Possible side effects of doing a typedef of a struct to an array of one element in C

Question

I came across this code.

typedef __mpz_struct MP_INT;
typedef __mpz_struct mpz_t[1];

Here the struct __mpz_struct is a struct that is typedefed to an array of single element. I understand that this is a trick to pass by reference in C. Then mpz_t has been used as a type to declare variables and pass them to function as parameters. Also, There was one more comment

/*
  MP_INT*, MP_RAT* and MP_FLOAT* are used because they don't have side-effects
  of single-element arrays mp*_t
*/

What kind of side effects are they talking about?

Answer 1

Passing an array to a function let's the array decay to a pointer to it's 1st element.

One can achieve the same effect by applying the Address-Of operator & to a simple variable of the same type as the array's elements.

Examples:

struct S
{
   int i;
   float f;
};

This

void set_S(struct S * ps)
{
  ps->i = 40;
  ps->f = 2.;
}

is equivalent to

void set_S(struct S ps[1])
{
  ps->i = 40;
  ps->f = 2.;
}

is equivalent to

void set_S(struct S * ps)
{
  ps[0].i = 40;
  ps[0].f = 2.;
}

is equivalent to

void set_S(struct S ps[1])
{
  ps[0].i = 40;
  ps[0].f = 2.;
}

One-Element-Array approach:

typedef struct S Array_of_S_with_size_1[1];

int main(void)
{
  Array_of_S_with_size_1 array_of_S_with_size_1;
  array_of_S_with_size_1[0].i = 0;
  array_of_S_with_size_1[0].f = 0.;

  set_S(array_of_S_with_size_1);

  ...
}

The above main() provides the same functionality as the following:

int main(void)
{
  struct S s;
  s.i = 0;
  s.f = 0.;

  set_S(&s);

  ...
}

I do not see any gain using the "One-Element-Array" approach. An expection might be if the &-key is broken on ones keyboard ... ;-)