Open Watcom FORTRAN 77 Language Reference

Functions and Subroutines
12.4 External Functions
An external function is a program unit that has a FUNCTION statement as its first statement. It is defined
externally to the program units that reference it. The form of a FUNCTION statement is defined in the
chapter entitled "FORTRAN Statements" on page 9.
The name of an external function is treated as if it was a variable. It is through the function name that the
result of an external function becomes defined. This variable must become defined before the execution of
the external function is completed. Once defined, it can be referenced or redefined. The value of this
variable when a RETURN or END statement is executed is the result returned by the external function.
Example:
INTEGER FUNCTION VECSUM( A, N )
INTEGER A(N), I
VECSUM = 0
DO 10 I = 1, N
VECSUM = VECSUM + A(I)
10 CONTINUE
END
If the variable representing the return value of the external function is of type CHARACTER with a length
specification of (*), it must not be the operand of a concatenation operator unless it appears in a character
assignment statement.
It is also possible for an external function to return results through its dummy arguments by assigning to
them. The following example demonstrates this.
External Functions 271