tcsetattr - set the parameters associated with the terminal
int tcsetattr(int fildes, int optional_actions,
const struct termios *termios_p);
The tcsetattr() function shall set the parameters associated with the terminal referred to by the open file descriptor fildes (an open file descriptor associated with a terminal) from the termios structure referenced by termios_p as follows:
If optional_actions is TCSANOW, the change shall occur immediately.
If optional_actions is TCSADRAIN, the change shall occur after all output written to fildes is transmitted. This function should be used when changing parameters that affect output.
If optional_actions is TCSAFLUSH, the change shall occur after all output written to fildes is transmitted, and all input so far received but not read shall be discarded before the change is made.
If the output baud rate stored in the termios structure pointed to by termios_p is the zero baud rate, B0, the modem control lines shall no longer be asserted. Normally, this shall disconnect the line.
If the input baud rate stored in the termios structure pointed to by termios_p is 0, the input baud rate given to the hardware is the same as the output baud rate stored in the termios structure.
The tcsetattr() function shall return successfully if it was able to perform any of the requested actions, even if some of the requested actions could not be performed. It shall set all the attributes that the implementation supports as requested and leave all the attributes not supported by the implementation unchanged. If no part of the request can be honored, it shall return -1 and set errno to [EINVAL]. If the input and output baud rates differ and are a combination that is not supported, neither baud rate shall be changed. A subsequent call to tcgetattr() shall return the actual state of the terminal device (reflecting both the changes made and not made in the previous tcsetattr() call). The tcsetattr() function shall not change the values found in the termios structure under any circumstances.
The effect of tcsetattr() is undefined if the value of the termios structure pointed to by termios_p was not derived from the result of a call to tcgetattr() on fildes; an application should modify only fields and flags defined by this volume of POSIX.1-2017 between the call to tcgetattr() and tcsetattr(), leaving all other fields and flags unmodified.
No actions defined by this volume of POSIX.1-2017, other than a call to tcsetattr(), a close of the last file descriptor in the system associated with this terminal device, or an open of the first file descriptor in the system associated with this terminal device (using the O_TTY_INIT flag if it is non-zero and the device is not a pseudo-terminal), shall cause any of the terminal attributes defined by this volume of POSIX.1-2017 to change.
If tcsetattr() is called from a process which is a member of a background process group on a fildes associated with its controlling terminal:
If the calling thread is blocking SIGTTOU signals or the process is ignoring SIGTTOU signals, the operation completes normally and no signal is sent.
Otherwise, a SIGTTOU signal shall be sent to the process group.
Upon successful completion, 0 shall be returned. Otherwise, -1 shall be returned and errno set to indicate the error.
The tcsetattr() function shall fail if:
- The fildes argument is not a valid file descriptor.
- A signal interrupted tcsetattr().
- The optional_actions argument is not a supported value, or an attempt was made to change an attribute represented in the termios structure to an unsupported value.
- The process group of the writing process is orphaned, the calling thread is not blocking SIGTTOU, and the process is not ignoring SIGTTOU.
- The file associated with fildes is not a terminal.
If trying to change baud rates, applications should call tcsetattr() then call tcgetattr() in order to determine what baud rates were actually selected.
In general, there are two reasons for an application to change the parameters associated with a terminal device:
The device already has working parameter settings but the application needs a different behavior, such as non-canonical mode instead of canonical mode. The application sets (or clears) only a few flags or c_cc values. Typically, the terminal device in this case is either the controlling terminal for the process or a pseudo-terminal.
The device is a modem or similar piece of equipment connected by a serial line, and it was not open before the application opened it. In this case, the application needs to initialize all of the parameter settings ``from scratch''. However, since the termios structure may include both standard and non-standard parameters, the application cannot just initialize the whole structure in an arbitrary way (e.g., using memset()) as this may cause some of the non-standard parameters to be set incorrectly, resulting in non-conforming behavior of the terminal device. Conversely, the application cannot just set the standard parameters, assuming that the non-standard parameters will already have suitable values, as the device might previously have been used with non-conforming parameter settings (and some implementations retain the settings from one use to the next). The solution is to open the terminal device using the O_TTY_INIT flag to initialize the terminal device to have conforming parameter settings, obtain those settings using tcgetattr(), and then set all of the standard parameters to the desired settings.
The tcsetattr() function can be interrupted in the following situations:
It is interrupted while waiting for output to drain.
It is called from a process in a background process group and SIGTTOU is caught.
See also the RATIONALE section in tcgetattr.
Using an input baud rate of 0 to set the input rate equal to the output rate may not necessarily be supported in a future version of this volume of POSIX.1-2017.
XBD General Terminal Interface, <termios.h>
First released in Issue 3. Included for alignment with the POSIX.1-1988 standard.
The following new requirements on POSIX implementations derive from alignment with the Single UNIX Specification:
In the DESCRIPTION, text previously conditional on _POSIX_JOB_CONTROL is now mandated. This is a FIPS requirement.
The [EIO] error is added.
In the DESCRIPTION, the text describing use of tcsetattr() from a process which is a member of a background process group is clarified.
Austin Group Interpretation 1003.1-2001 #144 is applied.
POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0652 , XSH/TC1-2008/0653 , and XSH/TC1-2008/0654  are applied.
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