signal was caught during execution.
- 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 path name exceeded 1023 characters.
- ENOENT
- The named file does not exist.
- ENOTDIR
- A component of the path prefix is not a directory.
- 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
fd:
- EBADF
- The fd is not a valid descriptor.
- EBADF or EINVAL
- The fd is not open for writing.
- EINVAL
- The fd does not reference a regular file.
4.4BSD, SVr4 (these function calls first appeared in BSD 4.2). POSIX 1003.1-1996
has
ftruncate. POSIX 1003.1-2001 also has
truncate, as an XSI
extension.
SVr4 documents additional
truncate error conditions EMFILE, EMULTIHP,
ENFILE, ENOLINK. SVr4 documents for
ftruncate an additional EAGAIN
error condition.
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. (Most Unices follow the XSI requirement.)
open(2),
path_resolution(2)
