How can a Unix program display output on screen even when stdout and stderr are redirected?

Question

I was running a program (valgrind, actually) on my Ubuntu machine, and had redirected both stdout and stderr to different files. I was surprised to see a short message appea

Answer 1

Here is some sample code that does exactly what was asked (thanks to earlier answers pointing me in the right direction). Both are compiled with g++, and will print a message to the screen even when stdout and stderr are redirected.

For Linux (Ubuntu 14):

#include 
#include 
#include 
#include 

int main( int, char *[]) {

    printf("This goes to stdout\n");

    fprintf(stderr, "This goes to stderr\n");

    int ttyfd = open("/dev/tty", O_RDWR);
    const char *msg = "This goes to screen\n";
    write(ttyfd, msg, strlen(msg));
}

For Windows 7, using MinGW:

#include 
#include 
#include 
#include 

void writeConsole( const char *s) {
    while( *s) {
        putch(*(s++));
    }
}

int main( int, char *[]) {  
    printf("This goes to stdout\n");

    fprintf(stderr, "This goes to stderr\n");

    writeConsole( "This goes to screen\n");
}