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Old 08-14-2009, 01:10 PM   #1
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Registered: Aug 2009
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emulating files or char devices

Hi, i need to make a program, which "emulates" a file (char device) to another program (which would run as child process).
the data coming from stdin should go to that file (if the child process reads from that file) and the output of the child written into that file should go to stdout.

like this:
$ echo "input" | emufile testfile.txt bash -c "base64 testfile.txt > testfile.txt"

or, to simulate a char device (this example would forward the webcam device over network via ssh):
$ mkfifo fifo1 fifo2
$ ssh -c "cat /dev/video0" host > fifo1 &
$ ssh -c "cat > /dev/video0" host < fifo2 &
$ cat fifo1 | emufile /dev/video0 cheese | cat fifo2

so, what i try to do is something like padsp (which simulates a OSS (/dev/dsp) device in an PulseAudio environment), but not for a specific file, and i also have to "forward" stdio from/to that file.
i looked at the padsp sources, and it seems that it defines custom fopen/fwrite/fclose/etc... syscalls, and somehow uses them as wrappers for his child process.

if anyone knows howto do that, or knows an tutorial/howto on simulating files to child processes, please tell me.
Old 08-14-2009, 02:08 PM   #2
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Not sure I understand the question, but I think that pseudo-ttys will do much of what you want. Check the pty(7) man page for capabilities and usage.
Old 08-14-2009, 02:29 PM   #3
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can make use a pseudotty called /dev/video0 and pipe stdin of an process to it, and stdout of it to another process? if yes, then it could work...
but i thought of making a real "virtual file", which is not on the harddisk or ramdisk, but only exists in an wrapper function which overwrites the stdio of the child process. i mean if using pseudo-tty is so simple, why should padsp then replace syscalls?

oh, i have an idea: could it be possible to compile a modified version of iostream.h into a lib, and then just call my "child process" with LD_PRELOAD or LD_LIBRARY_PATH (or how that environment variable is called), so it uses my modifies lib for file access?

i just got another idea using the Unix98PTY: i could create an PTY, then create a symlink to the name the file should be called, run the program, wait for the program quitting, remove the symlink and then close the PTY.
do you think this can be done in a bash script? because i like bash scripting more than coding C/C++ for such "small" tasks.

Last edited by nonchip; 08-14-2009 at 02:48 PM.
Old 08-14-2009, 04:33 PM   #4
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ok, update: i have the following code now:
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>
int main(int argc, char** argv) {
  if(argc > 2){
    int         fdm,          //master  FD
                fds;          //slave   FD
    char        *slavename;   //slave   filename

    int         fdm2,         //master2 FD
                fds2;         //slave2  FD
    char        *slavename2;  //slave2  filename
    extern char *ptsname();

    int         fdio= open("/dev/stdio", O_RDWR);

    int pid=fork();
    if(pid==0) { //fdm -> fdm2
      fdm = open("/dev/ptmx", O_NOCTTY);
      slavename = ptsname(fdm);
      char *buf_ln_1=malloc(strlen("ln -s ")+strlen(slavename)+strlen(" ")+strlen(argv[1])+1);
      strcpy(buf_ln_1,"ln -s ");strcat(buf_ln_1,slavename);strcat(buf_ln_1," ");strcat(buf_ln_1,argv[1]);
      char *buf=malloc(2);
        write(fdio,buf,1); //debug
    }else{ //fdm2 -> fdm
      fdm2 = open("/dev/ptmx", O_NOCTTY);
      slavename2 = ptsname(fdm2);
      char *buf_ln_2=malloc(strlen("ln -s ")+strlen(slavename2)+strlen(" ")+strlen(argv[2])+1);
      strcpy(buf_ln_2,"ln -s ");strcat(buf_ln_2,slavename2);strcat(buf_ln_2," ");strcat(buf_ln_2,argv[2]);
      char *buf=malloc(2);
        write(fdio,buf,1); //debug
    char *buf_ln_3=malloc(strlen("rm ")+strlen(argv[1])+strlen(argv[2])+1);
if you compile it, run like this: "a.out virtualFile1 virtualFile2" (when a.out is the binary).

it does create 2 PTYs, and symlinks to virtualFile1 and virtualFile2.
but what i want (in the while loops) is that it "connects" the two PTYs (so that input from pty1 goes to pty2's output and input from pty2 will be put out on pty1). it does'nt work.
i tried "cat > virtualFile1" on one xterm while doing "cat virtualFile2" in the other one, then typed into the first one, but the input didn't appear in the second one or in the debug lines
Old 08-15-2009, 12:17 AM   #5
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Registered: Aug 2009
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ok, i just saw there IS a program doing exactly what i want, creating chardevices (PTYs) and connecting them:

i use this one now, that's better than my quick&dirty code.


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