'\" te
.\" Copyright (c) 2000, 2001, 2002, 2003, 2004 by Martin C. Shepherd. All Rights Reserved.
.\" Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, provided that the above copyright notice(s) and this permission notice appear in all copies of the Software and that both the above copyright notice(s) and this permission notice appear in supporting documentation.  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT OF THIRD PARTY RIGHTS. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR HOLDERS INCLUDED IN THIS NOTICE BE LIABLE FOR ANY CLAIM, OR ANY SPECIAL INDIRECT OR CONSEQUENTIAL DAMAGES, OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.  Except as contained in this notice, the name of a copyright holder shall not be used in advertising or otherwise to promote the sale, use or other dealings in this Software without prior written authorization of the copyright holder.
.\" Portions Copyright (c) 2007, Sun Microsystems, Inc. All Rights Reserved.
.TH gl_io_mode 3TECLA "1 Jun 2004" "SunOS 5.11" "Interactive Command-line Input Library Functions"
.SH NAME
gl_io_mode, gl_raw_io, gl_normal_io, gl_tty_signals, gl_abandon_line, gl_handle_signal, gl_pending_io \- use \fBgl_get_line()\fR from an external event loop
.SH SYNOPSIS
.LP
.nf
cc [ \fIflag\fR\&.\|.\|. ] \fIfile\fR\&.\|.\|. \fB-ltecla\fR [ \fIlibrary\fR\&.\|.\|. ] 
#include <libtecla.h>

\fBint\fR \fBgl_io_mode\fR(\fBGetLine *\fR\fIgl\fR, \fBGlIOMode\fR \fImode\fR);
.fi

.LP
.nf
\fBint\fR \fBgl_raw_io\fR(\fBGetLine *\fR\fIgl\fR);
.fi

.LP
.nf
\fBint\fR \fBgl_normal_io\fR(\fBGetLine *\fR\fIgl\fR);
.fi

.LP
.nf
\fBint\fR \fBgl_tty_signals\fR(\fBvoid (*\fR\fIterm_handler\fR)(int), \fBvoid (*\fR\fIsusp_handler\fR)(int),
     \fBvoid (*\fR\fIcont_handler\fR)(int), \fBvoid (*\fR\fIsize_handler\fR)(int));
.fi

.LP
.nf
\fBvoid\fR \fBgl_abandon_line\fR(\fBGetLine *\fR\fIgl\fR);
.fi

.LP
.nf
\fBvoid\fR \fBgl_handle_signal\fR(\fBint\fR \fIsigno\fR, \fBGetLine *\fR\fIgl\fR, \fBint\fR \fIngl\fR);
.fi

.LP
.nf
\fBGlPendingIO\fR \fBgl_pending_io\fR(\fBGetLine *\fR\fIgl\fR);
.fi

.SH DESCRIPTION
.sp
.LP
The \fBgl_get_line\fR(3TECLA) function supports two different I/O modes. These are selected by calling the \fBgl_io_mode()\fR function. The \fImode\fR argument of \fBgl_io_mode()\fR specifies the new I/O mode and must be one of the following.
.sp
.ne 2
.mk
.na
\fB\fBGL_NORMAL_MODE\fR\fR
.ad
.RS 18n
.rt  
Select the normal blocking-I/O mode. In this mode \fBgl_get_line()\fR does not return until either an error occurs of the user finishes entering a new line.
.RE

.sp
.ne 2
.mk
.na
\fB\fBGL_SERVER_MODE\fR\fR
.ad
.RS 18n
.rt  
Select non-blocking server I/O mode. In this mode, since non-blocking terminal I/O is used, the entry of each new input line typically requires many calls to \fBgl_get_line()\fR from an external I/O-driven event loop.
.RE

.sp
.LP
Newly created GetLine objects start in normal I/O mode, so to switch to non-blocking server mode requires an initial call to \fBgl_io_mode()\fR.
.SS "Server I/O Mode"
.sp
.LP
In non-blocking server I/O mode, the application is required to have an event loop that calls \fBgl_get_line()\fR whenever the terminal file descriptor can perform the type I/O that \fBgl_get_line()\fR is waiting for. To determine which type of I/O \fBgl_get_line()\fR is waiting for, the application calls the \fBgl_pending_io()\fR function.  The return value is one of the following two enumerated values.
.sp
.ne 2
.mk
.na
\fB\fBGLP_READ\fR\fR
.ad
.RS 13n
.rt  
\fBgl_get_line()\fR is waiting to write a character to the terminal.
.RE

