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3.5 Parameter Conventions

When a GMP variable is used as a function parameter, it’s effectively a call-by-reference, meaning that when 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.

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);

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;

Our function foo works even if its caller 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.

Since GMP types are implemented as one-element arrays, using a GMP variable as a parameter passes a pointer to the object. Hence the call-by-reference.