Bit Flipping Programming Exercise in C

The comment at the top was not followed because I printed the contents. I just want an opinion on coding style.

    /*
     * Write a function setbits(x,p,n,y) that returns x with the n bits
     * that begin at position p set to the rightmost n bits of y, leaving
     * the other bits unchanged in the least number of lines.
     *
     * Note: function signatures and curly brackets dont count towards 
     * the number of lines. You must also declare your variables, and 
     * call the function
     *
     * build with:
     *   gcc -o bit_twiddle -Wall -g -lm ./bit_twiddle.c
     */

    #include <stdio.h>
    #include <math.h>
    #include <limits.h>

    unsigned setbits(unsigned x, unsigned p, unsigned n, unsigned y) {
        x |= (y & ~(~0 << n)) << p;
        size_t s = (int)(log(INT_MAX)/log(2)) + 1;
        printf("An int is %d bits\n", s);
        int mask = pow(2.0, (int)s);
        do {
            ((x & mask) == 0) ? printf("0") : printf("1");
            ((s%4)==0) ? printf(" ") : printf("");
            x <<= 1;
        } while (s--);
        printf("\n");
    }

    void main( ) {
        unsigned retrn=1, begin=3, nbits=3, input=7;
        unsigned x = setbits(retrn, begin, nbits, input);
    }

UPDATE

x |= (y & ~(0 << n)) << p --> x |= (y & ~(~0 << n)) << p