What does “%.6d” mean in printf

Question

What does %.6d mean in:

printf(\"%s.%.6d len:%d \", timestr, header->ts.tv_usec, header->len);

Is it a typo?

It seem

Answer 1

The former will pad with zeros, the latter with spaces.

#include 
int main(void) {
    printf ("%.6d\n", 123);
    printf ("%6d\n", 123);
    return 0;
}

Produces the following output,

000123
   123