Re: wait_for_file ?? [print via pipes - help testing on Windows?]

[Michael D Godfrey]

On 09/18/2010 07:59 AM, Ben Abbott wrote:

On Sep 18, 2010, at 12:33 AM, Marco Atzeri wrote:

> --- Ven 17/9/10, bpabbott <address@hidden> ha scritto:
> 
> This looks good to me. Does the short script below run and produce the "test.txt" file containg "Hello World"?
> 
>   pid = popen ("cat > test.txt", "w");
>   fprintf (pid, "Hello World");
>   waitpid (pid);
>   pclose (pid);
> 
> Ben
> 
> yes

I recall the gl2ps renderer includes an fclose(pid). What happens with ...

  pid = popen ("cat > test.txt", "w");
  fprintf (pid, "Hello World");
  fclose (pid);
  waitpid (pid);
  pclose (pid);

Ben

You may want to recall the requirement from "man popen":
       The return value from popen() is a normal standard I/O stream in all respects save that it  must  be
       closed  with  pclose() rather than fclose(3).  Writing to such a stream writes to the standard input
       of the command; the command’s standard output is the  same  as  that  of  the  process  that  called
       popen(),  unless this is altered by the command itself.  Conversely, reading from a "popened" stream
       reads the command’s standard output, and the command’s standard input is the same  as  that  of  the
       process that called popen().

       Note that output popen() streams are fully buffered by default.

       The  pclose()  function waits for the associated process to terminate and returns the exit status of
       the command as returned by wait4(2).

======================================
Therefore you should be using pclose() not fclose().  But also note, as Shai
pointed out, the call to the gl2ps code passes pid by value so the first of the 2 examples
above is more nearly correct.  Actually, the pclose is probably not necessary, and I think
that the gl2ps code does a waitpid before its pclose, so the waitpid may not be required
either.   However, if control returns from the fprintf() before the gl2ps
code has entered the section that uses the passed pid, the closes could be harmful.

Michael