User guide

2.1. LANGUAGE OVERVIEW 13

2.1.2 The GET Directive

A directives of the form GET "ﬁlename" is replaced by the contents of the named

ﬁle. Ear l y versions of the compiler only inserted the ﬁle up to the ﬁrst occurring

dot but now the entire ﬁle is inserted. By convention, GET directives n o r m al ly

appear on separate lines. If the ﬁlename does not end in .h or .b the extension

.h is added.

The name is looked up by ﬁrst s ear ching the current directory and t hen

the directori es speciﬁed by the environment variabl e whose nam e is held in the

rtn hdrsvar of the rootnode, b u t this can be overridden using the hdrs com-

piler option. The default environment variable for BCPL headers is BCPLHDRS

under Cintsys and POSHDRS under Cintpos. Header ﬁles are normally in the g/

directory in the root direct o r y of the curr ent syste m . To check whether the envi-

ronment variables are set cor r ect l y, enter cintsys or cintpos with the -f option

as suggested in Section 3.6.

2.1.3 Conditional Compilation

A simple mechanism, whose implementation takes fewer than 20 lines of code

in the lexical a n al ys er allows conditional skipping of lexical symbols. It uses

directives of the following form:

$$tag

$<tag

$~tag

$>tag

where tag is conditional compilatio n tag com posed of letters, digits, d ot s and

underlines. All tags are initially unset, but may be complemented usin g the $$tag

directive. All the lexical tokens between $<tag and $>tag are skipped (treated as

comments) unless the speciﬁed tag is set. All the lexical tokens between $~tag

and $>tag are skipped unless the speciﬁed tag is not set.

The following example shows how this conditional compilation feature can be

used.

$$Linux // Set the Linux conditional compilation tag

$<Linux // Include if the Linux tag is set

$<WinXP $$WinXP $>WinXP // Unset the WinXP tag if set

writef("This was compiled for Linux")

$>Linux

$<WinXP // Include if the WinXP tag is set

writef("This was compiled for Windows XP")

$>WinXP