C++ GMP weird memory management

Question

i am working with GMP lib for the first time and it looks like this code does some dirty memory cheats:

void addition (const point p,const point q,const mpz_t fp, point* result){
    mpz_t ld;
    mpz_init(ld);
    lambda_add(p,q,fp,ld);

    gmp_printf( "WTFCHECK1:  %Zx, %Zx\n", p.x ,p.y);
    mpz_mul(result->x,ld,ld);
    gmp_printf( "WTFCHECK2:  %Zx, %Zx\n", p.x ,p.y);
    mpz_sub(result->x,result->x,p.x);
    gmp_printf( "WTFCHECK3:  %Zx, %Zx\n", p.x ,p.y);
...
}

where lambda looks like

void lambda_add (const point p,const point q,const mpz_t fp, mpz_t result){
    mpz_t ydiff,xdiff;
    mpz_init (ydiff);       
    mpz_init (xdiff);

    mpz_sub (ydiff,q.y,p.y);
    mpz_sub (xdiff,q.x,p.x);
...//no more usage of p or q
}

But program output still looks like this:

WTFCHECK1:  7cf27b188d034f7e8a52380304b51ac3c08969e277f21b35a60b48fc47669978, 7775510db8ed040293d9ac69f7430dbba7dade63ce982299e04b79d227873d1
WTFCHECK2:  de8a6886d284cb85d49a93b707475ed3d277242444ba9e860207f9a59a9ef351, 7775510db8ed040293d9ac69f7430dbba7dade63ce982299e04b79d227873d1

As you can see, variable p.x of type mpz_t (x and y are mpz_t type in typedef structure point) has changed without me assigning anything into it. What is the reason of this behavior? Thanks.

You may need to post the setup of p, q, and result as well as the call to addition. What happens if you comment out the call to lambda_add? — Craig Estey
Can't say much without a MCVE. Note that you would likely save a lot of trouble if you used the header <gmpxx.h> that comes with GMP, and the type mpz_class it provides. — Marc Glisse

Smarty77 Smarty77 · Accepted Answer · 2016-03-26T10:25:01

Okay, :D problem was a bit on my side i was calling addition (const point p,const point q,const mpz_t fp, point* result) with same structure given as a point p and as a point* result, so along with modifying result i was also modifying point p. I appreciate your help anyway.

C++ GMP weird memory management

1 Answers