### 3.5 Parameter Conventions

When a GMP variable is used as a function parameter, it’s effectively a call-by-reference, meaning if the function stores a value there it will change the original in the caller. Parameters which are input-only can be designated `const` to provoke a compiler error or warning on attempting to modify them.

When a function is going to return a GMP result, it should designate a parameter that it sets, like the library functions do. More than one value can be returned by having more than one output parameter, again like the library functions. A `return` of an `mpz_t` etc doesn’t return the object, only a pointer, and this is almost certainly not what’s wanted.

Here’s an example accepting an `mpz_t` parameter, doing a calculation, and storing the result to the indicated parameter.

    void
    foo (mpz_t result, const mpz_t param, unsigned long n)
    {
      unsigned long  i;
      mpz_mul_ui (result, param, n);
      for (i = 1; i < n; i++)
        mpz_add_ui (result, result, i*7);
    }

    int
    main (void)
    {
      mpz_t  r, n;
      mpz_init (r);
      mpz_init_set_str (n, "123456", 0);
      foo (r, n, 20L);
      gmp_printf ("%Zd\n", r);
      return 0;
    }

`foo` works even if the mainline passes the same variable for `param` and `result`, just like the library functions. But sometimes it’s tricky to make that work, and an application might not want to bother supporting that sort of thing.

For interest, the GMP types `mpz_t` etc are implemented as one-element arrays of certain structures. This is why declaring a variable creates an object with the fields GMP needs, but then using it as a parameter passes a pointer to the object. Note that the actual fields in each `mpz_t` etc are for internal use only and should not be accessed directly by code that expects to be compatible with future GMP releases.