1018 lines
30 KiB
Plaintext
1018 lines
30 KiB
Plaintext
Implementing a System Call on Linux 2.6 for i386
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Amit Choudhary
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<amit2030 AT gmail DOT com>
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2006-10-27
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Revision History
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Revision 1.0 2006-10-27 Revised by: ac
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official release
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This HOWTO describes information about implementing system calls in the
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Linux operating system for i386 architecture with 2.6 kernels. General
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information about the used and modified files is available as well as
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description of the source and makefiles and test user space C program too.
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-----------------------------------------------------------------------------
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Table of Contents
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1. Introduction
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2. System call
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3. List of files to be modified/created
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4. Kernel files to be modified
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5. syscall_table.S
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6. unistd.h
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7. syscalls.h
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8. Makefile
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9. New kernel files/directories to be created
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10. mycall
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11. mycall.c
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12. Makefile
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13. New user space files, to be created, to test our system call
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14. testmycall.h (new user space header file to be created)
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15. testmycall.c (new user space source file to be created)
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16. _syscallN macro
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17. Testing our new system call
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18. Questions And Suggestions
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19. Copyright
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20. License
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1. Introduction
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This document explains how to implement a new system call on linux 2.6. A
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system call is used by application (user) programs to request service from
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the operating system.
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-----------------------------------------------------------------------------
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2. System call
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A system call is used by application (user) programs to request service from
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the operating system. The following statements illustrate why system calls
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are needed. An operating system can access a system's hardware directly, but
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a user program is not given direct access to the hardware. This is done so
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that the kernel can keep the system safe and secure from malicious user
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programs. But often, a user program requires some information from the
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hardware (e.g., from a web camera to show you the picture), but it cannot get
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the information directly. So, it requests the operating system to supply it
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the information. This request is made by using an appropriate system call.
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A system call executes in the kernel mode. Every system call has a number
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associated with it. This number is passed to the kernel and that's how the
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kernel knows which system call was made. When a user program issues a system
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call, it is actually calling a library routine. The library routine issues a
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trap to the Linux operating system by executing INT 0x80 assembly
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instruction. It also passes the system call number to the kernel using the
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EAX register. The arguments of the system call are also passed to the kernel
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using other registers (EBX, ECX, etc.). The kernel executes the system call
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and returns the result to the user program using a register. If the system
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call needs to supply the user program with large amounts of data, it will use
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another mechanism (e.g., copy_to_user call).
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-----------------------------------------------------------------------------
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3. List of files to be modified/created
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Assume that your linux source base directory is /usr/src/linux.
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Kernel files to be modified are listed below:
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1. /usr/src/linux/arch/i386/kernel/syscall_table.S
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2. /usr/src/linux/include/asm-i386/unistd.h
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3. /usr/src/linux/include/linux/syscalls.h
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4. /usr/src/linux/Makefile
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New kernel files/directories to be created are listed below:
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1. /usr/src/linux/mycall - Directory that will contain the source file,
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header file and the Makefile for our system call (You can also implement your
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system call in an existing file).
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2. /usr/src/linux/mycall/mycall.c - Source file containing our system call
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code.
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3. /usr/src/linux/mycall/Makefile - Makefile
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New user space files, to be created, to test our system call are listed
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below:
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1. testmycall.c – Source file that will call our system call.
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2. testmycall.h - Header file.
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-----------------------------------------------------------------------------
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4. Kernel files to be modified
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-----------------------------------------------------------------------------
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5. syscall_table.S
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Full path of the file - /usr/src/linux/arch/i386/kernel/syscall_table.S
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This file contains system call names.
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1. Add a line to the end of this file (Let's assume that the name of our
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system call is mycall).
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2. Add ".long sys_mycall" at the end of the list.
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-----------------------------------------------------------------------------
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6. unistd.h
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Full path of the file - /usr/src/linux/include/asm-i386/unistd.h
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This file contains the system call number that is passed to the kernel
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through the register (EAX) when a system call is invoked.
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1. Add "#define __NR_mycall <Last_System_Call_Num + 1>" at the end of the
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list.
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If the last system call defined here is:
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"#define __NR_vmsplice316", then add:
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"#define __NR_mycall317" at the end of the list.
