truncate(2) System Calls Manual truncate(2)
NAME
truncate, ftruncate - Changes file length
SYNOPSIS
#include <unistd.h>
int truncate( const char *path, off_t length);
int ftruncate( int filedes, off_t length);
STANDARDS
Interfaces documented on this reference page conform to industry standards as follows:
ftruncate(), truncate(): XSH4.2
Refer to the standards(5) reference page for more information about industry standards and associated tags.
PARAMETERS
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.
DESCRIPTION
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 parame-
ter.
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.
If the file has enforced file locking enabled and there are file locks on the file, the truncate() or ftruncate() function fails.
RETURN VALUES
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.
ERRORS
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 permis-
sion 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 fol-
lowing 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. A component of the specified pathname does not exist, or the path parameter points to an empty string. A compo-
nent of the path prefix is not a directory. The file resides on a read-only file system.
RELATED INFORMATION
Functions: chmod(2), fcntl(2), open(2)
Standards: standards(5) delim off
truncate(2)