Linux Kernel Architecture

Mauerer app03.tex V1 - 09/04/2008 6:11pm Page 1206

Appendix C: Notes on C

The program produces the following output:

wolfgang@meitner>and

And and

Because both numbers have no matching bit values at any bit position, bitwise AND returns 0 as

its result.

If 4 and 5 are used as input instead, both operators return a true value because the bits at position 2 in

both numbers are equal to 1 — in any case, both are greater than 0 (for&&).

The moral of the story (and please excuse the word play) is that AND and AND AND^8 are not always

the same.

Finally, notice that the kernel defines the auxiliary functionDECLARE_BITMAPto create a bitmap with

sufficient space to store the number of bits given by thebitsparameter:

<types.h>

#define DECLARE_BITMAP(name,bits) \

unsigned long name[BITS_TO_LONGS(bits)]

The macro automatically computes the required number oflongs in the array such that sufficient space

for all bits is available.

C.2.5 Pre-Processor Tricks

Most programmers are familiar with the pre-processor. However, the kernel uses two constructions that

are not usually needed and are therefore worthy of discussion.

Macro arguments that occur within strings are normally not replaced. If a string is to be generated from

a parameter, it is necessary to use a special pre-processor function known asstringification.Arguments

within strings that are to be replaced with their macro parameters must be prefixed by a hash mark, as in

the following example:

#define warning(text)\

printf("Warning: " #text "\n")

If the macro is used as follows:

warning(foobar not found);

the pre-processor generates the following output:

printf("Warning: " "foobar not found" "\n");

If functions (whose names are to be specified in part by macro parameters) are defined with the help of

the pre-processor, it is necessary to make use of the (concatenation) capability of the pre-processor. This

is illustrated by the following example used in the kernel to define functions for port IO with various

data types. Two hash marks are used to merge two consecutive tokens into a compound token once all

pre-processor replacements have been carried out.

(^8) Which reminds of the PL/I constructIF IF = THEN THEN THEN = ELSE ELSE ELSE = IF;.