fs/compat_ioctl.c:1651:2: error: bit-field '<anonymous>' width not an integer constant

Is this a known problem?

I'm getting this error on linus' kernel tree build:
fs/compat_ioctl.c:1651:2: error: bit-field '<anonymous>' width not an integer constant

The offending code is:
1646 static int compat_ioctl_check_table(unsigned int xcmd) 1647 {
1648 int i;
1649 const int max = ARRAY_SIZE(ioctl_pointer) - 1;
1650
1651 BUILD_BUG_ON(max >= (1 << 16));

This "looks" right to me. Everything is a constant other than "max". Since max is declared "const", I *expect* max to behave like a constant.

I used "make V=1" to get the command than used that to preprocess the code:

hppa64-linux-gnu-gcc -Wp,-MD,fs/.compat_ioctl.o.d -nostdinc -isystem /home/dave/opt/gnu64/gcc-4.5.0/lib/gcc/hppa64-linux-gnu/4.5.0/include -I/usr/src/linux-2.6/arch/parisc/include -Iinclude -include include/generated/autoconf.h -D__KERNEL__ -Wall -Wundef -Wstrict-prototypes -Wno-trigraphs -fno-strict-aliasing -fno-common -Werror-implicit-function-declaration -Wno-format-security -fno-delete-null-pointer-checks -Os -pipe -mno-space-regs -mfast-indirect-calls -mdisable-fpregs -ffunction-sections -march=2.0 -mschedule=8000 -Wframe-larger-than=2048 -fno-stack-protector -fomit-frame-pointer -Wdeclaration-after-statement -Wno-pointer-sign -fno-strict-overflow -fno-dwarf2-cfi-asm -fconserve-stack -D"KBUILD_STR(s)=#s" -D"KBUILD_BASENAME=KBUILD_STR(compat_ioctl)" -D"KBUILD_MODNAME=KBUILD_STR(c
ompat_ioctl)" -E -o fs/.tmp_compat_ioctl.P fs/compat_ioctl.c

and I believe the same code in fs/.tmp_compat_ioctl.P is:

static int compat_ioctl_check_table(unsigned int xcmd)
{
int i;
const int max = (sizeof(ioctl_pointer) / sizeof((ioctl_pointer)[0]) + (sizeof(struct { int:-!!(__builtin_types_compatible_p(typeof(ioctl_pointer), typeof(&ioctl_pointer[0]))); }))) - 1;

((void)(sizeof(struct { int:-!!(max >= (1 << 16)); })));

I can't make sense of this since I have no clue what "int:-!!" means.

BTW, I also looked in include/linux/kernel.h:

/* Force a compilation error if condition is true */ #define BUILD_BUG_ON(condition) ((void)BUILD_BUG_ON_ZERO(condition))

...
/* Force a compilation error if condition is true, but also produce a
result (of value 0 and type size_t), so the expression can be used e.g. in a structure initializer (or where-ever else comma expressions aren't permitted). */
#define BUILD_BUG_ON_ZERO(e) (sizeof(struct { int:-!!(e); })) #define BUILD_BUG_ON_NULL(e) ((void *)sizeof(struct { int:-!!(e); }))

Source this comes from is gsyprf11:/usr/src/linux-2.6 and I just did a git pull. Please let me know if I just got unlucky and should pull again.

hth,
grant

It basically means what the error above says. :) If the expr evaluates to non-zero, bound it to 1 (ie: !!x = 1, where x > 0) and the negative sign is to give an invalid field width, so we ftbfs.

Anyway, it's basically checking to see how many elements there are in the table, and failing if there are more than expected. It looks kind of failure that these are magic numbers.

regards, Kyle