Chapter 3. Language Elements and Source Form

3.1. Cray Fortran Character Set

The Cray Fortran character set contains the following characters:

• The Fortran character set, which includes the 26 uppercase letters of the alphabet, the corresponding 26 lowercase letters, and several special characters.

• The numerical digits 0, 1, 2, 3, 4, 5, 6, 7, 8, 9.

• The underscore character (_).

• The newline and tab characters, which are control characters and have no graphic representation. These characters are part of the processor-dependent control characters as allowed by the standard.

The following table shows the special characters of the Cray Fortran character set.

The 52 letters define the syntax class letter.

In most cases, the compiler is not case sensitive; lowercase letters are considered the same as uppercase letters. In the following cases, however, lowercase and uppercase letters are considered to have different data values:

• Within a character constant

• Within a quotation mark, apostrophe, or H edit descriptor

• Within a Hollerith constant (nonstandard)

The Cray Fortran Compiler considers the following two statements to be equivalent:

PRINT *, N Print *, n

The Cray Fortran Compiler distinguishes between uppercase and lowercase letters in the FILE= or NAME= specifier in an OPEN or an INQUIRE statement.

The digits 0 through 9 define the syntax class digit. The digits are assumed to be decimal numbers when used to describe a numeric value, except in binary, octal, and hexadecimal (BOZ) literal constants or input/output (I/O) records corresponding to B, O, or Z edit descriptors.

For example, in the following DATA statement, the digits of the first constant are decimal digits, those of the second constant are binary digits, and those of the third are hexadecimal digits:

DATA X, I, J / 4.89, B'1011', Z'BAC91' /

The underscore can be used to make names more readable. For example, in the identifier NUMBER_OF_CARS, each underscore is used to separate the obvious English words. It is a significant character in any name. An underscore cannot be used as the first character of a name, but it can be the last character. An underscore is also used to separate the kind value from the actual value of a literal constant (for example, 123_2).

There are 22 special characters used for operators like multiply and add. They are also used as separators or delimiters in Fortran statements. Separators and delimiters make the form of a statement unambiguous.

Fortran's treatment of uppercase and lowercase letters can lead to portability problems when calling subprograms written in other languages. The problem occurs because the standard does not specify the case of letters used for external names. The following program fragment illustrates the problem:

EXTERNAL FOO . . . CALL FOO . . . END

The Cray Fortran Compiler uses uppercase for external names, so the Cray Fortran Compiler would use FOO as the external name. If the subprogram were written in C, which is case sensitive, and if foo were written in lowercase, the external name used in C would then be different from the name produced by the Cray Fortran Compiler.

The NAME compiler directive allows you to specify a case-sensitive external name in a Fortran program. You can use this directive, for example, when writing calls to C routines. For more information on the NAME directive, see the Cray Fortran Compiler Commands and Directives Reference Manual.

The BIND(C) attribute allows you to specify case sensitive external names for Fortran and C. Refer to Section 5.6.5.