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1. Increment the "NR_syscalls" by 1. So, if NR_syscalls is defined as:
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"#define NR_syscalls 317", then change it to:
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"#define NR_syscalls 318"
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-----------------------------------------------------------------------------
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7. syscalls.h
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Full path of the file - /usr/src/linux/include/linux/syscalls.h
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This file contain the declarations for system calls.
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1. Add the following line at the end of the file:
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"asmlinkage long sys_mycall(int i);"
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-----------------------------------------------------------------------------
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8. Makefile
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Full path of the file - /usr/src/linux/Makefile
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1. Add mycall/ to core-y (Search for regex: core-y.*+=). You will be
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creating this directory. This directory will contain the source file,
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header file and the Makefile for our system call.
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-----------------------------------------------------------------------------
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9. New kernel files/directories to be created
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-----------------------------------------------------------------------------
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10. mycall
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Full path of the file - /usr/src/linux/mycall
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1. Create a new directory in /usr/src/linux and name it "mycall".
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-----------------------------------------------------------------------------
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11. mycall.c
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Full path of the file - /usr/src/linux/mycall/mycall.c
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1. Create a source file named "mycall.c" in dir "mycall". mycall.c will have
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the code for our system call. The definition of the system call in the
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source file would be asmlinkage long sys_mycall(...){...} . It should
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include the file linux/linkage.h So, the file "mycall.c" will look like:
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/*---Start of mycall.c----*/
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#include<linux/linkage.h>
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asmlinkage long sys_mycall(int i)
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{
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return i+10;
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}
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/*---End of mycall.c------*/
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What is asmlinkage?
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Asmlinkage is used to look for the arguments on the kernel stack.
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-----------------------------------------------------------------------------
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12. Makefile
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Full path of the file - /usr/src/linux/mycall/Makefile
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The Makefile in dir "mycall" will have only one line:
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#####Makefile Start#####
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obj-y := mycall.o
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#####Makefile End#######
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-----------------------------------------------------------------------------
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13. New user space files, to be created, to test our system call
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-----------------------------------------------------------------------------
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14. testmycall.h (new user space header file to be created)
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testmycall.h
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1. Create a header file called testmycall.h. This header file should be
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included by any program calling our system call.
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2. Add three lines to it
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1. Line 1: This is needed because we need the definition of _syscall1.
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#include<linux/unistd.h>
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1. Line 2: This is needed because we need the number of our system call.
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#define __NR_mycall 317
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1. Line 3: This is needed for system calls with 1 argument. It is explained
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in detail below.
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_syscall1(long, mycall, int, i)
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So, our user header file looks like:
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/*---Start of header file------*/
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#include<linux/unistd.h>
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#define __NR_mycall 317
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_syscall1(long, mycall, int, i)
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/*---End of header file--------*/
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-----------------------------------------------------------------------------
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15. testmycall.c (new user space source file to be created)
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testmycall.c
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1. Create a C file called testmycall.c in the same directory as
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testmycall.h. The C file will look like:
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/*---Start of C file------*/
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#include<stdio.h>
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#include "testmycall.h"
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int main(void)
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{
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printf("%d\n", mycall(15));
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}
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/*---End of C file------*/
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-----------------------------------------------------------------------------
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16. _syscallN macro
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1. _syscall0(int,mycall) indicates that:
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2. The name of the system call is mycall.
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3. It takes zero arguments.
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4. It returns an int.
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1. _syscall1(int,mycall,int,number) indicates that:
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2. The name of the system call is mycall.
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3. It takes one argument.
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4. The argument is an int named number.
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5. It returns an int.
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When you expand _syscall1(long,mycall,int,i), you get the following code:
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long mycall(int i)
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{
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return syscall(__NR_mycall, i);
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}
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But the definition of _syscallN macros are different in the kernel. You can
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look at /usr/src/linux/include/asm-i386/unistd.h for the definition.
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-----------------------------------------------------------------------------
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17. Testing our new system call
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1. Step 1: Recompile and install the new kernel so that our system call
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becomes available to the operating system.
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2. Step 2: Compile and execute the user space C file (testmycall.c) that we
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created above.
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3. RESULT: You should see the output as 25. This has been tested on kernel
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2.6.17.13.
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-----------------------------------------------------------------------------
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18. Questions And Suggestions
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1. Please send a mail to Amit Choudhary (amit2030 AT gmail DOT com).
