Linux Kernel Architecture

Mauerer app03.tex V1 - 09/04/2008 6:11pm Page 1207

Appendix C: Notes on C

include/asm-x86/io_32.h
#define BUILDIO(bwl,bw,type) \
static inline void out##bwl##_local(unsigned type value, int port) { \
asm volatile("out" #bwl " %" #bw "0, %w1" : : "a"(value), "Nd"(port)); \
}

bwlaccepts one of the three valuesb,l,orwdepending on the data type for which the function is defined.

typespecifies the corresponding C data type. The macro is called as follows to definecharoperations or

byte operations:

BUILDIO(b,b,char)

After processing by the pre-processor, the C file contains the following code (extra line breaks have been

added to improve readability):

static inline void outb_local(unsigned char value, int port) { _

_asm__ __volatile__("out" "b" " %" "b" "0, %w1"

:

: "a"(value), "Nd"(port));

}

C.2.6 Miscellaneous

There are three further items that do not fit into any of the previous categories.

Macros in the kernel very often include constructions of the following kind:

drivers/block/ataflop.c

#define FDC_WRITE(reg,val) \

do { \

dma_wd.dma_mode_status = 0x80 | (reg); \

udelay(25); \

dma_wd.fdc_acces_seccount = (val); \

MFPDELAY(); \

} while(0)

Thedostatement formally ensures that the code is executed just once when the macro is ‘‘called‘‘ and

does not alter the semantics as compared to a variant without an enclosingdoloop. The advantage

of this construction becomes clear when the macro is used inifqueries or similar language elements

as shown here:

if (condition)

FDC_WRITE(a,b);

Atfirstreading,thecodeappearstobecorrectbecausesingle-lineifbodies can — and in the kernel

usually are — used without braces. However, after macro expansion there would be a problem if the

enclosingdoconstruction were not present:

if (condition)

dma_wd.dma_mode_status = 0x80 | (reg);

udelay(25);

dma_wd.fdc_acces_seccount = (val);

MFPDELAY();