nydick@psc.edu (Dan Nydick) (01/10/91)
I have an application where I'd like to have a macro expand into
a #pragma. I tried the following:
----
#define THISISAPRAGMA #pragma something
main()
{
/* blah blah */
THISISAPRAGMA
/* blah blah */
}
----
I could imagine several interpretations for this...
1) insert and recognize the pragma in "main".
2) complain about the # in the macro definition.
3) insert "#pragma something" in main, but don't
recognize it as a pragma.
4) don't expand THISISAPRAGMA at all.
A quick scan of the C standard says that the # is illegal in
a "functionlike" definition, but didn't rule it out in an
"objectlike" definition.
Anyway, various compilers I've tried have resulted in 2, 3,
and 4. I haven't found a compiler which does option 1.
I am able to run the code through the preprocessor and then
feed the output back into the compiler to get the effect of
option 1 with one of my compilers.
Should this work? Which option is correct? How would one
define a macro which expands into a pragma? Or even, how
could one write a conditional expression so the pragma
only appears if certain symbols are defined (plain ifdefs
around the pragma are not good enough since some non-ansi
compilers will complain about #pragma even inside a "false"
condition)? Is a #pragma supposed to be significant to the
compiler or to the pre-processor?
My current workaround is to conditionally include a file
which contains the #pragma line.
Thanks in advance.
Dan Nydick, Pittsburgh Supercomputing Center
nydick@psc.eduhenry@zoo.toronto.edu (Henry Spencer) (01/12/91)
In article <1991Jan10.153117.29024@fs7.ece.cmu.edu> nydick@psc.edu (Dan Nydick) writes: >I have an application where I'd like to have a macro expand into >a #pragma... Can't be done. The wording of the standard could use a bit of cleanup, but 3.8.3 refers you to 3.8.3.4 for rescanning of both kinds of macros, and 3.8.3.4 is quite explicit: "The resulting completely macro-replaced preprocessing token sequence is not processed as a preprocessing directive even if it resembles one." >... Or even, how >could one write a conditional expression so the pragma >only appears if certain symbols are defined (plain ifdefs >around the pragma are not good enough since some non-ansi >compilers will complain about #pragma even inside a "false" >condition)? Try indenting the #pragma; most pre-ANSI compilers will not "see" a directive unless the `#' is the first character on the line: #ifdef foobar #pragma pre-ANSI compilers probably will not see this #endif >Is a #pragma supposed to be significant to the >compiler or to the pre-processor? Yes. :-) The standard does not require the separation between compiler and preprocessor; indeed, there are many implementations which integrate the preprocessor into the compiler's scanner. -- If the Space Shuttle was the answer, | Henry Spencer at U of Toronto Zoology what was the question? | henry@zoo.toronto.edu utzoo!henry
gwyn@smoke.brl.mil (Doug Gwyn) (01/12/91)
In article <1991Jan10.153117.29024@fs7.ece.cmu.edu> nydick@psc.edu (Dan Nydick) writes: >#define THISISAPRAGMA #pragma something The result of macro expansion is never rescanned to identify preprocessor directives. Find another method.