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c++gmparbitrary-precision

Initialization of variables in GMP

According to the manual of GMP, "The functions for integer arithmetic assume that all integer objects are initialized. You do that by calling the function mpz_init."

For example,

int main(){
    mpz_t n1, n2, result;
    mpz_init_set_str(n1, "465860", 10);
    mpz_init_set_str(n2, "167", 10);
    mpz_init(result);
    mpz_mod(result, n1, n2);
    mpz_out_str(stdout, 10, result);
    return 0;
}

if you remove mpz_init(result); from the code above, the compiler will produce an segmentation fault error. So, my question, what exactly is difference between these two type of definition: mpz_t result and mpz_init(result). Why is mpz_init also required?

Solution

  • mpz_t result only declares a variable. The type mpz_t is a typedef for struct __mpz_struct. This structure contains three variables which are initialized by mpz_init(). Hope that helps.