help on interprocess communication

[Conor P. Cahill]

In article <6456 at cps3xx.UUCP> bruey at cpsin2.cps.msu.edu () writes:
> pipe(filedes);
> if (fork())
>   {
>    printf("%d %d are filedes \n",filedes[0],filedes[1]);
>    buf = "first message"; 
>    write(filedes[1],buf,14);
>    if(fork()) 
>     { 
>      read(filedes[1],buf1,14); 
>       printf("in second   %s \n",buf1);    
>     } 
>   }

Read PIPE(2) which should state something like:

filedes[0] is opened for reading and filedes[1] is opened for writing.

There are also several comments that could be made as to the error 
checking that is going on (or isn't going on), but I will just shut up
and give you  an example of the use of fork() and pipe() from the 
imake sources:

cppit(Imakefile, template, outfd)
	char	*Imakefile;
	char	*template;
	FILE	*outfd;
{
	FILE	*pipeFile;
	int	pid, pipefd[2];
#ifdef SYSV
	int	status;
#else	/* !SYSV */
	union wait	status;
#endif	/* !SYSV */
	char	*cleanedImakefile;

	/*
	 * Get a pipe.
	 */
	if (pipe(pipefd) < 0)
		LogFatal("Cannot make a pipe.", "");

	/*
	 * Fork and exec cpp
	 */
	pid = vfork();
	if (pid < 0)
		LogFatal("Cannot fork.", "");
	if (pid) {	/* parent */
		close(pipefd[0]);
		cleanedImakefile = CleanCppInput(Imakefile);
		if ((pipeFile = fdopen(pipefd[1], "w")) == NULL)
			LogFatalI("Cannot fdopen fd %d for output.", pipefd[1]);
		fprintf(pipeFile, "#define IMAKE_TEMPLATE\t\"%s\"\n",
			template);
		fprintf(pipeFile, "#define INCLUDE_IMAKEFILE\t\"%s\"\n",
			cleanedImakefile);
		fprintf(pipeFile, "#include IMAKE_TEMPLATE\n");
		fclose(pipeFile);
		while (wait(&status) > 0) {
			errno = 0;
#ifdef SYSV
			if ((status >> 8) & 0xff)
				LogFatalI("Signal %d.", (status >> 8) & 0xff);
			if (status & 0xff)
				LogFatalI("Exit code %d.", status & 0xff);
#else	/* !SYSV */
			if (status.w_termsig)
				LogFatalI("Signal %d.", status.w_termsig);
			if (status.w_retcode)
				LogFatalI("Exit code %d.", status.w_retcode);
#endif	/* !SYSV */
		}
		CleanCppOutput(outfd);
	} else {	/* child... dup and exec cpp */
		if (verbose)
			showargs(cpp_argv);
		dup2(pipefd[0], 0);
		dup2(fileno(outfd), 1);
		close(pipefd[1]);
		execv(cpp, cpp_argv);
		LogFatal("Cannot exec %s.", cpp);
	}
}

-- 
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| Conor P. Cahill     uunet!virtech!cpcahil      	703-430-9247	!
| Virtual Technologies Inc.,    P. O. Box 876,   Sterling, VA 22170     |
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