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Old 08-28-2008, 04:36 PM   #1
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'\n' eats line when stdout opened from file descriptor

I have the following code

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

int main(void){
    int fildes = dup(1);
    printf("On terminal\n");
    freopen("file.txt", "w", stdout);
    printf("To file");
    stdout = fdopen(fildes, "w");
    printf("Back to terminal?");
    freopen("file.txt", "a", stdout);
    printf("Back to file?\n");
    stdout = fdopen(fildes, "w");
    printf("Back to terminal?\n");
return 0;
this results in output on the terminal of

On terminal
Back to terminal?!!!

the ???? does not appear. This code is the result of fiddling around a little, and from that it appears that after stdout has been redirected back to the terminal from a previously saved file descriptor, the \n causes any text printed with it to be eaten (not displayed). This happens whether fdopen is passed "w" "wb" or "w+b". Anybody know what's going on?
Old 08-28-2008, 06:51 PM   #2
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Distribution: FreeBSD 9.1, Kubuntu 12.10
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I'm not sure what a form feed does in a terminal, but carriage return should make the next print go to the beginning of the same line. What does it do without \f?

Looks like the buffer catches some of the output. It generally won't flush until a newline, so some of the printfs might be held until after the file change. You're changing stdout in a non-standard way, though.

Last edited by ta0kira; 08-28-2008 at 06:55 PM.
Old 08-30-2008, 08:08 PM   #3
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Thanks for the reply. What you say about changing stdout in a nonstandard way probably has something to do with the problem, because when I tried compiling my code on another operating system it complained about "stdout" being an invalid lvalue. Changing the code such that the stdout reassignment line is

*stdout = *fdopen(fildes, "w");
gets it to compile and solves the problem I was having. Not sure exactly why, but...
Old 08-30-2008, 08:30 PM   #4
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Your solution makes sense; think of it this way: you're modifying a FILE*, not a FILE**, so you can't have any permanent effect on the variable itself although you can alter what it points to.

Last edited by pinniped; 08-30-2008 at 08:46 PM.
Old 08-31-2008, 12:14 AM   #5
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Registered: Sep 2004
Distribution: FreeBSD 9.1, Kubuntu 12.10
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You're replacing the pointer to the FILE buffer itself, but like the other compiler demonstrates, it doesn't have to be a writable pointer. You should really use the open system call instead of freopen, then replace standard output with dup2(fd, STDOUT_FILENO); and close(fd);. That's the *nix way, and is the commonly-accepted way of replacing standard output. When you do that, you don't need to fclose(stdout); because the operation is beneath the C library; therefore, the state of the FILE object remains exactly the same. You should fflush(stdout); right before the switch, however.


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