practical examples use dup or dup2

Question

I know what dup / dup2 does, but I have no idea when it would be used.

Any practical examples?

Thanks.

Answer 1

One practical example is redirecting output messages to some other stream like some log file. Here is a sample code for I/O redirection.
Please refer to original post here

#include 

main()
{
    int    fd;
    fpos_t pos;

    printf("stdout, ");

    fflush(stdout);
    fgetpos(stdout, &pos);
    fd = dup(fileno(stdout));
    freopen("stdout.out", "w", stdout);

    f();

    fflush(stdout);
    dup2(fd, fileno(stdout));
    close(fd);
    clearerr(stdout);
    fsetpos(stdout, &pos);        /* for C9X */

    printf("stdout again\n");
}

f()
{
printf("stdout in f()");
}