Fortran User's Guide

Directives

A compiler directive directs the compiler to do some special action. Directives are also called pragmas.

A compiler directive is inserted into the source program as one or more lines of text. Each line looks like a comment, but has additional characters that identify it as more than a comment for this compiler. For most other compilers, it is treated as a comment, so there is some code portability.

Sun-style directives are the default with f90 (and f77). To switch to Cray-style directives, use the -mp=cray compiler command-line flag.

Form of General Directive Lines

General directives have the following syntax.

!DIR$ d1, d2, ...

A general directive line is defined as follows.

A directive line starts with the 5 characters CDIR$ or !DIR$, followed by:
- A space
- A directive

Spaces before, after, or within a directive are ignored.

Letters of a directive line can be in uppercase, lowercase, or mixed.

The form varies for fixed-form and free-form source as follows.

Fixed-Form Source

Put CDIR$ or !DIR$ in columns 1 through 5.
Directives are listed in columns 7 and beyond.
Columns beyond 72 are ignored.
An initial directive line has a blank in column 6.
A continuation directive line has a nonblank in column 6.

Free-Form Source

Put !DIR$ followed by a space anywhere in the line. The !DIR$ characters are the first nonblank characters in the line (actually, non-whitespace).

Directives are listed after the space.

An initial directive line has a blank, tab, or newline in the position immediately after the !DIR$.

A continuation directive line has a character other than a blank, tab, or newline in the position immediately after the !DIR$.

Thus, !DIR$ in columns 1 through 5 works for both free-form source and fixed-form source.

`FIXED` and `FREE` Directives

These directives specify the source form of lines following the directive line.

Scope

They apply to the rest of the file in which they appear, or until the next FREE or FIXED directive is encountered.

Uses

They allow you to switch source forms within a source file.

They allow you to switch source forms for an INCLUDE file. You insert the directive at the start of the INCLUDE file. After the INCLUDE file has been processed, the source form reverts back to the form being used prior to processing the INCLUDE file.

Restrictions

The FREE/FIXED directives:

Each must appear alone on a compiler directive line (not continued).

Each can appear anywhere in your source code. Other directives must appear within the program unit they affect.

Example: A FREE directive.

!DIR$ FREE
	DO i = 1, n
		a(i) = b(i) * c(i)
	END DO

Parallelization Directives

A parallelization directive is a special comment that directs the compiler to attempt to parallelize the next DO loop. Currently there is only one parallel directive, DOALL.

The DOALL directive tells the compiler to parallelize the next loop it finds, if possible.

Form of Parallelization Directive Lines

Parallel directives have the following syntax.

!MIC$ DOALL [general parameters] [scheduling parameter]

A parallelization directive line is defined as follows.

A parallel directive starts with the CMIC$ or !MIC$, followed by:
- A space
- A directive
- For some directives, one or more parameters

Spaces before, after, or within a directive are ignored.

Letters of a parallelization directive line can be in uppercase, lowercase, or mixed.

The form varies for fixed-form and free-form source as follows.

Fixed

Put CMIC$ or !MIC$ in columns 1 through 5.
Directives are listed in columns 7 and beyond.
Columns beyond 72 are ignored.
An initial directive line has a blank in column 6.
A continuation directive line has a nonblank in column 6.

Free

Put !MIC$ followed by a space anywhere in the line. The !MIC$ characters are the first nonblank characters in the line (actually, non-whitespace).

Directives are listed after the space.

An initial directive line has a blank, tab, or newline in the position immediately after the !MIC$.

A continuation directive line has a character other than a blank, tab, or newline in the position immediately after the !MIC$.

Thus, !MIC$ in columns 1 through 5 works for both free and fixed.

Example: Directive with continuation lines (DOALL directive and parameters.)

C$PAR DOALL
!MIC$&   SHARED( a, b, c, n )
!MIC$&   PRIVATE( i )
	DO i = 1, n
		a(i) = b(i) * c(i)
	END DO

Example: Same directive and parameters, with no continuation lines.

C$PAR DOALL  SHARED( a, b, c, n )  PRIVATE( i )
	DO i = 1, n
		a(i) = b(i) * c(i)
	END DO