undup2?

If I redirect stdout into a pipe with dup2, how can I get it back again?

Code:

---

#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>

int main () {
        FILE *pin, *pout;
        int pp[2], i, chr;
        pipe(pp);
        pout = fdopen(pp[0], "r");
        pin = fdopen(pp[1], "w");
        close(pp[0]);
        dup2(pp[1],1);
        puts("hello");
        printf("test");
        fclose(pin); //  ??
        close(pp[1]);  //  ??
        while((chr=fgetc(pout)) != EOF) printf("%c",chr);
        close(pp[0]);
        fclose(pout);      // everything closed
        puts("hello");
        fflush(NULL);    // and flushed
}

---

(there is no output)

I recognize that "It is not very useful for a single process to use a pipe to talk to itself", however, I'm trying to get the logic of c redirection. In other contexts (eg. shell scripting) redirecting stdin and stdout is always simple and straightforward -- if you exec 3>$1 you can later exec 1>&3. But in this example, stdout never comes back, even if the file descriptor it was redirected into gets closed, etc.

Also strange to me is that stderr seems to be included (puts).

puts doesn't write to stderr, it writes to stdout.

I don't think I have any good answers for you -- you can try assigning stdout to a FILE* before you dup2, and switching back to that, but I have a sinking suspicion that won't work very well.

Edit: Of course that's a stupid suggestion, since they'll share a file pointer and will travel (duplicate, whatever) together. I'll be interested to see a solution should you or someone come up with one.

Quote:

---

Originally Posted by tabstop

I don't think I have any good answers for you -- you can try assigning stdout to a FILE* before you dup2, and switching back to that, but I have a sinking suspicion that won't work very well.

---

No doubt. I am willing to abandon such fruitless researchs immediately. There are some strange things like this (about c) that i guess reflect the fact that no one would really want to do this that way anyway.

I'm pretty sure you can do something like this:

Code:

---

newdes = dup(STDIN);
dup2(STDIN, pipefd);
... stuff that reads the pipe.
dup2(STDIN, newdes);
close(newdes);

---

--
Mats

I tried

Code:

---

dbout=dup(1);    // at beginning
dup2(1,dbout);  // later

---

hoping this would reinstate stdout normally but to no avail, probably because whatever happened to it (attached to closed pipe) also happened to dbout.

Inspired by http://cboard.cprogramming.com/showthread.php?t=107253 I was screwing around with this & pipe & fork all weekend and the ratio of things "I'm pretty sure you can do" to things "actually not really" was rather poor, I think there is a string of my posts in here about such stuff.

Basically, I bet you can't write a set of forked processes that successfully pipes "ls" to "grep" to another "grep" (or sort, or sed, or whatever). It doesn't matter, because system("ls | grep e | grep c") works (ie. bash has to do it itself). But it certainly sounds easy, and I thought I came up with a lot of code that looks nice.