tcflow

Synopsis

#include <termios.h>

int tcflow(int fildes, int action);

Status

Partially implemented

Conformance

IEEE Std 1003.1-2017

Description

The tcflow() function shall suspend or restart transmission or reception of data on the object referred to by fildes, depending on the value of action. The fildes argument is an open file descriptor associated with a terminal.

  • If action is TCOOFF, output shall be suspended.

  • If action is TCOON, suspended output shall be restarted.

  • If action is TCIOFF, the system shall transmit a STOP character, which is intended to cause the terminal device to stop transmitting data to the system.

  • If action is TCION, the system shall transmit a START character, which is intended to cause the terminal device to start transmitting data to the system.

If fildes is associated with a pseudo-terminal, the STOP and START characters need not be transmitted.

The default on the opening of a terminal file is that neither its input nor its output are suspended.

Attempts to use tcflow() from a process which is a member of a background process group on a fildes associated with its controlling terminal, shall cause the process group to be sent a SIGTTOU signal. If the calling thread is blocking SIGTTOU signals or the process is ignoring SIGTTOU signals, the process shall be allowed to perform the operation, and no signal is sent.

Return value

Upon successful completion, 0 shall be returned. Otherwise, -1 shall be returned and errno set to indicate the error.

Errors

The tcflow() function shall fail if:

  • EBADF - The fildes argument is not a valid file descriptor.

  • EINVAL - The action argument is not a supported value.

  • EIO - The process group of the writing process is orphaned, the calling thread is not blocking SIGTTOU, and the process is not ignoring SIGTTOU.

  • ENOTTY - The file associated with fildes is not a terminal.

  • EINVAL - The file associated with fildes is not a terminal, and the server handling it reports unsupported requests with EINVAL (see POSIX deviations).

POSIX deviations

If the file associated with fildes is not a terminal, the error reported by tcflow() depends on the server handling the file. The function is implemented on top of ioctl(), and the kernel forwards every request to the server without checking the type of the file. Servers that do not handle terminal requests report them either with ENOTTY (for example files in jffs2) or with EINVAL (for example pipes and files in dummyfs).

Job control is not supported (_POSIX_JOB_CONTROL is not defined), so SIGTTOU is not sent when tcflow() is called from a background process group, and EIO is not reported for an orphaned process group.

Known bugs