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 );
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 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.
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.
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.
CONFORMING TO
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.
NOTES
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.)
