1 '\" te
   2 .\" Copyright (c) 2000, 2001, 2002, 2003, 2004 by Martin C. Shepherd.
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  28 .TH GL_IO_MODE 3TECLA "January 18, 2020"
  29 .SH NAME
  30 gl_io_mode, gl_raw_io, gl_normal_io, gl_tty_signals, gl_abandon_line,
  31 gl_handle_signal, gl_pending_io \- use \fBgl_get_line()\fR from an external
  32 event loop
  34 .nf
  35 cc [ \fIflag\fR\&.\|.\|. ] \fIfile\fR\&.\|.\|. \fB-ltecla\fR [ \fIlibrary\fR\&.\|.\|. ]
  36 #include <libtecla.h>
  38 \fBint\fR \fBgl_io_mode\fR(\fBGetLine *\fR\fIgl\fR, \fBGlIOMode\fR \fImode\fR);
  39 .fi
  41 .LP
  42 .nf
  43 \fBint\fR \fBgl_raw_io\fR(\fBGetLine *\fR\fIgl\fR);
  44 .fi
  46 .LP
  47 .nf
  48 \fBint\fR \fBgl_normal_io\fR(\fBGetLine *\fR\fIgl\fR);
  49 .fi
  51 .LP
  52 .nf
  53 \fBint\fR \fBgl_tty_signals\fR(\fBvoid (*\fR\fIterm_handler\fR)(int), \fBvoid (*\fR\fIsusp_handler\fR)(int),
  54      \fBvoid (*\fR\fIcont_handler\fR)(int), \fBvoid (*\fR\fIsize_handler\fR)(int));
  55 .fi
  57 .LP
  58 .nf
  59 \fBvoid\fR \fBgl_abandon_line\fR(\fBGetLine *\fR\fIgl\fR);
  60 .fi
  62 .LP
  63 .nf
  64 \fBvoid\fR \fBgl_handle_signal\fR(\fBint\fR \fIsigno\fR, \fBGetLine *\fR\fIgl\fR, \fBint\fR \fIngl\fR);
  65 .fi
  67 .LP
  68 .nf
  69 \fBGlPendingIO\fR \fBgl_pending_io\fR(\fBGetLine *\fR\fIgl\fR);
  70 .fi
  73 The \fBgl_get_line\fR(3TECLA) function supports two different I/O modes. These
  74 are selected by calling the \fBgl_io_mode()\fR function. The \fImode\fR
  75 argument of \fBgl_io_mode()\fR specifies the new I/O mode and must be one of
  76 the following.
  77 .sp
  78 .ne 2
  79 .na
  81 .ad
  82 .RS 18n
  83 Select the normal blocking-I/O mode. In this mode \fBgl_get_line()\fR does not
  84 return until either an error occurs of the user finishes entering a new line.
  85 .RE
  87 .sp
  88 .ne 2
  89 .na
  91 .ad
  92 .RS 18n
  93 Select non-blocking server I/O mode. In this mode, since non-blocking terminal
  94 I/O is used, the entry of each new input line typically requires many calls to
  95 \fBgl_get_line()\fR from an external I/O-driven event loop.
  96 .RE
  98 .sp
  99 .LP
 100 Newly created GetLine objects start in normal I/O mode, so to switch to
 101 non-blocking server mode requires an initial call to \fBgl_io_mode()\fR.
 102 .SS "Server I/O Mode"
 103 In non-blocking server I/O mode, the application is required to have an event
 104 loop that calls \fBgl_get_line()\fR whenever the terminal file descriptor can
 105 perform the type of I/O that \fBgl_get_line()\fR is waiting for. To determine
 106 which type of I/O \fBgl_get_line()\fR is waiting for, the application calls the
 107 \fBgl_pending_io()\fR function.  The return value is one of the following two
 108 enumerated values.
 109 .sp
 110 .ne 2
 111 .na
 112 \fB\fBGLP_READ\fR\fR
 113 .ad
 114 .RS 13n
 115 \fBgl_get_line()\fR is waiting to write a character to the terminal.
