bug#14572: Goto Label Bug

[Mark H Weaver]

Shane Celis <address@hidden> writes:

> If a goto label is followed by an SCM variable declaration, a
> compilation error will result.  I've written up this piece of code
> that demonstrates the behavior and how to work around it, available
> here:
>
> https://gist.github.com/shanecelis/5728982

In the future, please send the relevant information as attachments, so
that it's recorded in our bug tracking system.  For posterity, here it
is:

--8<---------------cut here---------------start------------->8---
/*
   This file demonstrates a bug with C goto labels and Guile.

$ gcc `pkg-config guile-2.0 --cflags --libs` goto_label_bug.c -o goto_label_bug
goto_label_bug.c: In function 'main':
goto_label_bug.c:10: error: expected expression before 'SCM'

$ gcc `pkg-config guile-2.0 --cflags --libs` goto_label_bug.c -o goto_label_bug 
-D INSERT_NOOP
$ # No problem.

Shane Celis

 */

#include <libguile.h>

int main(int argc, char *argv[]) {
  scm_init_guile();
  goto end;

end:
#ifdef INSERT_NOOP
  ;                             /* No error with dummy statement. */
#endif
  SCM dummy = SCM_BOOL_T;       /* Causes error. */

  return 0;
}
--8<---------------cut here---------------end--------------->8---

This is unrelated to Guile.  The same problem happens with GCC 4.7 if
you make the following substitutions:

  libguile.h       --> stdio.h
  scm_init_guile() --> printf ("Test\n");
  SCM              --> int
  SCM_BOOL_T       --> 0

However, in that case the error message is more helpful:

  error: a label can only be part of a statement and a declaration is not a 
statement

Indeed, C99 section 6.8.1 specifies that labels may only precede
statements.

    Regards,
      Mark