.sp
.ne 2
.mk
.na
\fB\fBGLP_WRITE\fR\fR
.ad
.RS 13n
.rt  
\fBgl_get_line()\fR is waiting to read a character from the keyboad.
.RE

.sp
.LP
If the application is using either the \fBselect\fR(3C) or \fBpoll\fR(2) function to watch for I/O on a group of file descriptors, then it should call the \fBgl_pending_io()\fR function before each call to these functions to determine which direction of I/O it should tell them to watch for, and configure their arguments accordingly. In the case of the \fBselect()\fR function, this means using the \fBFD_SET()\fR macro to add the terminal file descriptor either to the set of file descriptors to be watched for readability or the set to be watched for writability.
.sp
.LP
As in normal I/O mode, the return value of \fBgl_get_line()\fR is either a pointer to a completed input line or \fINULL\fR. However, whereas in normal I/O mode a \fINULL\fR return value always means that an error occurred, in non-blocking server mode, \fINULL\fR is also returned when \fBgl_get_line()\fR cannot read or write to the terminal without blocking. Thus in non-blocking server mode, in order to determine when a \fINULL\fR return value signifies that an error occurred or not, it is necessary to call the \fBgl_return_status()\fR function. If this function returns the enumerated value \fBGLR_BLOCKED\fR, \fBgl_get_line()\fR is waiting for I/O and no error has occurred.
.sp
.LP
When \fBgl_get_line()\fR returns \fINULL\fR and \fBgl_return_status()\fR indicates that this is due to blocked terminal I/O, the application should call \fBgl_get_line()\fR again when the type of I/O reported by \fBgl_pending_io()\fR becomes possible. The \fIprompt\fR, \fIstart_line\fR and \fIstart_pos\fR arguments of \fBgl_get_line()\fR will be ignored on these calls. If you need to change the prompt of the line that is currently being edited, you can call the \fBgl_replace_prompt\fR(3TECLA) function between calls to \fBgl_get_line()\fR.
.SS "Giving Up The Terminal"
.sp
.LP
A complication that is unique to non-blocking server mode is that it requires that the terminal be left in raw mode between calls to \fBgl_get_line()\fR. If this were not the case, the external event loop would not be able to detect individual key-presses, and the basic line editing implemented by the terminal driver would clash with the editing provided by \fBgl_get_line()\fR. When the terminal needs to be used for purposes other than entering a new input line with \fBgl_get_line()\fR, it needs to be restored to a usable state. In particular, whenever the process is suspended or terminated, the terminal must be returned to a normal state. If this is not done, then depending on the characteristics of the shell that was used to invoke the program, the user could end up with a hung terminal. To this end, the \fBgl_normal_io()\fR function is provided for switching the terminal back to the state that it was in when raw mode was last established.
.sp
.LP
The \fBgl_normal_io()\fR function first flushes any pending output to the terminal, then moves the cursor to the start of the terminal line which follows the end of the incompletely entered input line. At this point it is safe to suspend or terminate the process, and it is safe for the application to read and write to the terminal. To resume entry of the input line, the application should call the \fBgl_raw_io()\fR function.
.sp
.LP
The \fBgl_normal_io()\fR function starts a new line, redisplays the partially completed input line (if any), restores the cursor position within this line to where it was when \fBgl_normal_io()\fR was called, then switches back to raw, non-blocking terminal mode ready to continue entry of the input line when \fBgl_get_line()\fR is next called.
.sp
.LP
Note that in non-blocking server mode, if \fBgl_get_line()\fR is called after a call to \fBgl_normal_io()\fR, without an intervening call to \fBgl_raw_io()\fR, \fBgl_get_line()\fR will call \fBgl_raw_mode()\fR itself, and the terminal will remain in this mode when \fBgl_get_line()\fR returns.
.SS "Signal Handling"
.sp
.LP
In the previous section it was pointed out that in non-blocking server mode, the terminal must be restored to a sane state whenever a signal is received that either suspends or terminates the process. In normal I/O mode, this is done for you by \fBgl_get_line()\fR, but in non-blocking server mode, since the terminal is left in raw mode between calls to \fBgl_get_line()\fR, this signal handling has to be done by the application. Since there are many signals that can suspend or terminate a process, as well as other signals that are important to \fBgl_get_line()\fR, such as the \fBSIGWINCH\fR signal, which tells it when the terminal size has changed, the \fBgl_tty_signals()\fR function is provided for installing signal handlers for all pertinent signals.
.sp
.LP
The \fBgl_tty_signals()\fR function uses \fBgl_get_line()\fR's internal list of signals to assign specified signal handlers to groups of signals. The arguments of this function are as follows.
.sp
.ne 2
.mk
.na
\fB\fIterm_handler\fR\fR
.ad
.RS 16n
.rt  
This is the signal handler that is used to trap signals that by default terminate any process that receives them (for example, \fBSIGINT\fR or \fBSIGTERM\fR).
.RE

