Question

Basically I want to do in C (and without buffering) the same as this bash-script:

#!/bin/sh
cat ./fifo_in | myprogram > ./fifo_out

In other words I want to exec “myprogram” and redirect its stdin and stdout to two pipes which have been created previously.

Another program is feeding data into fifo_in and reading out of fifo_out.

Of course it would be easy to just read from ./fifo_in, buffer it in the parent and write to myprogram’s stdin (and reverse for stdout and ./fifo_out) but I think there is probably a way to let “myprogram” read/write directly from/to the fifos without buffering in the parent process.

Edit:

Eugen’s answer seems to be the correct one, but I cannot get it to work.

I use this function on the C-side, which seems correct to me:

pid_t execpipes(const char *wd, const char *command, const char *pipename)
{
char pipename_in[FALK_NAMESIZE];
char pipename_out[FALK_NAMESIZE];
strcpy(pipename_in, FALKPATH);
strcat(pipename_in, "/");
strcat(pipename_in, FALK_FIFO_PATH);
strcat(pipename_in, "/");
strncat(pipename_in, pipename, FALK_NAMESIZE-2);
strcpy(pipename_out, pipename_in);
strcat(pipename_out, "R");

pid_t pid;
pid = fork();
if (pid < 0)
{   //Error occured
    perror("fork");
    exit(1);
}
if (pid == 0)
{
    chdir(wd);
    d("execpipes: pipename_in=\"%s\"\n", pipename_in);
    d("          pipename_out=\"%s\"\n", pipename_out);
    freopen(pipename_in,"r",stdin);
    freopen(pipename_out,"w",stdout);

    d("execpipes: command=\"%s\"\n", command);

    execl("/bin/sh", "sh", "-c", command, (char *)NULL); // using execv is probably faster
    // Should never get here
    perror("execl");
    exit(1);
}
return pid;
}

I read and write the pipes from a PHP-script (only relevant part posted):

$pipe_in = fopen($fp.$pipename, "w");
$DEBUG .= "Write to pipe_in\n";
$ret = fwrite($pipe_in, $in);

$pipe_out = fopen($fp.$pipename.'R', "r");
$DEBUG .= "Read from pipe_out\n";
$atext = fread($pipe_out, 200000);  // Program hangs here

The program is started correctly, receives the input via $pipe_in correctly, processes the data correctly and (because it ran fine for many months) I assume it puts out the data correctly to stdout, but when I try to read from $pipe_out, it hangs. I know that the pipes themselves are set up correctly because if I don’t open $pipe_out, the program does not get any input – which makes sense because there is no reader for $pipe_out and therefore the pipeline is not complete. So I can open $pipe_out, but I cannot read anything from it, which is quite strange.

Edit2:

Program works now, thanks guys – For some reason the first pipe has to be closed before you can read from the second pipe:

$pipe_in = fopen($fp.$pipename, "w");
$pipe_out = fopen($fp.$pipename.'R', "r");
$DEBUG .= "Write to pipe_in\n";
$ret = fwrite($pipe_in, $in);
fclose($pipe_in);

$DEBUG .= "Read from pipe_out\n";
$atext = fread($pipe_out, 200000);
fclose($pipe_out);

unlink($fp.$pipename);
unlink($fp.$pipename.'R');

Answer 1

I’d write a small wrapper for myprogram, that does

freopen("./fifo_in","r",stdin)
freopen("./fifo_out","w",stdout)

(Ofcourse not with constant paths!), then execve myprogram