pipes and X applications

alphisb0t · 12-06-2012, 03:30 PM

Hello.

I'm writing a small C program that forks(), dups(), and execve()s an X application that reads its input from its stdin (as opposed to params).

I can spawn it and its definitely running, however the parent process waits for input and never gets it as the X application is waiting for me to select an option. The problem is....the visual X interface never appears! Its just hidden!

I wrote directly to the X applications's STDOUT file description via /proc and then terminated the process which yielded the correct response in my waiting parent process. So it should be working but the X application just stays invisible for some reason.

Is there any way to make this window show?

Thanks for any help!

theNbomr · 12-09-2012, 09:52 AM

Your description of the problem is confusing. Some code (in [CODE] [/CODE] tags, of course) that demonstrates the problem would be most useful here.

--- rod.

alphisb0t · 02-09-2013, 01:55 PM

I'm trying to write to stdin in a program called dmenu. i can't seem to get it to work at all. here is the code.

Code:

pid_t pid;
        char *exe_buffer[64] = { "/usr/bin/dmenu" };
        char rbuffer[1024];
        int wres;

        pid=fork();

        if(pid==-1) { printf("error forking\n"); return; }

        if(pid==0)
        {
                //debug
                printf("execve: %s %s %s\n",exe_buffer[0],exe_buffer[1],exe_buffer[2]);

                //child
                dup2(pwrite[0],0);
                dup2(pread[1],1);
                dup2(pread[1],2);

                //execve
                execve(exe_buffer[0],exe_buffer,environ);
                printf("..uh..\n");
        }
        else
        {
                sleep(2);
                printf("writing to child..\n");
                //write(pwrite[1],menu,strlen(menu));
                wres=write(pwrite[1],"item1\nitem2\n",12);
                printf("write result: %i\n",wres);
                fsync(pwrite[1]);
                printf("waiting for child..\n");
                wait();
                printf("reading from child..\n");
                read(pread[0],rbuffer,1024);
                printf("read from child: %s\n",rbuffer);
        }

I don't see why reading/writing to an external program via pipes is so....complicated. If anyone can tell me what I'm doing wrong it would be very helpful. I've read the man pages of the functions I'm using and still don't see what the problem is.

ntubski · 02-09-2013, 03:36 PM

Quote:

Originally Posted by alphisb0t

I'm trying to write to stdin in a program called dmenu.

dmenu waits for an EOF on stdin before displaying anything. In terms of your C program, you have to close(pwrite[1]), before read(pread[0]).

alphisb0t · 02-09-2013, 10:40 PM

Ah indeed EOF IS needed (and man page even mentions this too). I added and it didn't fix the problem. For *SOME* reason I had to close the parent only FDs in the child and magically it works!

Here is the code (from child process)

Code:

if(pid==0)
        {
                //debug
                printf("execve: %s %s %s\n",exe_buffer[0],exe_buffer[1],exe_buffer[2]);

                //child
                dres=dup2(pwrite[0],STDIN_FILENO);
                if(dres==-1) { printf("error duping handle!\n"); return; }
                dres=dup2(pread[1],STDOUT_FILENO);
                if(dres==-1) { printf("error duping handle!\n"); return; }
                dres=dup2(pread[1],STDERR_FILENO);
                if(dres==-1) { printf("error duping handle!\n"); return; }

                //close parent only used fds
                close(pread[0]);
                close(pwrite[1]);

                //execve
                execve(exe_buffer[0],exe_buffer,environ);
                printf("..uh..\n");
        }

Notice the new lines close() for the pread[0] and pwrite[1]. Adding this makes it work! Commenting them out again makes it not work! I don't understand why this is necessary seeing as how the child (dmenu) only reads from stdin and writes to stdout. I get the EOF thing in the parent (not shown) but don't get why this additional change it needed.

ntubski · 02-10-2013, 08:46 AM

Ah, the explanation is in pipe(7):

Quote:

If all file descriptors referring to the write end of a pipe have been closed, then an attempt to read(2) from the pipe will see end-of-file (read(2) will return 0). ... An application that uses pipe(2) and fork(2) should use suitable close(2) calls to close unnecessary duplicate file descriptors; this ensures that end-of-file and SIGPIPE/EPIPE are delivered when appropriate.

So you need close(pwrite[1]) in the child else close(pwrite[1]) in the parent has no effect.