https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87593
Bug ID: 87593
Summary: conflicting format_arg attributes on a declaration
accepted
Product: gcc
Version: 9.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: msebor at gcc dot gnu.org
Target Milestone: ---
As mentioned in pr87592, when the format_arg attribute is specified with
different arguments on distinct declarations of the same function they are all
(counter-inuitively) accepted. Clang accepts just the last one.
It's possible to have a format_arg function return one or the other argument so
allowing format_arg to specify multiple arguments isn't completely meaningless,
but it seems unlikely, and it's almost certainly a mistake when the attributes
are on distinct declarations of the same function. So issuing a warning
pointing out this likely mistake would seem appropriate. Documenting the
effect of such apparently conflicting attributes would be worthwhile in any
case.
$ cat y.c && gcc -S -Wall y.c
__attribute__ ((format_arg (2))) char* g (char*, char*); // accepted
__attribute__ ((format_arg (1))) char* g (char *s, char *t) // accepted
{ return t; }
void h (void)
{
__builtin_printf (g ("%i", "%s"), 123); // -Wformat
__builtin_printf (g ("%i", "%s"), "abc"); // also -Wformat
}
y.c: In function ‘h’:
y.c:7:32: warning: format ‘%s’ expects argument of type ‘char *’, but argument
2 has type ‘int’ [-Wformat=]
7 | __builtin_printf (g ("%i", "%s"), 123); // -Wformat
| ~^ ~~~
| | |
| | int
| char *
| %d
y.c:8:26: warning: format ‘%i’ expects argument of type ‘int’, but argument 2
has type ‘char *’ [-Wformat=]
8 | __builtin_printf (g ("%i", "%s"), "abc"); // also -Wformat
| ~^ ~~~~~
| | |
| int char *
| %s
