LINUX System Process in Detail

1. LINUX SystemProcess in Detail Bong-Soo Sohn

2. Overview • Process Control Block • process table • u area • Process context • User space • Kernel space • HW • Process management algorithm • Fork() • Exec() • Exit() • Wait()

3. Process Control Block • Process Table Entry • u area

4. Kernel data structure – PTE(Process Table Entry) • Process Identification • Process State • Scheduling parameter • The process priority, user-mode scheduling priority, recent CPU utilization, and amount od of time spent sleeping • Signal State • Signal pending delivery, signal mask, and summary of signal actions • Timer : Real-time timer and CPU-utilization counters • Event Descriptor • Realted Process Ids • Serveral User IDs & Group IDs : uid, euid, gid, egid

5. Kernel data structure – u area • Pointer to PTE • Timer : spent in kernel and user mode • Signal Handler • The control terminal • Error • Return value • I/O parameters • Current directory and Current root • User File Descriptor table • Limit fields restrict the size of process and the size of a file it can write • Permission modes field masks mode setting on the files the process creats.

6. Process Context • User level context (User address space) • System level context (Kernel Data Structures, Kernel Space) • HW registers

7. Process Context argc, argv env. variables memory mapping tables text table proc table u area kernel stack stack heap PC (prog counter) SP (stack pointer) register Uninitialized data Initialized data text (code) <kernel space> <hardware> <user space>

8. CPU execution mode • place restrictions on the operations that can be performed by the process currently running in the CPU • Kernel mode • When the CPU is in kernel mode, it is assumed to be executing trustedsoftware, and thus it can execute any instructions and reference any memory addresses (i.e., locations in memory). • The kernel (which is the core of the operating system and has complete control over everything that occurs in the system) is trusted software, but all other programs are considered untrusted software. • User mode • It is a non-privileged mode in which each process (i.e., a running instance of a program) starts out. It is non-privileged in that it is forbidden for processes in this mode to access those portions of memory (i.e., RAM) that have been allocated to the kernel or to other programs.

9. System Calls Dealing with Memory Management System Calls Dealing with Synchronization Miscellaneous fork exec brk exit wait signal kill setpgrp setuid dupreg attachreg detachreg allocreg attachreg growreg loadreg mapreg growreg detachreg Process Control • Control of Process Context • fork : create a new process • exit : terminate process execution • wait : allow parent process to synchronize its execution with the exit of a child process • exec : invoke a new program • brk : allocate more memory dynamically • signal : inform asynchronous event • major loop of the shell andof init

10. Sequence of Operations for fork 1. It allocates a slot in process table for the new process 2. It assigns a unique ID number to the child process 3. It makes a logical copy of the context of the parent process. Since certain portion of process, such as the text region, may be shared between processes, the kernel can sometimes increment a region reference count instead of copying the region to a new physical location in memory 4. It increments file and inode table counters for files associated with the process. 5. It returns the ID number of child process to the parent process, and a 0 value to the child process.

11. Algorithm for fork input : none output : to parent process, child PID number to child process, 0 { check for available kernel resources; get free process table slot, unique PID number; check that user not running too many process; mark child state "being created"; copy data from parent process table to new child slot; increment counts on current directory inode and changed root(if applicable); increment open file counts in file table; make copy of parent context(u area, text, data, stack) in memory; push dummy system level context layer onto child system level context; dummy context contains data allowing child process to recognize itself, and start from here when scheduled;

12. Algorithm for fork(Cont.) if ( executing process is parent ) { change child state to "ready to run"; return(child ID); /* from system to user */ } else /* executing process is the child process */ { initialize u area timing fields; return(0); /* to user */ } }

13. Parent Process File Table U Area Per Process Region Table Parent Data Open Files Current Directory Changed Root Parent User Stack . . . . . Kernel Stack Shared Text Inode Table U Area Per Process Region Table Child Data Open Files Current Directory Changed Root Child User Stack . . . . . Kernel Stack Child Process Fork Creating a New Process Context