 116 .RE
 118 .sp
 119 .ne 2
 120 .na
 121 \fB\fBGLP_WRITE\fR\fR
 122 .ad
 123 .RS 13n
 124 \fBgl_get_line()\fR is waiting to read a character from the keyboard.
 125 .RE
 127 .sp
 128 .LP
 129 If the application is using either the \fBselect\fR(3C) or \fBpoll\fR(2)
 130 function to watch for I/O on a group of file descriptors, then it should call
 131 the \fBgl_pending_io()\fR function before each call to these functions to
 132 determine which direction of I/O it should tell them to watch for, and
 133 configure their arguments accordingly. In the case of the \fBselect()\fR
 134 function, this means using the \fBFD_SET()\fR macro to add the terminal file
 135 descriptor either to the set of file descriptors to be watched for readability
 136 or the set to be watched for writability.
 137 .sp
 138 .LP
 139 As in normal I/O mode, the return value of \fBgl_get_line()\fR is either a
 140 pointer to a completed input line or \fINULL\fR. However, whereas in normal I/O
 141 mode a \fINULL\fR return value always means that an error occurred, in
 142 non-blocking server mode, \fINULL\fR is also returned when \fBgl_get_line()\fR
 143 cannot read or write to the terminal without blocking. Thus in non-blocking
 144 server mode, in order to determine when a \fINULL\fR return value signifies
 145 that an error occurred or not, it is necessary to call the
 146 \fBgl_return_status()\fR function. If this function returns the enumerated
 147 value \fBGLR_BLOCKED\fR, \fBgl_get_line()\fR is waiting for I/O and no error
 148 has occurred.
 149 .sp
 150 .LP
 151 When \fBgl_get_line()\fR returns \fINULL\fR and \fBgl_return_status()\fR
 152 indicates that this is due to blocked terminal I/O, the application should call
 153 \fBgl_get_line()\fR again when the type of I/O reported by
 154 \fBgl_pending_io()\fR becomes possible. The \fIprompt\fR, \fIstart_line\fR and
 155 \fIstart_pos\fR arguments of \fBgl_get_line()\fR will be ignored on these
 156 calls. If you need to change the prompt of the line that is currently being
 157 edited, you can call the \fBgl_replace_prompt\fR(3TECLA) function between calls
 158 to \fBgl_get_line()\fR.
 159 .SS "Giving Up The Terminal"
 160 A complication that is unique to non-blocking server mode is that it requires
 161 that the terminal be left in raw mode between calls to \fBgl_get_line()\fR. If
 162 this were not the case, the external event loop would not be able to detect
 163 individual key-presses, and the basic line editing implemented by the terminal
 164 driver would clash with the editing provided by \fBgl_get_line()\fR. When the
 165 terminal needs to be used for purposes other than entering a new input line
 166 with \fBgl_get_line()\fR, it needs to be restored to a usable state. In
 167 particular, whenever the process is suspended or terminated, the terminal must
 168 be returned to a normal state. If this is not done, then depending on the
 169 characteristics of the shell that was used to invoke the program, the user
 170 could end up with a hung terminal. To this end, the \fBgl_normal_io()\fR
 171 function is provided for switching the terminal back to the state that it was
 172 in when raw mode was last established.
 173 .sp
 174 .LP
 175 The \fBgl_normal_io()\fR function first flushes any pending output to the
 176 terminal, then moves the cursor to the start of the terminal line which follows
 177 the end of the incompletely entered input line. At this point it is safe to
 178 suspend or terminate the process, and it is safe for the application to read
 179 and write to the terminal. To resume entry of the input line, the application
 180 should call the \fBgl_raw_io()\fR function.
 181 .sp
 182 .LP
 183 The \fBgl_normal_io()\fR function starts a new line, redisplays the partially
 184 completed input line (if any), restores the cursor position within this line to
 185 where it was when \fBgl_normal_io()\fR was called, then switches back to raw,
 186 non-blocking terminal mode ready to continue entry of the input line when
 187 \fBgl_get_line()\fR is next called.
 188 .sp
 189 .LP
 190 Note that in non-blocking server mode, if \fBgl_get_line()\fR is called after a
 191 call to \fBgl_normal_io()\fR, without an intervening call to \fBgl_raw_io()\fR,
 192 \fBgl_get_line()\fR will call \fBgl_raw_mode()\fR itself, and the terminal will
 193 remain in this mode when \fBgl_get_line()\fR returns.