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-----------------------------------------------------------------------------
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19. Copyright
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Copyright (c) 2006 by Amit Choudhary.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.2
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or any later version published by the Free Software Foundation;
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with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
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Texts. A copy of the license is included in the section entitled "GNU
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Free Documentation License".
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-----------------------------------------------------------------------------
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20. License
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GNU Free Documentation License
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Version 1.2, November 2002
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Copyright (C) 2000,2001,2002 Free Software Foundation, Inc.
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51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
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Everyone is permitted to copy and distribute verbatim copies
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of this license document, but changing it is not allowed.
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0. PREAMBLE
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The purpose of this License is to make a manual, textbook, or other
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functional and useful document "free" in the sense of freedom: to
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assure everyone the effective freedom to copy and redistribute it,
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with or without modifying it, either commercially or noncommercially.
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Secondarily, this License preserves for the author and publisher a way
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to get credit for their work, while not being considered responsible
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for modifications made by others.
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This License is a kind of "copyleft", which means that derivative
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works of the document must themselves be free in the same sense. It
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complements the GNU General Public License, which is a copyleft
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license designed for free software.
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We have designed this License in order to use it for manuals for free
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software, because free software needs free documentation: a free
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program should come with manuals providing the same freedoms that the
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software does. But this License is not limited to software manuals;
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it can be used for any textual work, regardless of subject matter or
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whether it is published as a printed book. We recommend this License
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principally for works whose purpose is instruction or reference.
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1. APPLICABILITY AND DEFINITIONS
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This License applies to any manual or other work, in any medium, that
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contains a notice placed by the copyright holder saying it can be
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distributed under the terms of this License. Such a notice grants a
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world-wide, royalty-free license, unlimited in duration, to use that
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refers to any such manual or work. Any member of the public is a
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licensee, and is addressed as "you". You accept the license if you
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copy, modify or distribute the work in a way requiring permission
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under copyright law.
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A "Modified Version" of the Document means any work containing the
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Document or a portion of it, either copied verbatim, or with
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modifications and/or translated into another language.
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A "Secondary Section" is a named appendix or a front-matter section of
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the Document that deals exclusively with the relationship of the
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publishers or authors of the Document to the Document's overall subject
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textbook of mathematics, a Secondary Section may not explain any
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mathematics.) The relationship could be a matter of historical
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connection with the subject or with related matters, or of legal,
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The "Invariant Sections" are certain Secondary Sections whose titles
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The "Cover Texts" are certain short passages of text that are listed,
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as Front-Cover Texts or Back-Cover Texts, in the notice that says that
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the Document is released under this License. A Front-Cover Text may
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be at most 5 words, and a Back-Cover Text may be at most 25 words.
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A "Transparent" copy of the Document means a machine-readable copy,
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represented in a format whose specification is available to the
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An image format is not Transparent if used for any substantial amount
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Examples of suitable formats for Transparent copies include plain
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include proprietary formats that can be read and edited only by
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The "Title Page" means, for a printed book, the title page itself,
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The Document may include Warranty Disclaimers next to the notice which
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Disclaimers are considered to be included by reference in this
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no effect on the meaning of this License.
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2. VERBATIM COPYING
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You may copy and distribute the Document in any medium, either
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commercially or noncommercially, provided that this License, the
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copyright notices, and the license notice saying this License applies
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to the Document are reproduced in all copies, and that you add no other
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conditions whatsoever to those of this License. You may not use
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You may also lend copies, under the same conditions stated above, and
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you may publicly display copies.
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3. COPYING IN QUANTITY
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If you publish printed copies (or copies in media that commonly have
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Document's license notice requires Cover Texts, you must enclose the
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Texts: Front-Cover Texts on the front cover, and Back-Cover Texts on
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||
visible. You may add other material on the covers in addition.
|
||
|
||
Copying with changes limited to the covers, as long as they preserve
|
||
|
||
the title of the Document and satisfy these conditions, can be treated
|
||
|
||
as verbatim copying in other respects.
|
||
|
||
If the required texts for either cover are too voluminous to fit
|
||
|
||
legibly, you should put the first ones listed (as many as fit
|
||
|
||
reasonably) on the actual cover, and continue the rest onto adjacent
|
||
|
||
pages.