.sp
.ne 2
.mk
.na
\fB\fIsusp_handler\fR\fR
.ad
.RS 16n
.rt  
This is the signal handler that is used to trap signals that by default suspend any process that receives them, (for example, \fBSIGTSTP\fR or \fBSIGTTOU\fR).
.RE

.sp
.ne 2
.mk
.na
\fB\fIcont_handler\fR\fR
.ad
.RS 16n
.rt  
This is the signal handler that is used to trap signals that are usually sent when a process resumes after being suspended (usually \fBSIGCONT\fR). Beware that there is nothing to stop a user from sending one of these signals at other times.
.RE

.sp
.ne 2
.mk
.na
\fB\fIsize_handler\fR\fR
.ad
.RS 16n
.rt  
This signal handler is used to trap signals that are sent to processes when their controlling terminals are resized by the user (for example, \fBSIGWINCH\fR).
.RE

.sp
.LP
These arguments can all be the same, if so desired, and \fBSIG_IGN\fR (ignore this signal) or \fBSIG_DFL\fR (use the system-provided default signal handler) can be specified instead of a function where pertinent. In particular, it is rarely useful to trap \fBSIGCONT\fR, so the \fIcont_handler\fR argument will usually be \fBSIG_DFL\fR or \fBSIG_IGN\fR.
.sp
.LP
The \fBgl_tty_signals()\fR function uses the POSIX \fBsigaction\fR(2) function to install these signal handlers, and it is careful to use the \fIsa_mask\fR member of each \fBsigaction\fR structure to ensure that only one of these signals is ever delivered at a time. This guards against different instances of these signal handlers from simultaneously trying to write to common global data, such as a shared \fBsigsetjmp\fR(3C) buffer or a signal-received flag. The signal handlers installed by this function should call the \fBgl_handle_signal()\fR.
.sp
.LP
The \fIsigno\fR argument tells this function which signal it is being asked to respond to, and the \fIgl\fR argument should be a pointer to the first element of an array of \fIngl\fR \fBGetLine\fR objects. If your application has only one of these objects, pass its pointer as the \fIgl\fR argument and specify \fIngl\fR as 1.
.sp
.LP
Depending on the signal that is being handled, this function does different things.
.SS "Process termination signals"
.sp
.LP
If the signal that was caught is one of those that by default terminates any process that receives it, then \fBgl_handle_signal()\fR does the following steps.
.RS +4
.TP
1.
First it blocks the delivery of all signals that can be blocked (ie. \fBSIGKILL\fR and \fBSIGSTOP\fR cannot be blocked).
.RE
.RS +4
.TP
2.
Next it calls \fBgl_normal_io()\fR for each of the ngl GetLine objects. Note that this does nothing to any of the GetLine objects that are not currently in raw mode.
.RE
.RS +4
.TP
3.
Next it sets the signal handler of the signal to its default, process-termination disposition.
.RE
.RS +4
.TP
4.
Next it re-sends the process the signal that was caught.
.RE
.RS +4
.TP
5.
Finally it unblocks delivery of this signal, which results in the process being terminated.
.RE
.SS "Process suspension signals"
.sp
.LP
If the default disposition of the signal is to suspend the process, the same steps are executed as for process termination signals, except that when the process is later resumed, \fBgl_handle_signal()\fR continues, and does the following steps.
.RS +4
.TP
1.
It re-blocks delivery of the signal.
.RE
.RS +4
.TP
2.
It reinstates the signal handler of the signal to the one that was displaced when its default disposition was substituted.
.RE
.RS +4
.TP
3.
For any of the GetLine objects that were in raw mode when \fBgl_handle_signal()\fR was called, \fBgl_handle_signal()\fR then calls \fBgl_raw_io()\fR, to resume entry of the input lines on those terminals.
.RE
.RS +4
.TP
4.
Finally, it restores the signal process mask to how it was when \fBgl_handle_signal()\fR was called.
.RE
.sp
.LP
Note that the process is suspended or terminated using the original signal that was caught, rather than using the uncatchable \fBSIGSTOP\fR and \fBSIGKILL\fR signals. This is important, because when a process is suspended or terminated, the parent of the process may wish to use the status value returned by the wait system call to figure out which signal was responsible. In particular, most shells use this information to print a corresponding message to the terminal. Users would be rightly confused if when their process received a \fBSIGPIPE\fR signal, the program responded by sending itself a \fBSIGKILL\fR signal, and the shell then printed out the provocative statement, "Killed!".
