How to overwrite stdout in C

It’s not manipulating stdout — it’s overwriting the characters which have already been displayed by the terminal.

Try this:

#include <stdio.h>
#include <unistd.h>
static char bar[] = "======================================="
                    "======================================>";
int main() {
    int i;
    for (i = 77; i >= 0; i--) {
        printf("[%s]\r", &bar[i]);
        fflush(stdout);
        sleep(1);
    }
    printf("\n");
    return 0;
}

That’s pretty close to wget‘s output, right? \r is a carriage-return, which the terminal interprets as “move the cursor back to the start of the current line”.

Your shell, if it’s bash, uses the GNU Readline library, which provides much more general functionality, including detecting terminal types, history management, programmable key bindings, etc.

One more thing — when in doubt, the source for your wget, your shell, etc. are all available.