fcntl(TCL)

fcntl -- set or clear a file option or return its current value

Syntax

fcntl fileId attribute [value]

Description

This command either sets or clears a file option or returns its current value. If value is not specified, then the current value of attribute is returned. The following attributes may be specified:

RDONLY: The file is opened for reading only. (Get only)
WRONLY: The file is opened for writing only. (Get only)
RDWR: The file is opened for reading and writing. (Get only)
READ: If the file is readable. (Get only).
WRITE: If the file is writable. (Get only).
APPEND: The file is opened for append-only writes. All writes will be forced to the end of the file.
NONBLOCK: The file is to be accessed with non-blocking I/O. See the read system call for a description of how it affects the behavior of file reads.
CLOEXEC: Close the file on a process exec. If the execl command or some other mechanism causes the process to do an exec, the file will be closed if this option is set.
NOBUF: The file is not buffered. If set, then there is no stdio buffering for the file.
LINEBUF: Output to the file is line buffered. The buffer is flushed when a newline is written, when the buffer is full, or when input is requested.

The APPEND, NONBLOCK, and CLOEXEC attributes may be set or cleared by specifying the attribute name and the value 1 to set the attribute or 0 to clear it.

The NOBUF and LINEBUF attributes can only be set (a value of 1), not cleared; only one of the options may be selected. Once set, it may not be changed. These options should be set before any I/O operations have been done on the file or data may be lost.