Programming

does popen print out the executed string result in stdout, or just evaluate it and not print the result?

I'm not sure what you're even asking. You fread() the output of the program from the handle it gives you if that's what you mean.

if making a function popen using pipes/forks, then when calling that function giving "r" and the string, that means that the result of the string will go to the write end of the pipe, and a pointer to the read end of the pipe will be returned?

Also if given "w" instead of "r", then where does the result of the string go? I think that its supposed to return a pointer to the write end of the pipe... But I don't understand where the string output goes...

Ah, yes.

If you open it with "r", you're given the read-end of a pipe connected to its stdout.

if you open it with "w", you're given the write-end of a pipe connected to its stdin. I think the output is discarded (but aren't 100% sure -- it may go to stdout). Stuff written to stdout isn't a 'string' by the way -- string means something specific and that's not it.

Note that this is not quite the usual file-handle. You have to call pclose() on it, not fclose(), and nonstandard operations on it like fileno() may not do quite what you expect.

I'm not sure if this is right...
for the 'w' part i am not doing execl since i dont know where it goes

You don't have to write your own popen(), stdio already has one.

Have you actually tried this code? It'll crash, because

Code:

return &fd[0];

That doesn't make a FILE pointer, just an integer pointer, since fd isn't a FILE. You convert a file number into a FILE * with a command like fdopen(fd, "r");

You also forgot to close the end of the pipe you're not using again.

This doesn't work either:

Code:

(*type == "r")

You don't compare strings that way.

Code:

if(strcmp(type, "r") == 0)
{
...
}

...and you have no way to wait for the process on close now, because you didn't store the PID anywhere. stdio popen() can do that and remember, but you can't just from a FILE *.

Honestly I think some review of the C language would do you good before you try to tackle multiprocessing.

so the returns statements are like this: (unless i did something wrong again)
return fdopen(fd[0], "r");
return fdopen(fd[1], "w");
respectively


and just to be clear again, when given 'w', the output of the string gets discarded or printed to stdout?