Weird bug in c implementing schoolbook multiplication using gmp

Question

I have a weird problem.

I am trying to implement the schoolbook multiplication. I am aware that the function mpz_mul does that for me but it is my task to implement it myself as a homework.

So here is my code:

void mpz_school_mul(mpz_t c, mpz_t a, mpz_t b)
{
    size_t i;
    mp_limb_t b_i;
    mpz_t c_part;

    mpz_init(c_part);

    /* Backup a for the special case a := a * b. */
    mpz_t a_backup;

    mpz_init(a_backup);
    mpz_set(a_backup, a);
    /* Clear the result */
    mpz_set_ui(c,0);

    gmp_printf("i = %zx, size(b) = %zx, a = %Zx, b = %Zx\n", i, mpz_size(b), a, b);

    for(i = 0; i < mpz_size(b); i++)
    {
        printf("test\n");
        b_i = mpz_getlimbn(b,i);
        /* c = a*b_i*B^i + ... + a*b_0*B^0 */

            /* Calculate a*b_i for every round. */
            mpz_mul_limb(c_part,a_backup,b_i); 

            /* Shift it to the right position (*B^i). */
            mpz_mul_base(c_part,c_part,i);

            /* Sum all a*b_i*B^i */
            mpz_school_add(c, c, c_part);   
    }

    mpz_clear(a_backup);
    mpz_clear(c_part);
}

This code works well for me and i can test it with several parameters. The result is correct so I don't think I need to change to much in the calculation part. ;)

As example: This parameters work as intended.

mpz_set_str(a, "ffffffff00000000abcdabcd", 16);
mpz_set_str(b, "cceaffcc00000000abcdabcd", 16);
mpz_school_mul(c,a,b);

Now to the bug:

When i run the program with a parameter b with a zero limb (I'm using a 32 bit VM) at the end the program crashes:

mpz_set_str(a, "ffffffff00000000abcdabcd", 16);
mpz_set_str(b, "cceaffcc00000000", 16);
mpz_school_mul(c,a,b);

The output with this parameter b_0 = 0 is:

i = 0, size(b) = 2, a = ffffffff00000000abcdabcd, b = cceaffcc00000000

I think the for-loop stucks because the printf("test\n"); does not show up in this run.

Thanks for your help ;)

Answer 1

The bug in the problem is fixed now.

Here is the solution:

I tested out to use fprintf(stderr, "test\n"); rather than printf("test\n"); and tested the code. It magically showed up the "test" in my console. It may have to do with the wrong inclusion order of the and file.

As I had this problem with the print I didn't check my other functions. Since I figured out, that the for-loop wasn't the problem I tested several prints after each command. I was able to detect the error in the void mpz_mul_base(mpz_t c, mpz_t a, mp_size_t i) function where I didn't check the case with c_part = 0. With this parameter the following code of the mpz_mul_base(c_part,c_part,i); function ran into an endless loop:

if(mpz_size(c) >= n)
    for(i = mpz_size(c); i >= n; i--)
    {
    mpz_setlimbn(c, clear, i);
    }

I replaced the >= with > and everything works fine now.