Question

Code below redirects stdout to a file fname & then redirects back to original stdout. It works fine for me.
But I am not able to understand how it actually works. If anyone can help me understand I will appreiciate it.

    printf("\n This is console");
    fflush(stdout);
    fgetpos(stdout, &pos);
    fd = dup(fileno(stdout));
    freopen(fname, "a+", stdout);   

    printf("inside file op");  

    fflush(stdout);
    dup2(fd,fileno(stdout));
    close(fd);
    clearerr(stdout);
    fsetpos(stdout, &pos);
    printf("\nBack to Console");

Answer 1

Let’s go through it line by line. The first line prints something to stdout:

printf("\n This is console");

Then it flushes stdout so all the remaining data in the buffer gets sent to stdout and won’t get mixed up with the file data:

fflush(stdout);

Now we store the current position of ourselves in stdout because otherwise if stdout was already directed to a file, we might (?) overwrite earlier parts of it.

fgetpos(stdout, &pos);

Now we clone the file descriptor of what’s currently stdout. Since we’re about to change where stdout points to, we need to keep a copy of the original:

fd = dup(fileno(stdout));

Now that we have everything preserved, we can reopen stdout as the file:

freopen(fname, "a+", stdout);

At this point, stdout has been redirected to the file. We can now print to it:

printf("inside file op");  

Now we’re done printing to the file. We need to flush stdout (now the file) so it doesn’t get mixed up with the normal stdout data:

fflush(stdout);

After that, we clone the original stdout file descriptor over the current stdout descriptor.

dup2(fd,fileno(stdout));

The cloned one can be closed now:

close(fd);

I’m not quite sure why this is here but this clears any errors that occurred writing to the file:

clearerr(stdout);

Now we restore our position in stdout. Again, as far as I know, this is only useful if it was originally redirected to a file:

fsetpos(stdout, &pos);

Now we’re back to the original stdout, so we can print again:

printf("\nBack to Console");