 194 .SS "Signal Handling"
 195 In the previous section it was pointed out that in non-blocking server mode,
 196 the terminal must be restored to a sane state whenever a signal is received
 197 that either suspends or terminates the process. In normal I/O mode, this is
 198 done for you by \fBgl_get_line()\fR, but in non-blocking server mode, since the
 199 terminal is left in raw mode between calls to \fBgl_get_line()\fR, this signal
 200 handling has to be done by the application. Since there are many signals that
 201 can suspend or terminate a process, as well as other signals that are important
 202 to \fBgl_get_line()\fR, such as the \fBSIGWINCH\fR signal, which tells it when
 203 the terminal size has changed, the \fBgl_tty_signals()\fR function is provided
 204 for installing signal handlers for all pertinent signals.
 205 .sp
 206 .LP
 207 The \fBgl_tty_signals()\fR function uses \fBgl_get_line()\fR's internal list of
 208 signals to assign specified signal handlers to groups of signals. The arguments
 209 of this function are as follows.
 210 .sp
 211 .ne 2
 212 .na
 213 \fB\fIterm_handler\fR\fR
 214 .ad
 215 .RS 16n
 216 This is the signal handler that is used to trap signals that by default
 217 terminate any process that receives them (for example, \fBSIGINT\fR or
 218 \fBSIGTERM\fR).
 219 .RE
 221 .sp
 222 .ne 2
 223 .na
 224 \fB\fIsusp_handler\fR\fR
 225 .ad
 226 .RS 16n
 227 This is the signal handler that is used to trap signals that by default suspend
 228 any process that receives them, (for example, \fBSIGTSTP\fR or \fBSIGTTOU\fR).
 229 .RE
 231 .sp
 232 .ne 2
 233 .na
 234 \fB\fIcont_handler\fR\fR
 235 .ad
 236 .RS 16n
 237 This is the signal handler that is used to trap signals that are usually sent
 238 when a process resumes after being suspended (usually \fBSIGCONT\fR). Beware
 239 that there is nothing to stop a user from sending one of these signals at other
 240 times.
 241 .RE
 243 .sp
 244 .ne 2
 245 .na
 246 \fB\fIsize_handler\fR\fR
 247 .ad
 248 .RS 16n
 249 This signal handler is used to trap signals that are sent to processes when
 250 their controlling terminals are resized by the user (for example,
 251 \fBSIGWINCH\fR).
 252 .RE
 254 .sp
 255 .LP
 256 These arguments can all be the same, if so desired, and \fBSIG_IGN\fR (ignore
 257 this signal) or \fBSIG_DFL\fR (use the system-provided default signal handler)
 258 can be specified instead of a function where pertinent. In particular, it is
 259 rarely useful to trap \fBSIGCONT\fR, so the \fIcont_handler\fR argument will
 260 usually be \fBSIG_DFL\fR or \fBSIG_IGN\fR.
 261 .sp
 262 .LP
 263 The \fBgl_tty_signals()\fR function uses the POSIX \fBsigaction\fR(2) function
 264 to install these signal handlers, and it is careful to use the \fIsa_mask\fR
 265 member of each \fBsigaction\fR structure to ensure that only one of these
 266 signals is ever delivered at a time. This guards against different instances of
 267 these signal handlers from simultaneously trying to write to common global
 268 data, such as a shared \fBsigsetjmp\fR(3C) buffer or a signal-received flag.
 269 The signal handlers installed by this function should call the
 270 \fBgl_handle_signal()\fR.
 271 .sp
 272 .LP
 273 The \fIsigno\fR argument tells this function which signal it is being asked to
 274 respond to, and the \fIgl\fR argument should be a pointer to the first element
 275 of an array of \fIngl\fR \fBGetLine\fR objects. If your application has only
 276 one of these objects, pass its pointer as the \fIgl\fR argument and specify
 277 \fIngl\fR as 1.
 278 .sp
 279 .LP
 280 Depending on the signal that is being handled, this function does different
 281 things.
