http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50581
Bug #: 50581 Summary: stdarg doesn't support array types Classification: Unclassified Product: gcc Version: 4.5.3 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c AssignedTo: unassig...@gcc.gnu.org ReportedBy: wolfg...@solfrank.net __builtin_va_arg doesn't work correctly if given an array type, as it doesn't take the promotion of array to pointer into account. When called without some -std=xxx argument, gcc complains with the message "invalid use of non-lvalue array". If it is called with e.g. -std=c99, it doesn't complain, but generates incorrect code nevertheless. The problem can be reproduced by the following program: ------------------------------------------- typedef char array[10]; array arr; void f(char *fmt, ...) { __builtin_va_list ap; __builtin_va_start(ap, fmt); __builtin_memcpy(arr, __builtin_va_arg(ap, array), sizeof arr); __builtin_va_end(ap); } ------------------------------------------- Fixing this is especially useful for types the user doesn't even know whether it is an array or not, like e.g. __builtin_va_list itself to allow for recursive variadic argument passing: ------------------------------------------- void f(char *fmt, ...) { __builtin_va_list ap, ap1; __builtin_va_start(ap, fmt); __builtin_va_copy(ap1, __builtin_va_arg(ap, __builtin_va_list)); __builtin_va_end(ap); __builtin_va_copy(ap, ap1); __builtin_va_end(ap1); __builtin_va_end(ap); } ------------------------------------------- The fix is quite easy. Just apply the following diff to gcc/c-common.c: ------------------------------------------- Index: gcc/c-common.c =================================================================== RCS file: /cvsroot/src/external/gpl3/gcc/dist/gcc/c-common.c,v retrieving revision 1.1.1.1 diff -u -r1.1.1.1 c-common.c --- gcc/c-common.c 21 Jun 2011 01:20:03 -0000 1.1.1.1 +++ gcc/c-common.c 30 Sep 2011 15:12:23 -0000 @@ -5124,6 +5124,14 @@ tree build_va_arg (location_t loc, tree expr, tree type) { + /* + * Since arrays are converted to pointers when given as function arguments, + * here we have to do the same with the type of the argument. + * XXX Does it make sense to do the same with function type arguments? + */ + if (TREE_CODE(type) == ARRAY_TYPE) + type = build_pointer_type(strip_array_types(type)); + expr = build1 (VA_ARG_EXPR, type, expr); SET_EXPR_LOCATION (expr, loc); return expr; -------------------------------------------