1 Introduction to UNIX 2 Ke Liu

2 Last Time SSH, Login and Log out Shells and Shell commands man, ls, cd, pwd, mkdir, rmdir, rm, cp, mv, chmod, ps, kill File System: /, directory, file, pathname Editors C Compiler: gcc Foreground / Background Processes

3 Process Control 3 primary functions: fork( ) exec( ) wait( )

4 Fork( ) The only way of creating a new process in Unix. New process is called “child process”. Fork returns twice !! Once in the parent returning the child process ID. Once in child returning 0.

5 Fork example 1 #include 2 #include 3 #include 4 main( ) 5 { 6 int pid; 7 pid = fork( );

6 Fork example 8if(pid = = 0) 9 printf(“Hello from the child\n"); 10while(1); 11else 12 printf("This is parent\n"); 13 }

7 Fork example bingsun2% gcc –o hello hello.c or gcc hello.c –o hello bingsun2% test Hello from the child Hello from the parent What about files opened in parent?

8 Exec( ) Executes commands. Normally, the parent process does a fork( ), creates a child and the child process does an exec( ) exec( ) never returns if successful !! 6 different ways.

9 Exec( ) example execvp call 1 #include 2 #include 3 #include 4 main() 5 { 6 int pid; 7 char * command[2];

10 Exec( ) example 8 command[0] = (char*)malloc(10); 9 command[1] = (char*)malloc(10); 10 strcpy(command[0], "ls"); 11 strcpy(command[1], "-l"); 12 command[2] = (char*)0; 13 pid = fork(); 14 if(pid == 0) 15 { 16 printf("This is child\n");

11 Exec( ) example 17 execvp(*command, command); 18 } 19 else 20 printf("This is parent\n"); 21 }

12 Wait( ) Wait for a process to terminate. Generally used in parent process which waits for the child to terminate waitpid( ) waits for a specific process. Example programs for wait()

13 Inter-process communication. Signals: software interrupts generated when certain asynchronous events occur. E.g.: when you press Control C to stop a program. Pipes.

14 Inter-process communication. Semaphores: mutual exclusion. Shared memory: multiple processes share a common region in memory. Message queues: linked-list of messages.

15 Debugging An easy way to debug in my opinion is printf statements. GDB: Gnu Debugger. Help for gdb available on class homepage –Invest some time learning it; it will make your life much easier