 282 .SS "Process termination signals"
 283 If the signal that was caught is one of those that by default terminates any
 284 process that receives it, then \fBgl_handle_signal()\fR does the following
 285 steps.
 286 .RS +4
 287 .TP
 288 1.
 289 First it blocks the delivery of all signals that can be blocked (ie.
 290 \fBSIGKILL\fR and \fBSIGSTOP\fR cannot be blocked).
 291 .RE
 292 .RS +4
 293 .TP
 294 2.
 295 Next it calls \fBgl_normal_io()\fR for each of the ngl GetLine objects. Note
 296 that this does nothing to any of the GetLine objects that are not currently in
 297 raw mode.
 298 .RE
 299 .RS +4
 300 .TP
 301 3.
 302 Next it sets the signal handler of the signal to its default,
 303 process-termination disposition.
 304 .RE
 305 .RS +4
 306 .TP
 307 4.
 308 Next it re-sends the process the signal that was caught.
 309 .RE
 310 .RS +4
 311 .TP
 312 5.
 313 Finally it unblocks delivery of this signal, which results in the process
 314 being terminated.
 315 .RE
 316 .SS "Process suspension signals"
 317 If the default disposition of the signal is to suspend the process, the same
 318 steps are executed as for process termination signals, except that when the
 319 process is later resumed, \fBgl_handle_signal()\fR continues, and does the
 320 following steps.
 321 .RS +4
 322 .TP
 323 1.
 324 It re-blocks delivery of the signal.
 325 .RE
 326 .RS +4
 327 .TP
 328 2.
 329 It reinstates the signal handler of the signal to the one that was displaced
 330 when its default disposition was substituted.
 331 .RE
 332 .RS +4
 333 .TP
 334 3.
 335 For any of the GetLine objects that were in raw mode when
 336 \fBgl_handle_signal()\fR was called, \fBgl_handle_signal()\fR then calls
 337 \fBgl_raw_io()\fR, to resume entry of the input lines on those terminals.
 338 .RE
 339 .RS +4
 340 .TP
 341 4.
 342 Finally, it restores the signal process mask to how it was when
 343 \fBgl_handle_signal()\fR was called.
 344 .RE
 345 .sp
 346 .LP
 347 Note that the process is suspended or terminated using the original signal that
 348 was caught, rather than using the uncatchable \fBSIGSTOP\fR and \fBSIGKILL\fR
 349 signals. This is important, because when a process is suspended or terminated,
 350 the parent of the process may wish to use the status value returned by the wait
 351 system call to figure out which signal was responsible. In particular, most
 352 shells use this information to print a corresponding message to the terminal.
 353 Users would be rightly confused if when their process received a \fBSIGPIPE\fR
 354 signal, the program responded by sending itself a \fBSIGKILL\fR signal, and the
 355 shell then printed out the provocative statement, "Killed!".
 356 .SS "Interrupting The Event Loop"
 357 If a signal is caught and handled when the application's event loop is waiting
 358 in \fBselect()\fR or \fBpoll()\fR, these functions will be aborted with
 359 \fBerrno\fR set to \fBEINTR\fR. When this happens the event loop should call
 360 \fBgl_pending_io()\fR before calling \fBselect()\fR or \fBpoll()\fR again. It
 361 should then arrange for \fBselect()\fR or \fBpoll()\fR to wait for the type of
 362 I/O that \fBgl_pending_io()\fR reports. This is necessary because any signal
 363 handler that calls \fBgl_handle_signal()\fR will frequently change the type of
 364 I/O that \fBgl_get_line()\fR is waiting for.
 365 .sp
 366 .LP
 367 If a signal arrives between the statements that configure the arguments of
 368 \fBselect()\fR or \fBpoll()\fR and the calls to these functions, the signal
 369 will not be seen by these functions, which will then not be aborted. If these
 370 functions are waiting for keyboard input from the user when the signal is
 371 received, and the signal handler arranges to redraw the input line to
 372 accommodate a terminal resize or the resumption of the process. This redisplay
 373 will be delayed until the user presses the next key. Apart from puzzling the
 374 user, this clearly is not a serious problem. However there is a way, albeit
 375 complicated, to completely avoid this race condition. The following steps
 376 illustrate this.
