https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109516
Bug ID: 109516
Summary: warning: format '%lx' expects argument of type 'long
unsigned int', but argument 2 has type 'const long
unsigned int:48' [-Wformat=]
Product: gcc
Version: 13.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: yann at droneaud dot fr
Target Milestone: ---
I've discovered today an unexpected format related warning when asking to print
a bitfield over unsigned long:
$ cat > fmt-bitfield.c <<EOF
#include <inttypes.h>
#include <stdint.h>
#include <stdio.h>
struct struct24 { uint32_t uint24:24; };
void print24(const struct struct24 *s)
{ printf("%" PRIx32 "\n", s->uint24); }
struct struct48 { uint64_t uint48:48; };
void print48(const struct struct48 *s)
{ printf("%" PRIx64 "\n", s->uint48); }
EOF
$ gcc -Wformat -c fmt-bitfield.c
fmt-bitfield.c: In function 'print48':
fmt-bitfield.c:13:10: warning: format '%lx' expects argument of type 'long
unsigned int', but argument 2 has type 'long unsigned int:48' [-Wformat=]
13 | { printf("%" PRIx64 "\n", s->uint48); }
| ^~~ ~~~~~~~~~
| |
| long unsigned int:48
In file included from fmt-bitfield.c:1:
/usr/include/inttypes.h:121:41: note: format string is defined here
121 | # define PRIx64 __PRI64_PREFIX "x"
See https://godbolt.org/z/r44e5rfnh
I understood that, according to C standard, bitfield over type other that int
or bool is an implementation defined behavior.
But GCC is expected to support bitfield over long as well:
https://gcc.gnu.org/onlinedocs/gcc/Structures-unions-enumerations-and-bit-fields-implementation.html
- Allowable bit-field types other than _Bool, signed int, and unsigned int
(C99 and C11 6.7.2.1).
Other integer types, such as long int, and enumerated types are permitted
even in strictly conforming mode.
So I believe the warning is spurious, and formatting a value whose type is a
bitfield over long should not trigger the warning when using %l format
modifier.
(For the record, clang doesn't report such warning).
