'\n' eats line when stdout opened from file descriptor

rsood · 08-28-2008, 04:36 PM

I have the following code

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");
    fclose(stdout);
    stdout = fdopen(fildes, "w");
    printf("Back to terminal?");
    printf("!!!\r\f");
    printf("????\n");
/*
    freopen("file.txt", "a", stdout);
    printf("Back to file?\n");
    fclose(stdout);
    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?

ta0kira · 08-28-2008, 06:51 PM

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?
ta0kira

edit:
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.

rsood · 08-30-2008, 08:08 PM

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

Code:

*stdout = *fdopen(fildes, "w");

gets it to compile and solves the problem I was having. Not sure exactly why, but...

pinniped · 08-30-2008, 08:30 PM

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.

ta0kira · 08-31-2008, 12:14 AM

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.
ta0kira