|
||
|
||
If you publish or distribute Opaque copies of the Document numbering
|
||
|
||
more than 100, you must either include a machine-readable Transparent
|
||
|
||
copy along with each Opaque copy, or state in or with each Opaque copy
|
||
|
||
a computer-network location from which the general network-using
|
||
|
||
public has access to download using public-standard network protocols
|
||
|
||
a complete Transparent copy of the Document, free of added material.
|
||
|
||
If you use the latter option, you must take reasonably prudent steps,
|
||
|
||
when you begin distribution of Opaque copies in quantity, to ensure
|
||
|
||
that this Transparent copy will remain thus accessible at the stated
|
||
|
||
location until at least one year after the last time you distribute an
|
||
|
||
Opaque copy (directly or through your agents or retailers) of that
|
||
|
||
edition to the public.
|
||
|
||
It is requested, but not required, that you contact the authors of the
|
||
|
||
Document well before redistributing any large number of copies, to give
|
||
|
||
them a chance to provide you with an updated version of the Document.
|
||
|
||
4. MODIFICATIONS
|
||
|
||
You may copy and distribute a Modified Version of the Document under
|
||
|
||
the conditions of sections 2 and 3 above, provided that you release
|
||
|
||
the Modified Version under precisely this License, with the Modified
|
||
|
||
Version filling the role of the Document, thus licensing distribution
|
||
|
||
and modification of the Modified Version to whoever possesses a copy
|
||
|
||
of it. In addition, you must do these things in the Modified Version:
|
||
|
||
A. Use in the Title Page (and on the covers, if any) a title distinct
|
||
|
||
from that of the Document, and from those of previous versions
|
||
|
||
(which should, if there were any, be listed in the History section
|
||
|
||
of the Document). You may use the same title as a previous version
|
||
|
||
if the original publisher of that version gives permission.
|
||
|
||
B. List on the Title Page, as authors, one or more persons or entities
|
||
|
||
responsible for authorship of the modifications in the Modified
|
||
|
||
Version, together with at least five of the principal authors of the
|
||
|
||
Document (all of its principal authors, if it has fewer than five),
|
||
|
||
unless they release you from this requirement.
|
||
|
||
C. State on the Title page the name of the publisher of the
|
||
|
||
Modified Version, as the publisher.
|
||
|
||
D. Preserve all the copyright notices of the Document.
|
||
|
||
E. Add an appropriate copyright notice for your modifications
|
||
|
||
adjacent to the other copyright notices.
|
||
|
||
F. Include, immediately after the copyright notices, a license notice
|
||
|
||
giving the public permission to use the Modified Version under the
|
||
|
||
terms of this License, in the form shown in the Addendum below.
|
||
|
||
G. Preserve in that license notice the full lists of Invariant Sections
|
||
|
||
and required Cover Texts given in the Document's license notice.
|
||
|
||
H. Include an unaltered copy of this License.
|
||
|
||
I. Preserve the section Entitled "History", Preserve its Title, and add
|
||
|
||
to it an item stating at least the title, year, new authors, and
|
||
|
||
publisher of the Modified Version as given on the Title Page. If
|
||
|
||
there is no section Entitled "History" in the Document, create one
|
||
|
||
stating the title, year, authors, and publisher of the Document as
|
||
|
||
given on its Title Page, then add an item describing the Modified
|
||
|
||
Version as stated in the previous sentence.
|
||
|
||
J. Preserve the network location, if any, given in the Document for
|
||
|
||
public access to a Transparent copy of the Document, and likewise
|
||
|
||
the network locations given in the Document for previous versions
|
||
|
||
it was based on. These may be placed in the "History" section.
|
||
|
||
You may omit a network location for a work that was published at
|
||
|
||
least four years before the Document itself, or if the original
|
||
|
||
publisher of the version it refers to gives permission.
|
||
|
||
K. For any section Entitled "Acknowledgements" or "Dedications",
|
||
|
||
Preserve the Title of the section, and preserve in the section all
|
||
|
||
the substance and tone of each of the contributor acknowledgements
|
||
|
||
and/or dedications given therein.
|
||
|
||
L. Preserve all the Invariant Sections of the Document,
|
||
|
||
unaltered in their text and in their titles. Section numbers
|
||
|
||
or the equivalent are not considered part of the section titles.
|
||
|
||
M. Delete any section Entitled "Endorsements". Such a section
|
||
|
||
may not be included in the Modified Version.
|
||
|
||
N. Do not retitle any existing section to be Entitled "Endorsements"
|
||
|
||
or to conflict in title with any Invariant Section.
