The ccomp processor


Introduction

Questaal’s codes are compiled with a source code preprocessor, ccomp. This tool, written in C, performs a similar function to the conditional compilation preprocessing step found in modern programming languages, e.g.

# if expr
# else
# endif

It was written long before conditional compilation became part of FORTRAN. Questaal still uses it in part to retain backwards compatibility, but also because it retains some advantages (as well as some disadvantages).


How ccomp works

ccomp mimics the C language conditional compilation constructs such as the (#ifdef, #else, #endif) group, in a fortran-77 compatible way.

FORTRAN statements beginning with C# are preprocessor directives. The ones implemented are:

C#define   name
C#include  file-name
C#ifdef    expr
C#ifndef   expr
C#else
C#elseif   expr
C#endif 

Boolean expressions

C#define defines a name, to be used later in boolean expressions.

Names can also be defined through command line switches: when they appear in a boolean expression they evaluate to true if they exist, else false. Thus the expression john & bill evaluates to true if john and bill had both been previously defined; otherwise false. Boolean expressions can make use of the AND (&) and OR (|) operators. Precedence of operators is strictly left to right, so that john | bill & mike is equivalent to (john | bill) & mike, whereas john & bill | mike is equivalent to (john & bill) | mike.

Note: ccomp distinguishes case in the names (as does C).

Running ccomp

You can also define (and undefine) names on the command line. Names so defined (undefined) take precedence over subsequent occurences of C#define in the source file. Thus if a name is defined in the source code but undefined on the command line, it is undefined.

ccomp writes to standard output, unless a destination filename is supplied. For example:

ccomp -dMPI -uMPIK source.f dest.f

reads file source.f and writes the modified file to dest.f. MPI is defined, MPIK is undefined.

To see arguments ccomp, accepts, type ccomp --h.

Conditional commenting or uncommenting of lines

C#ifdef, C#ifndef, and C#elseif are followed by a boolean expression, e.g.

C#ifdef CRAY
C#ifndef john | bill & mike

At any point in parsing the source file, ccomp is in a true or false state, depending on the result of the last boolean expression evaluated. In addition to knowing the ‘current’ state, it knows what the ‘prior’ state of the original code is at that point, i.e. what the state would be if given the names as defined in the source file.

Any time a new directive such as C#ifdef is encountered, the current and prior states are recalculated. For lines between conditional compilation directives (e.g. a C#ifdef/C#else/C#endif block), ccomp does one of the following:

  • If the current and prior states are the same it outputs the line unmodified
  • If the current state is true and the prior is false, it comments out the line
  • If the current state is false and the prior is true, it removes a C from the first column.
  • If the C is missing, ccomp exits with an error message.

Comments use the fortran-77 convention (C in the first column).

Applications of ccomp

Programs lmf, lm, lmdos, lmchk, etc. derive the same source code, lmv7.f. Any one of them can be got from the other by suitable definitions of names. ccomp is used extensively to create new branches code, to and customize code specific to certain compilers, either for optimization purposes or to avoid compiler bugs.

Other resources

The source code to ccomp can be found here.


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