Argument Association

  The actual arguments of the function reference or CALL statement become associated with the corresponding dummy arguments of the FUNCTION or SUBROUTINE statement. The main rules are as follows:

• There must be the same number of actual and dummy arguments; they are associated solely by their position in the two lists. Optional arguments are not permitted in Fortran77.
• If the dummy argument is a variable, array, or procedure used as a function then the corresponding actual argument must have the same data type.
• If the dummy argument is an array then its array bounds must not be larger than those of the corresponding actual argument. Alternatively the dimension bounds of a dummy array can be passed in by means of other procedure arguments to form an adjustable. This option and the assumed-size array are both described in section 9.6.
• If the dummy argument is a character item then its length must not be greater than that of the corresponding actual argument. Alternatively there is a passed-length option for character arguments: see section 9.5.

Because program units are compiled independently, it is difficult for the compiler to check for mismatches in actual and dummy argument lists. Although mismatches could, in principle, be detected by the linker, this rarely seems to happen in practice. Errors, particularly mismatches of data type or array bounds, are especially easy to make but hard to detect. Sometimes the only indication is that the program produces the wrong answer. This shows how important it is to check procedure interfaces.