waitpid usage

[John S. Gwynne]

I'm trying to open a two-way pipe (with popen2) to create a dialog
with another application. The problem I'm having is trying to release
"waitpid" in between exchanged lines. In the example below, I send a
text string to a program that echos it back. After sending the string,
waitpid is called to wait on the child to echo. I can unblock waitpid
by first closing the "in" pipe (closing the app) and the example
finishes, but I need more than one exchange of dialog. I need to
unblock waitpid without closing the application or pipe. How can I do
that?

"man waitpid" says...

 option 0: wait until signal is received or a child process exits (this
         is the default if the OPTIONS argument is missing)

So... what signal can I send that will unblock, leave the pipe open,
and how should it be coded? I tried a killpg with a SIGCONT without
success and figured I need help at this point. HELP!! :)

TIA,
john gwynne


========================================================
================= begin example ptest1.m ===============
========================================================

function ptest1()

  [in out pid] = popen2("myEcho");
  if (pid == -1)
    error("Pipe failed\n");
  end;
  
  fputs(in,"Test 1\n");

  % works fine with an "fclose(in)" here, but that closes the pipe
  
  waitpid(pid); % would like to wait here until text is available
                % or some non-exit signal is received.
  fputs(stdout,fgets(out));
  fflush(stdout);
  
  % would like to put more fputs/fgets here for dialog with application
    
  fclose(in);
  fclose(out);

end;
  
========================================================
=================== end example ptest1.m ===============
========================================================

%%%%%%%%%%%%%%%%%%%%%%%%%%

===============================================
================ begin myEcho,c ===============
===============================================
#include <signal.h>
#include <stdio.h>

int main() {
  int ch;
  FILE *fid;

  fid = fopen("/tmp/hold","w");
  if (!fid)
    perror("fopen failed");

  while (EOF != (ch=getchar())) {
    fputc(ch,fid);
    fflush(fid);
    putchar((char)ch);
    if (ch == '\n') {
      fputc('.',fid);
      fflush(fid);
      if (killpg(0,18)) /* does this make sense? */
        perror("Signal failed");
    }
  }
}
===============================================
================== end myEcho,c ===============
===============================================



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