[Bug c++/68052] New: No printf format warnings in C++ code for macro in system headers

["gcc at blino dot org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68052

            Bug ID: 68052
           Summary: No printf format warnings in C++ code for macro in
                    system headers
           Product: gcc
           Version: 5.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gcc at blino dot org
  Target Milestone: ---

In gcc versions starting from 4.8, it seems that printf format warnings in
macros are inconsistent, depending on whether the header defining the macro is
included as a system header or not, and on the build mode (C or C++).

With gcc < 4.8 (tried gcc 4.6.3 and 4.7.2), there are printf format warnings
for both C and C++ languages, when the macro is in a system header or not.

With gcc >= 4.8 (tried 4.8.2 and 5.2.1 snapshot), there are no printf format
warnings when the build mode is C++ *and* when the macro is in a system header.
If the build mode is C or if the macro is in a non-system header, the printf
format warnings are emitted.

Here is a sample test case:
$ mkdir -p syshdr_dir
$ cat <<EOF > syshdr_dir/myprintf.h
#include <stdio.h>
#define MY_PRINTF(x, ...) printf(x, ## __VA_ARGS__);
EOF

$ cat <<EOF > test-format-printf.c
#include <myprintf.h>
int main(void) {
    long long ll = 0;
    MY_PRINTF("%ld", ll);
    return 0;
}
EOF

With gcc 4.6.3 and 4.7.2, I get printf format warnings for both C and C++
languages.

$ gcc -isystem $PWD/syshdr_dir -Wall -Werror -x c++ -c test-format-printf.c -o
/dev/null
test-format-printf.c: In function âint main()â:
test-format-printf.c:4:5: error: format â%ldâ expects argument of type âlong
intâ, but argument 2 has type âlong long intâ [-Werror=format]
cc1plus: all warnings being treated as errors

$ gcc -isystem $PWD/syshdr_dir -Wall -Werror -x c -c test-format-printf.c -o
/dev/null
test-format-printf.c: In function âmainâ:
test-format-printf.c:4:5: error: format â%ldâ expects argument of type âlong
intâ, but argument 2 has type âlong long intâ [-Werror=format]
cc1: all warnings being treated as errors


With gcc 4.8.2 and a 5.2.1 snapshot, there is a warning for C, but no warning
for C++ when the macro is in a system header.

$ gcc -isystem $PWD/syshdr_dir -Wall -Werror -x c++ -c test-format-printf.c -o
/dev/null

$ gcc -isystem $PWD/syshdr_dir -Wall -Werror -x c -c test-format-printf.c -o
/dev/null
In file included from test-format-printf.c:1:0:
test-format-printf.c: In function âmainâ:
test-format-printf.c:4:15: error: format â%ldâ expects argument of type âlong
intâ, but argument 2 has type âlong long intâ [-Werror=format=]
     MY_PRINTF("%ld", ll);
               ^
cc1: all warnings being treated as errors

$ gcc -I $PWD/syshdr_dir -Wall -Werror -x c++ -c test-format-printf.c -o
/dev/null
In file included from test-format-printf.c:1:0:
test-format-printf.c: In function âint main()â:
/home/sah0146/dev/workspaces/bcm7252/hgw/build/MAIN/syshdr_dir/myprintf.h:2:51:
error: format â%ldâ expects argument of type âlong intâ, but argument 2 has
type âlong long intâ [-Werror=format=]
 #define MY_PRINTF(x, ...) printf(x, ## __VA_ARGS__);
                                                   ^
test-format-printf.c:4:5: note: in expansion of macro âMY_PRINTFâ
     MY_PRINTF("%ld", ll);
     ^
cc1plus: all warnings being treated as errors