Compatible functions

For two function types to be compatible, they must meet the following requirements:

• They must agree in the number of parameters (and use of ellipsis).
• They must have compatible return types.
• The corresponding parameters must be compatible with the type that results from the application of the default argument promotions.

The composite type of two function types is determined as follows:

• If one of the function types has a parameter type list, the composite type is a function prototype with the same parameter type list.
• If both function types have parameter type lists, the composite type of each parameter is determined as follows:
  • The composite of parameters of different rank is the type that results from the application of the default argument promotions.
  • The composite of parameters with array or function type is the adjusted type.
  • The composite of parameters with qualified type is the unqualified version of the declared type.

For example, for the following two function declarations:

int f(int (*)(), double (*)[3]); 
int f(int (*)(char *), double (*)[]);

The resulting composite type would be:

int f(int (*)(char *), double (*)[3]);

If the function declarator is not part of the function declaration, the parameters may have incomplete type. The parameters may also specify variable length array types by using the [*] notation in their sequences of declarator specifiers. The following are examples of compatible function prototype declarators:

double maximum(int n, int m, double a[n][m]);
double maximum(int n, int m, double a[*][*]);
double maximum(int n, int m, double a[ ][*]);
double maximum(int n, int m, double a[ ][m]);