|
||
|
||
O. Preserve any Warranty Disclaimers.
|
||
|
||
If the Modified Version includes new front-matter sections or
|
||
|
||
appendices that qualify as Secondary Sections and contain no material
|
||
|
||
copied from the Document, you may at your option designate some or all
|
||
|
||
of these sections as invariant. To do this, add their titles to the
|
||
|
||
list of Invariant Sections in the Modified Version's license notice.
|
||
|
||
These titles must be distinct from any other section titles.
|
||
|
||
You may add a section Entitled "Endorsements", provided it contains
|
||
|
||
nothing but endorsements of your Modified Version by various
|
||
|
||
parties--for example, statements of peer review or that the text has
|
||
|
||
been approved by an organization as the authoritative definition of a
|
||
|
||
standard.
|
||
|
||
You may add a passage of up to five words as a Front-Cover Text, and a
|
||
|
||
passage of up to 25 words as a Back-Cover Text, to the end of the list
|
||
|
||
of Cover Texts in the Modified Version. Only one passage of
|
||
|
||
Front-Cover Text and one of Back-Cover Text may be added by (or
|
||
|
||
through arrangements made by) any one entity. If the Document already
|
||
|
||
includes a cover text for the same cover, previously added by you or
|
||
|
||
by arrangement made by the same entity you are acting on behalf of,
|
||
|
||
you may not add another; but you may replace the old one, on explicit
|
||
|
||
permission from the previous publisher that added the old one.
|
||
|
||
The author(s) and publisher(s) of the Document do not by this License
|
||
|
||
give permission to use their names for publicity for or to assert or
|
||
|
||
imply endorsement of any Modified Version.
|
||
|
||
5. COMBINING DOCUMENTS
|
||
|
||
You may combine the Document with other documents released under this
|
||
|
||
License, under the terms defined in section 4 above for modified
|
||
|
||
versions, provided that you include in the combination all of the
|
||
|
||
Invariant Sections of all of the original documents, unmodified, and
|
||
|
||
list them all as Invariant Sections of your combined work in its
|
||
|
||
license notice, and that you preserve all their Warranty Disclaimers.
|
||
|
||
The combined work need only contain one copy of this License, and
|
||
|
||
multiple identical Invariant Sections may be replaced with a single
|
||
|
||
copy. If there are multiple Invariant Sections with the same name but
|
||
|
||
different contents, make the title of each such section unique by
|
||
|
||
adding at the end of it, in parentheses, the name of the original
|
||
|
||
author or publisher of that section if known, or else a unique number.
|
||
|
||
Make the same adjustment to the section titles in the list of
|
||
|
||
Invariant Sections in the license notice of the combined work.
|
||
|
||
In the combination, you must combine any sections Entitled "History"
|
||
|
||
in the various original documents, forming one section Entitled
|
||
|
||
"History"; likewise combine any sections Entitled "Acknowledgements",
|
||
|
||
and any sections Entitled "Dedications". You must delete all sections
|
||
|
||
Entitled "Endorsements".
|
||
|
||
6. COLLECTIONS OF DOCUMENTS
|
||
|
||
You may make a collection consisting of the Document and other documents
|
||
|
||
released under this License, and replace the individual copies of this
|
||
|
||
License in the various documents with a single copy that is included in
|
||
|
||
the collection, provided that you follow the rules of this License for
|
||
|
||
verbatim copying of each of the documents in all other respects.
|
||
|
||
You may extract a single document from such a collection, and distribute
|
||
|
||
it individually under this License, provided you insert a copy of this
|
||
|
||
License into the extracted document, and follow this License in all
|
||
|
||
other respects regarding verbatim copying of that document.
|
||
|
||
7. AGGREGATION WITH INDEPENDENT WORKS
|
||
|
||
A compilation of the Document or its derivatives with other separate
|
||
|
||
and independent documents or works, in or on a volume of a storage or
|
||
|
||
distribution medium, is called an "aggregate" if the copyright
|
||
|
||
resulting from the compilation is not used to limit the legal rights
|
||
|
||
of the compilation's users beyond what the individual works permit.
|
||
|
||
When the Document is included in an aggregate, this License does not
|
||
|
||
apply to the other works in the aggregate which are not themselves
|
||
|
||
derivative works of the Document.
