bruey@cpsin2.cps.msu.edu (02/15/90)
Can any of you brainy netlanders give me a hint why this
doesn't work? I've been cursing at this code for days
and I can't get it to work. (I'm not even sure that the
forks are in the right order. So basically, I'm lost.)
#include<stdio.h>
main()
{
/* yes, I know I don't need all of these declarations, but
this isn't the only part of the program */
int pid;
int id1,id2,id3;
int rc=0,status,parent;
int filedes[2];
char *a,*buf1,*buf;
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);
}
}
} flc@n.sp.cs.cmu.edu (Fred Christianson) (02/19/90)
In article <6456@cps3xx.UUCP> bruey@cpsin2.cps.msu.edu () writes: > >Can any of you brainy netlanders give me a hint why this >doesn't work? ... > write(filedes[1],buf,14); > if(fork()) > { > read(filedes[1],buf1,14); ^ | 0 --- > printf("in second %s \n",buf1); > } > } > >} filedes[1] is for writing and filedes[0] is for reading. I haven't used pipes for a while, but that's what my man page says. ---- Fred
cpcahil@virtech.uucp (Conor P. Cahill) (02/20/90)
In article <6456@cps3xx.UUCP> bruey@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); } } -- +-----------------------------------------------------------------------+ | Conor P. Cahill uunet!virtech!cpcahil 703-430-9247 ! | Virtual Technologies Inc., P. O. Box 876, Sterling, VA 22170 | +-----------------------------------------------------------------------+
john@chinet.chi.il.us (John Mundt) (02/20/90)
In article <6456@cps3xx.UUCP> bruey@cpsin2.cps.msu.edu () writes: > >Can any of you brainy netlanders give me a hint why this >doesn't work? I've been cursing at this code for days >and I can't get it to work. (I'm not even sure that the >forks are in the right order. So basically, I'm lost.) > > >#include<stdio.h> >main() >{ > int pid; > int id1,id2,id3; >int rc=0,status,parent; >int filedes[2]; >char *a,*buf1,*buf; > > > 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); > } > } > >} When you fork, you have two processes, the child and the parent. The child process gets a return of 0 from fork if all goes well, the parent the process ID of the child. The normal way to do the above is to put it in a switch statement like this: switch(fork()) { case -1 : horrible_error_routine; exit(1); /* error */ case 0 : do_child_thing(); /* like write to pipe */ exit(0); /* and die right away */ default : do_parent_thing(); /* like read the pipe */ } Yours failed above since you never had a child process do anything. You never left the parent process. And, you only need one fork. Hope this helps. -- --------------------- John Mundt Teachers' Aide, Inc. P.O. Box 1666 Highland Park, IL john@admctr.chi.il.us *OR* fred@teacha.chi.il.us (312) 998-5007 (Day voice) || -432-8860 (Answer Mach) && -432-5386 Modem