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POWeird bug in c implementing schoolbook multiplication using gmp
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2013-12-30T10:19:30.747
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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: <pre><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); } </code></pre> 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. <pre><code>mpz_set_str(a, "ffffffff00000000abcdabcd", 16); mpz_set_str(b, "cceaffcc00000000abcdabcd", 16); mpz_school_mul(c,a,b); </code></pre> 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: <pre><code>mpz_set_str(a, "ffffffff00000000abcdabcd", 16); mpz_set_str(b, "cceaffcc00000000", 16); mpz_school_mul(c,a,b); </code></pre> The output with this parameter b_0 = 0 is: <pre><code>i = 0, size(b) = 2, a = ffffffff00000000abcdabcd, b = cceaffcc00000000 </code></pre> I think the for-loop stucks because the printf("test\n"); does not show up in this run. Thanks for your help ;)
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<c><for-loop><gmp>
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Weird bug in c implementing schoolbook multiplication using gmp
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