Multi process programming in C Post: 302894454

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Top Forums Programming Multi process programming in C Post 302894454 by alister on Tuesday 25th of March 2014 02:08:48 PM

03-25-2014

Registered User

As before, I did not compile or test the code. There may be other issues.

content is allocated and freed but never used.

fd will never contain an error code; it's always a pointer value. When that pointer is NULL, it indicates an error and the error code is in errno. That's what you should be inspecting during error handling. To convert the integer errno into a useful message, something like strerror() will help.

As you discovered, inserting arbitrary text into a script can present serious issues. The simplest solution is to write the reminder's text to a separate file.

strcpy of optarg to text is unsafe and can overflow. You can use strncpy, but then must be careful to ensure that the string is always null-terminated. strlcpy is a simpler alternative, if available. However, the simplest alternative in this case is to not copy at all.

Not only do you not need to copy, you don't need to allocate either. The reminder's text has already been allocated storage during startup. It's in argv. The only thing you need to do is pass around the pointer. The default value has also been stored away in the executable's image and it's location can also be passed around.

You implied that you are no longer waiting in the parent, but since the code is still there, a couple of notes about it.

You should always check WIFEXITED before using WEXITSTATUS (as you did with WIFSIGNALED before using WTERMSIG). An implementation is not forbidden from overloading bits for signal and status information (though I don't know if any implementation actually does so).

For portability and readability (especially for readability), it's a far better choice to use the macros in signal.h. Instead of 6, use SIGABRT. As far as I know, even though the kill(1) utility is required to recognize -6 as SIGABRT, nothing requires the kill(2) system call implementation to equate 6 with SIGABRT (although it almost certainly does).

Regarding your interprocess communication question, your pid file idea seems perfectly reasonable.

Regards,
Alister

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LEARN ABOUT OSF1

learn

learn(1)						      General Commands Manual							  learn(1)

NAME

       learn - Provides computer-aided instruction for the C shell

SYNOPSIS

       learn [-directory] [subject] [lesson]

       The learn command provides computer-aided instruction courses and practice in the use of Tru64 UNIX.

OPTIONS

       Allows you to exercise a script in a nonstandard place.

DESCRIPTION

       To  get	started,  enter learn; if this is the first time that you are invoking the learn command, you are guided through a series of ques-
       tions to determine what type of instruction you want to receive.

       If you have used learn before and left your last session without completing a subject, the program uses information  in	$HOME/.learnrc	to
       start you up in the same place you left off.

       To  bypass questions, enter a subject or lesson.  In order to enter a lesson, you must know the lesson number that you received in a previ-
       ous learn command session. If you do not know the lesson number, enter the lesson number as a subject.  The learn command searches for  the
       first  lesson  containing  the subject you specified. If the lesson is a - (dash), learn prompts for each lesson; this is useful for debug-
       ging.

       You can specify the following subjects:

       files editor vi morefiles macros eqn C

SUBCOMMANDS

       There are a few special commands.  The bye command terminates a learn session, and the where command tells you of your  progress  (where  m
       tells  you more.)  The again command redisplays the text of the lesson and again lesson lets you review lesson. The hint command prints the
       last part of the lesson script used to evaluate a response, while hint m prints the entire lesson script.  This	is  useful  for  debugging
       lessons and might possibly give you an idea about what is expected.

EXAMPLES

       To take the online lesson about files, enter: learn files

       You are then prompted for further input.

FILES

       Playpen directories.  Start-up information.

SEE ALSO

       Commands:  csh(1), ex(1)

																	  learn(1)