Linux Kernel Architecture

Mauerer app03.tex V1 - 09/04/2008 6:11pm Page 1205

Appendix C: Notes on C

As expected, the program produces the following output:

wolfgang@meitner>./bitmask

Bit 5 in val2 is set

It is, of course, possible to mask out not just one but several bits of a number by designing an appropriate
mask. If, for example, the last 5 bits of a number are to be analyzed, a mask can be designed in which the
first bit is moved to position 6 and then 1 is subtracted from the resulting number.

Bit position numbering begins at 0 as usual. Thebits at positions 0 to 5 are equal to 1 as a result of

subtraction, while the bits at positions 6 to 31 are equal to 0.

The desired bits in a bit string can be selected by ANDing as shown here:

int main() {

unsigned int val = 49;

unsigned int res;

unsigned int mask = 1;

mask <<= 5;

mask -= 1; printf("mask: %u\n", mask);

res = val & mask;

printf("val, res: %u, %u\n", val, res);

}

The program generates the following output:

wolfgang@meitner>./maskfive

mask: 31

val, res: 49, 17

A common programming error occurs when the operators&&(logical AND) and&

(bitwise AND) are confused. Because the former checks only whether both

arguments are greater than 0 and the latter performs a bitwise comparison, they

return different results.

The following example illustrates the difference between both operations:

int main() {

int val1 = 4;

int val2 = 8;

if (val1 & val2) {

printf("And\n");

}

if (val1 && val2) {

printf("And and\n");

}

}