 377 .RS +4
 378 .TP
 379 1.
 380 Block all of the signals that \fBgl_get_line()\fR catches, by passing the
 381 signal set returned by \fBgl_list_signals()\fR to \fBsigprocmask\fR(2).
 382 .RE
 383 .RS +4
 384 .TP
 385 2.
 386 Call \fBgl_pending_io()\fR and set up the arguments of \fBselect()\fR or
 387 \fBpoll()\fR accordingly.
 388 .RE
 389 .RS +4
 390 .TP
 391 3.
 392 Call \fBsigsetjmp\fR(3C) with a non-zero \fIsavemask\fR argument.
 393 .RE
 394 .RS +4
 395 .TP
 396 4.
 397 Initially this \fBsigsetjmp()\fR statement will return zero, indicating that
 398 control is not resuming there after a matching call to \fBsiglongjmp\fR(3C).
 399 .RE
 400 .RS +4
 401 .TP
 402 5.
 403 Replace all of the handlers of the signals that \fBgl_get_line()\fR is
 404 configured to catch, with a signal handler that first records the number of the
 405 signal that was caught, in a file-scope variable, then calls \fBsiglongjmp()\fR
 406 with a non-zero \fIval\fR argument, to return execution to the above
 407 \fBsigsetjmp()\fR statement. Registering these signal handlers can conveniently
 408 be done using the \fBgl_tty_signals()\fR function.
 409 .RE
 410 .RS +4
 411 .TP
 412 6.
 413 Set the file-scope variable that the above signal handler uses to record any
 414 signal that is caught to -1, so that we can check whether a signal was caught
 415 by seeing if it contains a valid signal number.
 416 .RE
 417 .RS +4
 418 .TP
 419 7.
 420 Now unblock the signals that were blocked in step 1. Any signal that was
 421 received by the process in between step 1 and now will now be delivered, and
 422 trigger our signal handler, as will any signal that is received until we block
 423 these signals again.
 424 .RE
 425 .RS +4
 426 .TP
 427 8.
 428 Now call \fBselect()\fR or \fBpoll()\fR.
 429 .RE
 430 .RS +4
 431 .TP
 432 9.
 433 When select returns, again block the signals that were unblocked in step 7.
 434 .sp
 435 If a signal is arrived any time during the above steps, our signal handler will
 436 be triggered and cause control to return to the \fBsigsetjmp()\fR statement,
 437 where this time, \fBsigsetjmp()\fR will return non-zero, indicating that a
 438 signal was caught. When this happens we simply skip the above block of
 439 statements, and continue with the following statements, which are executed
 440 regardless of whether or not a signal is caught. Note that when
 441 \fBsigsetjmp()\fR returns, regardless of why it returned, the process signal
 442 mask is returned to how it was when \fBsigsetjmp()\fR was called. Thus the
 443 following statements are always executed with all of our signals blocked.
 444 .RE
 445 .RS +4
 446 .TP
 447 10.
 448 Reinstate the signal handlers that were displaced in step 5.
 449 .RE
 450 .RS +4
 451 .TP
 452 11.
 453 Check whether a signal was caught, by checking the file-scope variable that
 454 the signal handler records signal numbers in.
 455 .RE
 456 .RS +4
 457 .TP
 458 12.
 459 If a signal was caught, send this signal to the application again and
 460 unblock only this signal so that it invokes the signal handler which was just
 461 reinstated in step 10.
 462 .RE
 463 .RS +4
 464 .TP
 465 13.
 466 Unblock all of the signals that were blocked in step 7.
 467 .RE
 468 .SS "Signals Caught By \fBgl_get_line()\fR"
 469 Since the application is expected to handle signals in non-blocking server
 470 mode, \fBgl_get_line()\fR does not attempt to duplicate this when it is being
 471 called. If one of the signals that it is configured to catch is sent to the
 472 application while \fBgl_get_line()\fR is being called, \fBgl_get_line()\fR
 473 reinstates the caller's signal handlers, then immediately before returning,
 474 re-sends the signal to the process to let the application's signal handler
 475 handle it. If the process is not terminated by this signal, \fBgl_get_line()\fR
 476 returns \fINULL\fR, and a following call to \fBgl_return_status()\fR returns
 477 the enumerated value \fBGLR_SIGNAL\fR.
