Q:

How to use format specifiers to set field width of a string?

printf("%5s\n", "#");

gives:
    #

Is their a way to set field width of this string using an integer format specifier?
Something like this,
printf("%%ds\n", 5, "#");

A:

From the printf manual

Instead of a decimal digit string one may write "*" or "*m$" (for some decimal integer m) to specify that the field width is given in the next argument, or in the m-th argument, respectively, which must be of type int. 

So in your example it would be:
printf("%*s\n", 5, "#");

