Re: Process Synchronization using Pipes

On Mar 26, 7:41 pm, j...@xxxxxxxxxxx (Jens Thoms Toerring) wrote:
Arturo <arturo.roto...@xxxxxxxxx> wrote:
I should have mentioned I'm using C/C++

Please devide if you use C or C++, they are different languages.

not shell script. OK so I'll
post the code so you can see how I'm approaching this. I'm a beginner
at this, the code may look a little ugly, but it's just a simple chain
where Producer forks to Filter1 whom forks to Filter2 whom forks to
Consumer.
The problem is ... The while loop seems to only execute once. I don't
understand why, I think it may have to do with close() cause I've
reading about how it deallocates the file descriptor. If this is the
case then how can I solve this?

There are some many things broken with your program that I don't see
any way to correct them. So instead here's a probably incomplete list:

0) Your main() isn't 'int main(void)' even though you never use
argc and argv.
1) You continue even if you can't open the input file (note that
perror() doesn't calll exit()).
2) You continue even if you are unable to create a pipe.
3) You don't seem to understand what feof() is good for (and that
you don't need it at all if you use fgets(), which returns NULL
when EOF is reached).
4) You use vfork().
5) You create a set new processes plus new pipes in each child for
each line you read in while a single set would do perfectly well
(or probaly better!) for all the lines if each child would loop
until it doesn't get any more input.
6) You're writing spaghetti code instead of a set of functions
that deal with the sub-tasks.
7) You don't check and retain the return value of a read() call
but instead seem to think that read() would treat a '\0'
character similarly to the way fgets() treats a '\n'. But that
isn't the case. To read() all character are just the same. If
you use write() and read() you usually need some kind of "pro-
tocol" that by which the sender informs the receiver of how
many bytes to expect, e.g. by first sending the number of bytes
and only then the data.
8) You seem to believe tht toupper() only would work with alphabe-
tical, lower-case characters.
9) You don't test if fopen() succeeds for the output file
10) You're closing the read end of the first pipe each time you
read a new line and then you start a newprocessfor the next
line which is going to try to read from this already closed
pipe descriptor. That never will work, once it's closed it
it's closed and won't suddenly be opened again just because
you try to write to it. So already when you start dealing
with the second line the top-most parentprocesswill die
on a SIGPIPE signal the moment it's going to try to write to
the pipe since it's child only has a closed pipe handle and
so there's actually no receiver.

Do yourself a favor and start with a simpler task, e.g. reading
from a file in the parentprocess, passing the data to the childprocessvia the pipe and there just write the data to another file.
The basic problem is the same but you at least will have a fighting
chance to get it right after some time (and you can simply test if
the output is ok by doing a 'diff' on the input and output file).
To get to that point read the man pages of the functions you use
very carefully, especially those for the low level I/O functions
like read().

Especially note that the number of bytes to read you pass to
read() is an upper limit only and that you never can assume that
you will get that many - read may return anything between zero bytes
and the upper limit you called it with (or even -1 if an error
occurred). For this reason read() is normally always embedded in a
loop to be able to restart read() if less bytes arrived than you
asked for. For that reason it probably will be simpler if you
restrict yourself to the use of the higher level I/O function like
fgets and fputs() for a start (there's that nice function called
fdopen() that gives you a FILE pointer when you pass it a file or
pipe handle as its argument).

And, BTW, if you post code it would be really helpful if you would
keep the line length down to say 72 characters and replace tabs by
spaces.
Regards, Jens
--
\ Jens Thoms Toerring ___ j...@xxxxxxxxxxx
\__________________________ http://toerring.de

Thanks to all, I got it to work finally.

Jens that was pretty harsh, but I took your advice about starting with
simply reading from a file and then writing the content to another
file,
and after getting that to work, I built on it and finally got it all
to work together.


.

Relevant Pages

  • Re: Process Synchronization using Pipes
    ... You continue even if you are unable to create a pipe. ... for all the lines if each child would loop ... To readall character are just the same. ... especially those for the low level I/O functions ...
    (comp.unix.programmer)
  • Re: Program hanging when read()ing from pipe
    ... stop the while) loop (on the pipe connected to stdout, ... This could cause a deadlock where the child is blocked writing to stderr ... That's what I plan to solve with a SIGCHLD handler. ... to spawn a "stub" program instead of the external program. ...
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    (microsoft.public.vc.mfc)
  • Re: How does one effect O_NONBLOCK?!
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  • Re: How does one effect O_NONBLOCK?!
    ... once I've created 3 pipes (for stdout, stderr and stdin) ... child and parent processes and then dup2the 3 descriptors into the ... corresponding ends of the pipes in the child before execve-ing. ... end side of the pipe that's connected to stdin of the child process. ...
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