|
||
|
||
If the Cover Text requirement of section 3 is applicable to these
|
||
|
||
copies of the Document, then if the Document is less than one half of
|
||
|
||
the entire aggregate, the Document's Cover Texts may be placed on
|
||
|
||
covers that bracket the Document within the aggregate, or the
|
||
|
||
electronic equivalent of covers if the Document is in electronic form.
|
||
|
||
Otherwise they must appear on printed covers that bracket the whole
|
||
|
||
aggregate.
|
||
|
||
8. TRANSLATION
|
||
|
||
Translation is considered a kind of modification, so you may
|
||
|
||
distribute translations of the Document under the terms of section 4.
|
||
|
||
Replacing Invariant Sections with translations requires special
|
||
|
||
permission from their copyright holders, but you may include
|
||
|
||
translations of some or all Invariant Sections in addition to the
|
||
|
||
original versions of these Invariant Sections. You may include a
|
||
|
||
translation of this License, and all the license notices in the
|
||
|
||
Document, and any Warranty Disclaimers, provided that you also include
|
||
|
||
the original English version of this License and the original versions
|
||
|
||
of those notices and disclaimers. In case of a disagreement between
|
||
|
||
the translation and the original version of this License or a notice
|
||
|
||
or disclaimer, the original version will prevail.
|
||
|
||
If a section in the Document is Entitled "Acknowledgements",
|
||
|
||
"Dedications", or "History", the requirement (section 4) to Preserve
|
||
|
||
its Title (section 1) will typically require changing the actual
|
||
|
||
title.
|
||
|
||
9. TERMINATION
|
||
|
||
You may not copy, modify, sublicense, or distribute the Document except
|
||
|
||
as expressly provided for under this License. Any other attempt to
|
||
|
||
copy, modify, sublicense or distribute the Document is void, and will
|
||
|
||
automatically terminate your rights under this License. However,
|
||
|
||
parties who have received copies, or rights, from you under this
|
||
|
||
License will not have their licenses terminated so long as such
|
||
|
||
parties remain in full compliance.
|
||
|
||
10. FUTURE REVISIONS OF THIS LICENSE
|
||
|
||
The Free Software Foundation may publish new, revised versions
|
||
|
||
of the GNU Free Documentation License from time to time. Such new
|
||
|
||
versions will be similar in spirit to the present version, but may
|
||
|
||
differ in detail to address new problems or concerns. See
|
||
|
||
http://www.gnu.org/copyleft/.
|
||
|
||
Each version of the License is given a distinguishing version number.
|
||
|
||
If the Document specifies that a particular numbered version of this
|
||
|
||
License "or any later version" applies to it, you have the option of
|
||
|
||
following the terms and conditions either of that specified version or
|
||
|
||
of any later version that has been published (not as a draft) by the
|
||
|
||
Free Software Foundation. If the Document does not specify a version
|
||
|
||
number of this License, you may choose any version ever published (not
|
||
|
||
as a draft) by the Free Software Foundation.
|
||
|
||
ADDENDUM: How to use this License for your documents
|
||
|
||
To use this License in a document you have written, include a copy of
|
||
|
||
the License in the document and put the following copyright and
|
||
|
||
license notices just after the title page:
|
||
|
||
Copyright (c) YEAR YOUR NAME.
|
||
|
||
Permission is granted to copy, distribute and/or modify this document
|
||
|
||
under the terms of the GNU Free Documentation License, Version 1.2
|
||
|
||
or any later version published by the Free Software Foundation;
|
||
|
||
with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.
|
||
|
||
A copy of the license is included in the section entitled "GNU
|
||
|
||
Free Documentation License".
|
||
|
||
If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts,
|
||
|
||
replace the "with...Texts." line with this:
|
||
|
||
with the Invariant Sections being LIST THEIR TITLES, with the
|
||
|
||
Front-Cover Texts being LIST, and with the Back-Cover Texts being LIST.
|
||
|
||
If you have Invariant Sections without Cover Texts, or some other
|
||
|
||
combination of the three, merge those two alternatives to suit the
|
||
|
||
situation.
|
||
|
||
If your document contains nontrivial examples of program code, we
|
||
|
||
recommend releasing these examples in parallel under your choice of
|
||
|
||
free software license, such as the GNU General Public License,
|
||
|
||
to permit their use in free software.
|