Is int main() { } (without “void”) valid and portable in ISO C?

Question

The C standard specifies two forms of definition for main for a hosted implementation:

int main(void) { /* ... */ }

and

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Answer 1

A strong indication that int main() is meant to be valid, regardless of whether the standard accurately gives the wording to make it valid, is the fact that int main() is occasionally used in the standard without anyone raising any objection. While examples are not normative, they do indicate intent.

6.5.3.4 The sizeof and _Alignof operators

8 EXAMPLE 3 In this example, the size of a variable length array is computed and returned from a function:

#include 

size_t fsize3(int n)
{
      char b[n+3];       // variable length array
      return sizeof b;   // execution time sizeof
}

int main()
{
      size_t size;
      size = fsize3(10); // fsize3 returns 13
      return 0;
}

6.7.6.3 Function declarators (including prototypes)

20 EXAMPLE 4 The following prototype has a variably modified parameter.

void addscalar(int n, int m,
      double a[n][n*m+300], double x);

int main()
{
      double b[4][308];
      addscalar(4, 2, b, 2.17);
      return 0;
}

void addscalar(int n, int m,
      double a[n][n*m+300], double x)
{
      for (int i = 0; i < n; i++)
            for (int j = 0, k = n*m+300; j < k; j++)
                  // a is a pointer to a VLA with n*m+300 elements
                  a[i][j] += x;
}

As for the actual normative text of the standard, I think too much is being read into "equivalent". It should be pretty clear that

int main (int argc, char *argv[]) {
    (void) argc; (void) argv;
    return 0;
}

is valid, and that

int main (int x, char *y[]) {
    (void) argc; (void) argv;
    return 0;
}

is invalid. Nonetheless, the standard explicitly states in the normative text that any names may be used, meaning that int main (int argc, char *argv[]) and int main (int x, char *y[]) count as equivalent for the purposes of 5.1.2.2.1. The strict English meaning of the word "equivalent" is not how it is meant to be read.

A somewhat looser interpretation of the word is what Keith Thompson suggests in his answer.

An equally valid even looser interpretation of the word does allow int main(): both int main(void) and int main() define main as a function returning int and taking no parameters.

Neither the standard nor any official DRs currently answer the question of which interpretation is intended, so the question is unanswerable, but the examples strongly suggest that last interpretation.