14. Example of Sharing File Access rdwt() { for(;;) { if (read(fdrd,&c,1)!=-1) return; write(fdwt,&c,1); } } #include <fcntl.h> int fdrd, fdwt; char c; main( argc,argv ) int argc; char *argv[]; { if ( argc != 3 ) exit(1); if ((fdrd=open(argv[1],O_RDONLY))==-1) exit(1); if ((fdwt=creat(argv[2],0666))==-1) exit(1); fork(); /*both process execute same code*/ rdwt(); exit(0); }

15. Example of Sharing File Access(Cont.) • fdrd for both process refer to the file table entry for the source file(argv[1]) • fdwt for both process refer to the file table entry for the target file(argv[2]) • two processes never read or write the same file offset values. User File Descriptor Table (Parent Process) Inode Table File Table fdrd Count Read 2 Only fdwt : : : ... (Child Process) Count Read 2 Write fdrd fdwt ...

16. Interprocess Communication • IPC using regular files • unrelated processes can share • fixed size • lack of synchronization • IPC using pipes • for transmitting data between related processes • can transmit an unlimited amount of data • automatic synchronization on open()

17. Pipe $ who | sort

18. pipes #include <unistd.h> int pipe(int fd[2]) Returns: 0 if OK, -1 on error • Two file descriptors are returned through the fd argument • fd[0]: can be used to read from the pipe, and • fd[1]: can be used to write to the pipe • Anything that is written on fd[1] may be read by fd[0]. • This is of no use in a single process. • However, between processes, it gives a method of communication • The pipe() system call gives parent-child processes a way to communicate with each other.

19. pipe after fork user process parent process child process fd[0] fd[1] fd[0] fd[1] fd[0] fd[1] pipe pipe kernel kernel

20. dup #include <unistd.h> int dup(int filedes); Int dup2(int filedes, int filedes2); Both return: new file descriptor if OK, -1 on error • The new file descriptor returned by dup is guaranteed to be the lowest numbered available file descriptor. • With dup2, we specify the value of the new descriptor with the filedes2 argument • The new file descriptor that is returned as the value of the functions shares the same file table entry as the filedes argument.

21. example • dup(1); File descriptor table File table entry fd 0 fd 1 fd 2 fd 3 fd 4 …

22. Use of Pipe, Dup, and Fork if (pid == 0) { dup2(pipefd[1], STDOUT_FILENO); close(pipefd[0]); close(pipefd[1]); if (execl(path, arg0, NULL) == -1) perror("execl"); } else { if (fork() == 0) { dup2(pipefd[0], STDIN_FILENO); close(pipefd[0]); close(pipefd[1]); if (execl("/bin/cat", "cat", NULL) == -1) perror("execl cat"); } else { close(pipefd[0]); close(pipefd[1]); wait(&status); wait(&status); } } } #include <sys/types.h> #include <unistd.h> #include <stdio.h> #include <errno.h> #include <sys/wait.h> int main(int argc, char *argv[]) { char *path = "/bin/ls"; char *arg0 = "ls"; pid_t pid; int pipefd[2]; int status; pipe(pipefd); pid = fork();

23. Process Termination • exit system call • process terminate by exit system call • enters the zombie status • relinquish resources (close all open files) • buffered output written to disk • dismantles its context except for its slot in the process table. • exit(status); • status : the value returned to parent process • can be used for unix shell (shell programming) • call exit explicitly or implicitly(by startup routine) at the end of program. • kernel may invoke internally on receipt of uncaught signals. In this case, the value of status is the signal number.