.SS "Interrupting The Event Loop"
.sp
.LP
If a signal is caught and handled when the application's event loop is waiting in \fBselect()\fR or \fBpoll()\fR, these functions will be aborted with \fBerrno\fR set to \fBEINTR\fR. When this happens the event loop should call \fBgl_pending_io()\fR before calling \fBselect()\fR or \fBpoll()\fR again. It should then arrange for \fBselect()\fR or \fBpoll()\fR to wait for the type of I/O that \fBgl_pending_io()\fR reports. This is necessary because any signal handler that calls \fBgl_handle_signal()\fR will frequently change the type of I/O that \fBgl_get_line()\fR is waiting for.
.sp
.LP
If a signal arrives between the statements that configure the arguments of \fBselect()\fR or \fBpoll()\fR and the calls to these functions, the signal will not be seen by these functions, which will then not be aborted. If these functions are waiting for keyboard input from the user when the signal is received, and the signal handler arranges to redraw the input line to accommodate a terminal resize or the resumption of the process. This redisplay will be delayed until the user presses the next key. Apart from puzzling the user, this clearly is not a serious problem. However there is a way, albeit complicated, to completely avoid this race condition. The following steps illustrate this.
.RS +4
.TP
1.
Block all of the signals that \fBgl_get_line()\fR catches, by passing the signal set returned by \fBgl_list_signals()\fR to \fBsigprocmask\fR(2).
.RE
.RS +4
.TP
2.
Call \fBgl_pending_io()\fR and set up the arguments of \fBselect()\fR or \fBpoll()\fR accordingly.
.RE
.RS +4
.TP
3.
Call \fBsigsetjmp\fR(3C) with a non-zero \fIsavemask\fR argument.
.RE
.RS +4
.TP
4.
Initially this \fBsigsetjmp()\fR statement will return zero, indicating that control is not resuming there after a matching call to \fBsiglongjmp\fR(3C).
.RE
.RS +4
.TP
5.
Replace all of the handlers of the signals that \fBgl_get_line()\fR is configured to catch, with a signal handler that first records the number of the signal that was caught, in a file-scope variable, then calls \fBsiglongjmp()\fR with a non-zero \fIval\fR argument, to return execution to the above \fBsigsetjmp()\fR statement. Registering these signal handlers can conveniently be done using the \fBgl_tty_signals()\fR function.
.RE
.RS +4
.TP
6.
Set the file-scope variable that the above signal handler uses to record any signal that is caught to -1, so that we can check whether a signal was caught by seeing if it contains a valid signal number.
.RE
.RS +4
.TP
7.
Now unblock the signals that were blocked in step 1. Any signal that was received by the process in between step 1 and now will now be delivered, and trigger our signal handler, as will any signal that is received until we block these signals again.
.RE
.RS +4
.TP
8.
Now call \fBselect()\fR or \fBpoll()\fR.
.RE
.RS +4
.TP
9.
When select returns, again block the signals that were unblocked in step 7.
.sp
If a signal is arrived any time during the above steps, our signal handler will be triggered and cause control to return to the \fBsigsetjmp()\fR statement, where this time, \fBsigsetjmp()\fR will return non-zero, indicating that a signal was caught. When this happens we simply skip the above block of statements, and continue with the following statements, which are executed regardless of whether or not a signal is caught. Note that when \fBsigsetjmp()\fR returns, regardless of why it returned, the process signal mask is returned to how it was when \fBsigsetjmp()\fR was called. Thus the following statements are always executed with all of our signals blocked.
.RE
.RS +4
.TP
10.
Reinstate the signal handlers that were displaced in step 5.
.RE
.RS +4
.TP
11.
Check wether a signal was caught, by checking the file-scope variable that the signal handler records signal numbers in.
.RE
.RS +4
.TP
12.
If a signal was caught, send this signal to the application again and unblock only this signal so that it invokes the signal handler which was just reinstated in step 10.
.RE
.RS +4
.TP
13.
Unblock all of the signals that were blocked in step 7.
.RE
.SS "Signals Caught By \fBgl_get_line()\fR"
