Linux Kernel Architecture

Mauerer runapp01.tex V1 - 09/04/2008 6:08pm Page 1120

Appendix A: Architecture Specifics

of the entire address space into a kernel address space and a user address space. However, this does

not apply on all architectures. (Sparc is one such exception, because it has two separate address spaces

for the kernel and userspace. AMD64 is another exception, because its virtual address space has a non-

addressable hole in the middle.) As a result, theTASK_SIZEconstant defined inasm-arch/process.h

must be used instead ofPAGE_OFFSETto determine the size of userspace.

A.5 System Calls

The mechanism for issuing system calls to perform a controlled switch from userspace to kernel space

differs on all supported platforms. However, a standard file named<asm-arch/unistd.h>is responsible

for the following two aspects relevant to system calls:

❑ It defines pre-processor constants to link the descriptors of all system calls with symbolic

constants. The constants have names such as__NR_chdirand__NR_send. Because the individual

architectures do their best to remain compatible with the descriptors of the specific native

operating system (for example, OSF/1 on Alpha, or Solaris on Sparc), the numeric values differ

from architecture to architecture.

❑ It defines functions to invoke system callsfrom within the kernel itself. Generally, a

pre-processor mechanism is used for this purpose together with an inline assembler for

automatic generation.

A.6 String Processing

Operations on strings are performed at various points in the kernel and are therefore particularly time-

critical. Because many architectures provide special assembler commands to carry out the requisite tasks,

or because manually optimized assembler code executes faster than compiler-generated code, all archi-

tectures may define various string operations of their own in<asm-arch/string.h>:

❑ int strcmp(const char * cs,const char * ct)compares two strings, character by character.

❑ int strncmp(const char * cs,const char * ct,size_t count)is similar tostrcmpbut compares

amaximumofcountcharacters.

❑ int strnicmp(const char *s1, const char *s2, size_t len)is similar tostrncmpbut compares

the individual characters regardless of case.

❑ char * strcpy(char * dest,const char *src)copies a null-terminated string fromsrctodest.

❑ char * strncpy(char * dest,const char *src,size_t count)is similar tostrcpybut restricts

the maximum copy length tocountbytes or characters.

❑ size_t strlcpy(char *dest, const char *src, size_t size)is similar tostrncpy,butthedes-

tination string is also null-terminated if the source string has more thansizecharacters.

❑ char * strcat(char * dest, const char * src)addssrctodest.

❑ char * strncat(char *dest, const char *src, size_t count)is similar tostrcat, but restricts

the operation to a maximum ofcountcopied bytes.

❑ size_t strlcat(char *dest, const char *src, size_t count)is similar tostrncat,but

restricts the length of the result (and not the number of copy operations) tocountbytes.