https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78000
Bug ID: 78000
Summary: -Wimplicit-function-declaration inhibited with macro
from system headers
Product: gcc
Version: 7.0
Status: UNCONFIRMED
Keywords: diagnostic
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: glisse at gcc dot gnu.org
Target Milestone: ---
i/a.h:
#define foo bar
x.c:
#include <a.h>
int main(){
foo(42);
}
$ gcc x.c -isystem i -c
$ gcc x.c -I i -c
In file included from x.c:1:0:
x.c: In function ‘main’:
i/a.h:1:13: warning: implicit declaration of function ‘bar’
[-Wimplicit-function-declaration]
#define foo bar
^
x.c:4:3: note: in expansion of macro ‘foo’
foo(42);
^~~
This problem occurs for instance with GMP headers (installed by the
distribution in /usr/include) that have
#define mpf_out_str __gmpf_out_str
but only declare the function if stdio.h was included first. Users don't get
any indication that they are using an undeclared function.
http://stackoverflow.com/q/40069876/1918193
At least in this case, it seems that it would be better to use the location of
'foo(42)' to determine if the issue is in a system header or not. Are there
cases where the current behavior is preferred?