.sp
.LP
Since the application is expected to handle signals in non-blocking server mode, \fBgl_get_line()\fR does not attempt to duplicate this when it is being called. If one of the signals that it is configured to catch is sent to the application while \fBgl_get_line()\fR is being called, \fBgl_get_line()\fR reinstates the caller's signal handlers, then immediately before returning, re-sends the signal to the process to let the application's signal handler handle it. If the process is not terminated by this signal, \fBgl_get_line()\fR returns \fINULL\fR, and a following call to \fBgl_return_status()\fR returns the enumerated value \fBGLR_SIGNAL\fR.
.SS "Aborting Line Input"
.sp
.LP
Often, rather than letting it terminate the process, applications respond to the \fBSIGINT\fR user-interrupt signal by aborting the current input line. This can be accomplished in non-blocking server-I/O mode by not calling \fBgl_handle_signal()\fR when this signal is caught, but by calling instead the \fBgl_abandon_line()\fR function. This function arranges that when \fBgl_get_line()\fR is next called, it first flushes any pending output to the terminal, discardes the current input line, outputs a new prompt on the next line, and finally starts accepting input of a new input line from the user.
.SS "Signal Safe Functions"
.sp
.LP
Provided that certain rules are followed, the \fBgl_normal_io()\fR, \fBgl_raw_io()\fR, \fBgl_handle_signal()\fR, and \fBgl_abandon_line()\fR functions can be written to be safely callable from signal handlers. Other functions in this library should not be called from signal handlers. For this to be true, all signal handlers that call these functions must be registered in such a way that only one instance of any one of them can be running at one time. The way to do this is to use the POSIX \fBsigaction()\fR function to register all signal handlers, and when doing this, use the \fIsa_mask\fR member of the corresponding \fBsigaction\fR structure to indicate that all of the signals whose handlers invoke the above functions should be blocked when the current signal is being handled. This prevents two signal handlers from operating on a \fBGetLine\fR object at the same time.
.sp
.LP
To prevent signal handlers from accessing a \fBGetLine\fR object while \fBgl_get_line()\fR or any of its associated public functions are operating on it, all public functions associated with \fBgl_get_line()\fR, including \fBgl_get_line()\fR itself, temporarily block the delivery of signals when they are accessing \fBGetLine\fR objects. Beware that the only signals that they block are the signals that \fBgl_get_line()\fR is currently configured to catch, so be sure that if you call any of the above functions from signal handlers, that the signals that these handlers are assigned to are configured to be caught by \fBgl_get_line()\fR. See \fBgl_trap_signal\fR(3TECLA).
.SS "Using Timeouts To Poll"
.sp
.LP
If instead of using \fBselect()\fR or \fBpoll()\fR to wait for I/O your application needs only to get out of \fBgl_get_line()\fR periodically to briefly do something else before returning to accept input from the user, use the \fBgl_inactivity_timeout\fR(3TECLA) function in non-blocking server mode to specify that a callback function that returns \fBGLTO_CONTINUE\fR should be called whenever \fBgl_get_line()\fR has been waiting for I/O for more than a specified amount of time. When this callback is triggered, \fBgl_get_line()\fR will return \fINULL\fR and a following call to \fBgl_return_status()\fR will return \fBGLR_BLOCKED\fR.
.sp
.LP
The \fBgl_get_line()\fR function will not return until the user has not typed a key for the specified interval, so if the interval is long and the user keeps typing, \fBgl_get_line()\fR might not return for a while. There is no guarantee that it will return in the time specified.
.SH ATTRIBUTES
.sp
.LP
See \fBattributes\fR(5) for descriptions of the following attributes:
.sp

.sp
.TS
tab() box;
cw(2.75i) |cw(2.75i) 
lw(2.75i) |lw(2.75i) 
.
ATTRIBUTE TYPEATTRIBUTE VALUE
_
Interface StabilityCommitted
_
MT-LevelMT-Safe
.TE

.SH SEE ALSO
.sp
.LP
\fBcpl_complete_word\fR(3TECLA), \fBef_expand_file\fR(3TECLA), \fBgl_get_line\fR(3TECLA), \fBlibtecla\fR(3LIB), \fBpca_lookup_file\fR(3TECLA), \fBattributes\fR(5), \fBtecla\fR(5)