24. Exit handler #include <stdlib.h> void atexit(void (*func)(void)); returns: 0 if OK, nonzero on error • Register exit handler • Register a function that is called when a program is terminated • Called in reverse order of registration

25. Exit handler Output : main is done first exit handler first exit handler second exit handler /* doatexit.c */ static void my_exit1(void), my_exit2(void); int main(void) { if (atexit(my_exit2) != 0) perror("can't register my_exit2"); if (atexit(my_exit1) != 0) perror("can't register my_exit1"); if (atexit(my_exit1) != 0) perror("can't register my_exit1"); printf("main is done\n"); return 0; } static void my_exit1(void) { printf("first exit handler\n"); } static void my_exit2(void) { printf("second exit handler\n"); }

26. Algorithm for Exit algorithm exit input : return code for parent process output : none { ignore all signals; if ( process group leader with associated control terminal ) { send hangup signal to all members of process group; reset process group for all members to 0; } close all open files(internal version of algorithm close) release current directory(algorithm iput); release current(changed) root, if exists (algorithm iput); free regions, memory associated with process(algorithm freereg); write accounting record; make process state zombie; assign parent process ID of all child processes to be init process(1); if any children were zombie, send death of child signal to init; send death of child signal to parent process; context switch; }

27. Awaiting Process Termination • wait system call • synchronize its execution with the termination of a child process • pid = wait(stat_addr); • pid : process id of the zombie child process • stat_addr : address of an integer to contain the exit status of the child

28. Algorithm for Awaiting Process Termination 1. searches the zombie child process 2. If no children, return error 3. if finds zombie children, extracts PID number and exit code 4. adds accumulated time the child process executes in the user and kernel mode to the fields in u area 5. Release process table slot

29. Algorithm for Wait algorithm wait input : address of variables to store status of exiting process output : child ID, child exit code { if (waiting process has no child process) return(error); for(;;) { if (waiting process has zombie child) { pick arbitrary zombie child; add child CPU usage to parent; free child process table entry; return(childID,child exit code); } if (process has no child process) return(error); sleep at interruptible priority(event child process exits); } }

30. Invoking Other Programs • exec system call • invokes another program, overlaying the memory space of a process with a copy of an executable file • execve(filename, argv, envp) • filename : the name of executable file being invoked • argv : a pointer to an array of character pointers that are parameters to the executable program • envp : a pointer array of character pointers that are environment of the executed program • several library functions that calls exec system call execl, execv, execle...

31. Algorithm for Exec algorithm exec input : (1) file name (2) parameter list (3) environment variables list output : none { get file inode(algorithm namei) verify file executable, user has permission to execute; read file headers, check that it is a load module; copy exec parameters from old address space to system space; for(every region attached to process) detach all old regions(algorithm detach); for(every region specified in load module) { allocate new regions(algorithm allocreg); attach the regions(algorithm attachreg); load region into memory if appropriate(algorithm loadreg); } copy exec parameters into new user stack region; special processing for setuid programs, tracing; initialize user register save area for return to user mode; release inode of file(iput); }

32. Environment variables • Environment variables(EV) are inherited from parent to child process • Generally, EV are set in .login or .cshrc $ env USER=ysmoon LOGNAME=ysmoon HOME=/home/prof/ysmoon PATH=/bin:/usr/bin:/usr/local/bin:/usr/ccs/bin:/usr/ucb:/usr/openwin/bin:/etc:. SHELL=/bin/csh ... ...

33. Environment list • Environment variables are accessed through global variable environ extern char ** environ; • Each element has a form of “Name=Value” • Each string ends with ‘\0’ • Last element of environ is NULL pointer

34. Environment list environment strings environment pointer environment list environ: "USER=bongbong" "LOGNAME=bongbong" "HOME=/home/prof/bongbong" "PATH=/bin:/usr/local…" "MAIL =/var/mail/bongbong" ... "SHELL=/bin/csh" NULL

35. getenv/putenv #include <stdlib.h> char *getenv(const char *name); Returns : pointer to value associated with name, NULL if not found #include <stdlib.h> int putenv(const char *str); // str : “name=value” Returns: 0 if OK, nonzero on error

36. setenv/unsetenv #include <stdlib.h> int setenv(const char *name, const char *value, int rewrite); Returns: 0 if OK, nonzero on error void unsetenv(const char *name);