 478 .SS "Aborting Line Input"
 479 Often, rather than letting it terminate the process, applications respond to
 480 the \fBSIGINT\fR user-interrupt signal by aborting the current input line. This
 481 can be accomplished in non-blocking server-I/O mode by not calling
 482 \fBgl_handle_signal()\fR when this signal is caught, but by calling instead the
 483 \fBgl_abandon_line()\fR function. This function arranges that when
 484 \fBgl_get_line()\fR is next called, it first flushes any pending output to the
 485 terminal, discards the current input line, outputs a new prompt on the next
 486 line, and finally starts accepting input of a new input line from the user.
 487 .SS "Signal Safe Functions"
 488 Provided that certain rules are followed, the \fBgl_normal_io()\fR,
 489 \fBgl_raw_io()\fR, \fBgl_handle_signal()\fR, and \fBgl_abandon_line()\fR
 490 functions can be written to be safely callable from signal handlers. Other
 491 functions in this library should not be called from signal handlers. For this
 492 to be true, all signal handlers that call these functions must be registered in
 493 such a way that only one instance of any one of them can be running at one
 494 time. The way to do this is to use the POSIX \fBsigaction()\fR function to
 495 register all signal handlers, and when doing this, use the \fIsa_mask\fR member
 496 of the corresponding \fBsigaction\fR structure to indicate that all of the
 497 signals whose handlers invoke the above functions should be blocked when the
 498 current signal is being handled. This prevents two signal handlers from
 499 operating on a \fBGetLine\fR object at the same time.
 500 .sp
 501 .LP
 502 To prevent signal handlers from accessing a \fBGetLine\fR object while
 503 \fBgl_get_line()\fR or any of its associated public functions are operating on
 504 it, all public functions associated with \fBgl_get_line()\fR, including
 505 \fBgl_get_line()\fR itself, temporarily block the delivery of signals when they
 506 are accessing \fBGetLine\fR objects. Beware that the only signals that they
 507 block are the signals that \fBgl_get_line()\fR is currently configured to
 508 catch, so be sure that if you call any of the above functions from signal
 509 handlers, that the signals that these handlers are assigned to are configured
 510 to be caught by \fBgl_get_line()\fR. See \fBgl_trap_signal\fR(3TECLA).
 511 .SS "Using Timeouts To Poll"
 512 If instead of using \fBselect()\fR or \fBpoll()\fR to wait for I/O your
 513 application needs only to get out of \fBgl_get_line()\fR periodically to
 514 briefly do something else before returning to accept input from the user, use
 515 the \fBgl_inactivity_timeout\fR(3TECLA) function in non-blocking server mode to
 516 specify that a callback function that returns \fBGLTO_CONTINUE\fR should be
 517 called whenever \fBgl_get_line()\fR has been waiting for I/O for more than a
 518 specified amount of time. When this callback is triggered, \fBgl_get_line()\fR
 519 will return \fINULL\fR and a following call to \fBgl_return_status()\fR will
 520 return \fBGLR_BLOCKED\fR.
 521 .sp
 522 .LP
 523 The \fBgl_get_line()\fR function will not return until the user has not typed a
 524 key for the specified interval, so if the interval is long and the user keeps
 525 typing, \fBgl_get_line()\fR might not return for a while. There is no guarantee
 526 that it will return in the time specified.
 528 See \fBattributes\fR(5) for descriptions of the following attributes:
 529 .sp
 531 .sp
 532 .TS
 533 box;
 534 c | c
 535 l | l .
 537 _
 538 Interface Stability     Evolving
 539 _
 540 MT-Level        MT-Safe
 541 .TE
 544 \fBcpl_complete_word\fR(3TECLA), \fBef_expand_file\fR(3TECLA),
 545 \fBgl_get_line\fR(3TECLA), \fBlibtecla\fR(3LIB), \fBpca_lookup_file\fR(3TECLA),
 546 \fBattributes\fR(5), \fBtecla\fR(5)