According to Linux's vsnprintf man page, va_start and va_end should occur within the for(;;) loop used to calculate the result buffer's length. Patching xdebug_str.c as follows fixes the problem, and I am now able to step through PHP scripts with Komodo(!!!) using 64-bit PHP CLI and Xdebug.

Note, though, that this may not work with older versions of glibc or with other architecutres' or operating systems' versions of vsnprintf.

Index: xdebug_str.c

RCS file: /repository/xdebug/xdebug_str.c,v
retrieving revision 1.5
diff -u -r1.5 xdebug_str.c
--- xdebug_str.c 17 Dec 2003 20:50:27 -0000 1.5
+++ xdebug_str.c 6 Oct 2004 18:24:19 -0000
@@ -88,9 +88,10 @@

    new_str = (char *) xdmalloc(size);

• va_start(args, fmt);
  for (;;) {
• va_start(args, fmt);
  int n = vsnprintf(new_str, size, fmt, args);
• va_end(args);
  if (n > -1 && n < size) {
  break;
  }
  @@ -101,7 +102,6 @@
  }
  new_str = (char *) xdrealloc(new_str, size);
  }

• va_end(args);

  return new_str;

  }