coproc(1fmli)

coproc: cocreate, cosend, cocheck, coreceive, codestroy -- communicate with a process

Synopsis

   cocreate [-r rpath] [-w wpath] [-i id] [-R refname] [-s send_string]
   	[-e expect_string] command
   
   cosend [-n] proc_id string
   

   cocheck proc_id
   

   coreceive proc_id
   

   codestroy [-R refname] proc_id [string]

Description

These co-processing functions provide a flexible means of interaction between FMLI and an independent process; especially, they enable FMLI to be responsive to asynchronous activity.

The cocreate function starts command as a co-process and initializes communications by setting up pipes between FMLI and the standard input and standard output of command. The argument command must be an executable and its arguments (if any). This means that command expects strings on its input (supplied by cosend) and sends information on its output that can be handled in various ways by FMLI. The following options can be used with cocreate.

-r rpath: If -r is specified, rpath is the pathname from which FMLI reads information. This option is usually used to set up communication with processes that naturally write to a certain path. If -r is not specified, cocreate will choose a unique path in /var/tmp.
-w wpath: If -w is specified, wpath is the pathname to which cosend writes information. This option is usually used so that one process can talk to many different FMLI processes through the same pipe. If -w is not specified, cocreate will choose a unique path in /var/tmp.
-i id: If -i is specified, id is an alternative name for the co-process initialized by this cocreate. If -i is not specified, id defaults to command. The argument id can later be used with the other co-processing functions rather than command. This option is typically used, since it facilitates the creation of two or more co-processes generated from the same command. (For example, cocreate -i ID1 program args and cocreate -i ID2 program different_args.)
-R refname: If -R is specified, refname is a local name for the co-process. Since the cocreate function can be issued more than once, a refname is useful when the same co-process is referenced a second or subsequent time. With the -R option, if the co-process already exists a new one will not be created: the same pipes will be shared. Then, refname can be used as an argument to the -R option to codestroy when you want to end a particular connection to a co-process and leave other connections undisturbed. (The co-process is only killed after codestroy -R has been called as many times as cocreate -R was called.)
-s send_string: The -s option specifies send_string as a string that will be appended to all output sent to the co-process using cosend. This option allows a co-process to know when input from FMLI has completed. The default send_string is a newline if -s is not specified.
-e expect_string: The -e option specifies expect_string as a string that identifies the end of all output returned by the co-process. (Note: expect_string need only be the initial part of a line, and there must be a newline at the end of the co-process output). This option allows FMLI to know when output from the co-process has completed. The default expect_string is a newline if -e is not specified.

The cosend function sends string to the co-process identified by proc_id via the pipe set up by cocreate (optionally wpath), where proc_id can be either the command or id specified in cocreate. By default, cosend blocks, waiting for a response from the co-process. Also by default, FMLI does not send a send_string and does not expect an expect_string (except a newline). That is, it reads only one line of output from the co-process. If -e expect_string was not defined when the pipe was created, then the output of the co-process is any single string followed by a newline: any other lines of output remain on the pipe. If the -e option was specified when the pipe was created, cosend reads lines from the pipe until it reads a line starting with expect_string. All lines except the line starting with expect_string become the output of cosend. The following option can be used with cosend:

-n: If the -n option is specified, cosend will not wait for a response from the co-process. It simply returns, providing no output. If the -n option is not used, a co-process that does not answer will cause FMLI to permanently hang, waiting for input from the co-process.

The cocheck function determines if input is available from the process identified by proc_id, where proc_id can be either the command or id specified in cocreate. It returns a Boolean value, which makes cocheck useful in if statements and in other backquoted expressions in Boolean descriptors. cocheck receives no input from the co-process; it simply indicates if input is available from the co-process. You must use coreceive to actually accept the input. The cocheck function can be called from a reread descriptor to force a frame to update when new data is available. This is useful when the default value of a field in a form includes coreceive.

The coreceive function is used to read input from the co-process identified by proc_id, where proc_id can be either the command or id specified in cocreate. It should only be used when it has been determined, using cocheck, that input is actually available. If the -e option was used when the co-process was created, coreceive will continue to return lines of input until expect_string is read. At this point, coreceive will terminate. The output of coreceive is all the lines that were read excluding the line starting with expect_string. If the -e option was not used in the cocreate, each invocation of coreceive will return exactly one line from the co-process. If no input is available when coreceive is invoked, it will simply terminate without producing output.

The codestroy function terminates the read/write pipes to proc_id, where proc_id can be either the command or id specified in cocreate. It generates a SIGPIPE signal to the (child) co-process. This kills the co-process, unless the co-process ignores the SIGPIPE signal. If the co-process ignores the SIGPIPE, it will not die, even after the FMLI process terminates (the parent process id of the co-process will be 1).

The optional argument string is sent to the co-process before the co-process dies. If string is not supplied, a NULL string is passed, followed by the normal send_string (newline by default). That is, codestroy will call cosend proc_id string: this implies that codestroy will write any output generated by the co-process to stdout. For example, if an interactive co-process is written to expect a ``quit'' string when the communication is over, the close descriptor could be defined;

   close=`codestroy ID 'quit' | message`

and any output generated by the co-process when the string quit is sent to it via codestroy (using cosend) would be redirected to the message line.

The codestroy function should usually be given the -R option, since you may have more than one process with the same name, and you do not want to kill the wrong one. codestroy keeps track of the number of refnames you have assigned to a process with cocreate, and when the last instance is killed, it kills the process (id) for you. codestroy is typically called as part of a close descriptor because close is evaluated when a frame is closed. This is important because the co-process will continue to run if codestroy is not issued.

When writing programs to use as co-processes, the following tips may be useful. If the co-process program is written in C language, be sure to flush output after writing to the pipe. [Currently, awk(1) and sed(1) cannot be used in a co-process program because they do not flush after lines of output.] Shell scripts are well-mannered, but slow. C language is recommended. If possible, use the default send_string, rpath and wpath. In most cases, expect_string will have to be specified. This, of course, depends on the co-process.

In the case where asynchronous communication from a co-process is desired, a co-process program should use vsig to force strings into the pipe and then signal FMLI that output from the co-process is available. This causes the reread descriptor of all frames to be evaluated immediately.

Examples

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init=`cocreate -i BIGPROCESS initialize`
close=`codestroy BIGPROCESS`
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reread=`cocheck BIGPROCESS`

name=`cosend -n BIGPROCESS field1`
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name="Receive field"
inactive=TRUE
value=`coreceive BIGPROCESS`

Notices

Co-processes for trusted FMLI applications should use named pipes created by the application with the appropriate permissions; the default pipes created by FMLI are readable and writable by everyone. Handshaking can also be used to enhance security.

If cosend is used without the -n option, a co-process that does not answer will cause FMLI to permanently hang.

The use of non-alphabetic characters in input and output strings to a co-process should be avoided because they may not get transferred correctly.