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I'm trying to compile linux kernel verison 4.14.41 and got the following error:

./arch/x86/include/asm/acpi.h: In function ‘void arch_acpi_set_pdc_bits(u32*)’:
./include/linux/build_bug.h:30:45: error: types may not be defined in ‘sizeof’ expressions
 #define BUILD_BUG_ON_ZERO(e) (sizeof(struct { int:(-!!(e)); }))
                                             ^
./arch/x86/include/asm/required-features.h:110:29: note: in expansion of macro ‘BUILD_BUG_ON_ZERO’
 #define REQUIRED_MASK_CHECK BUILD_BUG_ON_ZERO(NCAPINTS != 19)
                             ^~~~~~~~~~~~~~~~~
./arch/x86/include/asm/cpufeature.h:84:5: note: in expansion of macro ‘REQUIRED_MASK_CHECK’
     REQUIRED_MASK_CHECK       || \
     ^~~~~~~~~~~~~~~~~~~
./arch/x86/include/asm/cpufeature.h:111:32: note: in expansion of macro ‘REQUIRED_MASK_BIT_SET’
  (__builtin_constant_p(bit) && REQUIRED_MASK_BIT_SET(bit) ? 1 : \
                                ^~~~~~~~~~~~~~~~~~~~~
./arch/x86/include/asm/acpi.h:118:6: note: in expansion of macro ‘cpu_has’
  if (cpu_has(c, X86_FEATURE_EST))

I would like to know what the error means and what does sizeof(struct { int:(-!!(e)); }) returns. Is there any simpler alternative for the same.

  • What compiler are you using? – ilkkachu Jun 11 '18 at 17:23
  • I am adding c++ run time support to the library. So I'm using both g++ and gcc. This error occurred as a libsupc++ file has included the header file ./arch/x86/include/asm/acpi.h. The version of the compiler is 4.5 – Kommuru jai shankar reddy Jun 11 '18 at 17:37
2

That macro tries to produce a compile error if the argument is true (non-zero). It does that by defining a struct with a bitfield whose length becomes negative if e is true.

Based on the answers on this question on Stackoverflow, the issue is just that the trickery used in the macro isn't compatible with C++, for exactly the reason stated in the error message: unlike C, C++ doesn't allow defining a struct inside sizeof.

The Linux kernel only uses C, so they probably don't care about incompatibilities with C++ in the headers used by the kernel.

The answers to the linked question contains C++ compatible alternatives to the same effect.

  • So, what could be the alternative so that I can compile with C++. Basically what does sizeof(struct { int:(-!!(e)); }) return ? Does it return the size of an int and e combined in a struct ? please correct me if I am wrong – Kommuru jai shankar reddy Jun 11 '18 at 17:56
  • @Kommurujaishankarreddy, -(!!(e)) becomes -1 if e is non-zero, producing an error. The linked question has C++-compatible alternatives, too. – ilkkachu Jun 11 '18 at 18:13
  • Can it by doing this: #define BUILD_BUG_ON_ZERO(e) if( e!=0) return -1; – Kommuru jai shankar reddy Jun 11 '18 at 18:19
  • @ikkachu Can it be done by something like this '#ifdef __cplusplus #define BUILD_BUG_ON_ZERO(e) [ e ? -1 : 1 ] #else #define BUILD_BUG_ON_ZERO(e) (sizeof(struct { int:(-!!(e)); })) #endif – Kommuru jai shankar reddy Jun 11 '18 at 18:27

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