truncate, ftruncate - Change file length
#include <unistd.h>
int truncate(
const char *path,
off_t length ); int ftruncate(
int filedes,
off_t length );
Interfaces documented on this reference page conform to
industry standards as follows:
ftruncate(), truncate(): XSH4.2, XSH5.0
Refer to the standards(5) reference page for more information
about industry standards and associated tags.
Specifies the name of a file that is opened, truncated,
and then closed. The path parameter must point to a pathname
which names a regular file for which the calling process
has write permission. If the path parameter refers to
a symbolic link, the length of the file pointed to by the
symbolic link is truncated. Specifies the descriptor of a
file that must be open for writing. Specifies the new
length of the file in bytes.
The truncate() and ftruncate() functions change the length
of a file to the size in bytes specified by the length
parameter. If the new length is less than the previous
length, the truncate() and ftruncate() functions remove
all data beyond length bytes from the specified file. All
file data between the new end-of-file and the previous
end-of-file is discarded. If the new length is greater
than the previous length, one byte of zero (0x00) is written
at the offset of the new length. The space in between
the previous end-of-file and the new end-of-file is left
as a hole; that is, no blocks are allocated to the space
in between the previous last block and the new last block.
Full blocks are returned to the file system so that they
can be used again, and the file size is changed to the
value of the length parameter.
The truncate() and ftruncate() functions have no effect on
FIFO special files or directories. These functions do not
modify the seek pointer of the file.
Upon successful completion, the truncate() and ftruncate()
functions mark the st_ctime and st_mtime fields of the
file for update. If the file is a regular file, the ftruncate()
and truncate() functions clear the S_ISUID and
S_ISGID attributes of the file.
[Tru64 UNIX] If the file has enforced file locking
enabled and there are file locks on the file, the truncate()
or ftruncate() function fails.
Upon successful completion, a value of 0 (zero) is
returned. If the truncate() or ftruncate() function fails,
it returns a value of -1, and errno is set to indicate the
error.
The ftruncate() and truncate() functions set errno to the
specified values for the following conditions: [Tru64
UNIX] The write operation failed due to an enforced write
lock on the file.
[Tru64 UNIX] The file has enforced mode file locking
enabled and there are file locks on the file.
The length parameter was greater than the maximum
file size. A signal was caught during execution.
The length parameter was less than 0 (zero).
[Tru64 UNIX] The file is not a regular file. An
I/O error occurred while reading from or writing to
a file system. [Tru64 UNIX] The process' root or
current directory is located in a virtual file system
that has been unmounted.
In addition, the ftruncate() function sets errno to the
specified values for the following conditions: [Tru64
UNIX] Write access permission to the file was denied.
The filedes parameter is not a valid file descriptor open
for writing. The fildes parameter references a file that
was opened without write permission. [Tru64 UNIX] The
file resides on a read-only file system.
In addition, the truncate() function fails if errors occur
that apply to any service requiring pathname resolution,
or if one of the following are true: A component of the
path prefix denies search permission, or write permission
is denied on the file. The named file is a directory.
Too many symbolic links were encountered in resolving
path. The size of the pathname exceeds PATH_MAX or a
pathname component is longer than NAME_MAX.
Pathname resolution of a symbolic link produced an
intermediate result whose length exceeds PATH_MAX.
A component of the specified pathname does not
exist, or the path parameter points to an empty
string. A component of the path prefix is not a
directory. The file resides on a read-only file
system.
Functions: chmod(2), fcntl(2), open(2)
Standards: standards(5)
truncate(2)
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