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¹ This error make code not compilable, one of the reasons for closing the question. Let us count this as a copy/paste error. Still, better to be certain that the code posted compiles, not just the code on your computer.

¹ This error makes code not compilable, one of the reasons for closing the question. Let us count this as a copy/paste error. Still, better to be certain that the code posted compiles, not just the code on your computer.

In common C, one would not expect a to be modified by foo() as C is pass by value . Yet since a is an array, a is converted from its value as an array to the addresses of a[0]. It is that address that is passed by value to foo. Now foo() can change the caller's a.

It is quite unnecessary to pass 4 mpz_t* as that is 4 pointers to pointers. Code could have been the following which is also passing 4 pointers.

void dec_alt1(mpz_t m, const mpz_t c, const mpz_t d, const mpz_t n) {
  mpz_powm(m, c, d, n);
}

void dec_alt2(mpz_t *m, const mpz_t c, const mpz_t d, const mpz_t n) {
  mpz_powm(*m, c, d, n);
}

In common C, one would not expect a to be modified by foo() as C is pass by value . Yet since a is an array of __mpz_struct, a is converted from its value as an array to the addresses of a[0]. It is that address that is passed by value to foo. Now foo() can change the caller's a.

This should use const to indicate which may change.

void dec_alt1(mpz_t* m, const mpz_t* c, const mpz_t* d, const mpz_t* n) {
  mpz_powm(*m, *c, *d, *n);
}

It is quite unnecessary to pass 4 mpz_t* pointers. Code could have been the following which is also passing 4 pointers.

void dec_alt2(mpz_t m, const mpz_t c, const mpz_t d, const mpz_t n) {
  mpz_powm(m, c, d, n);
}

void dec_alt3(mpz_t *m, const mpz_t c, const mpz_t d, const mpz_t n) {
  mpz_powm(*m, c, d, n);
}

mpz_t

But what should OP's function do? Take advantage of the hidden "call-by-reference"?

Consider OP's dec()

void dec(mpz_t* m, mpz_t* c, mpz_t* d, mpz_t* n) {
  mpz_powm(*m, *c, *d, *n);
}

It is quite unnecessary to pass 4 mpz_t* as that is 4 pointers to pointers. Code could have been the following which is also passing 4 pointers.

void dec_alt1(mpz_t m, const mpz_t c, const mpz_t d, const mpz_t n) {
  mpz_powm(m, c, d, n);
}

Alternatively, code could avoided the hidden "pass-by-reference":

void dec_alt2(mpz_t *m, const mpz_t c, const mpz_t d, const mpz_t n) {
  mpz_powm(*m, c, d, n);
}