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Brief Introduction to System Calls and Process Management

Brief Introduction to System Calls and Process Management. COMP229 - System Software Edition 1.1 , March 11, 2002. Slides and examples could be found at: http://alcor.concordia.ca/~sa_mokho/comp229/. Overview. System Call Interface Process Management fork() exec() wait() exit().

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Brief Introduction to System Calls and Process Management

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  1. Brief Introduction to System Calls and Process Management COMP229 - System Software Edition 1.1, March 11, 2002 Slides and examples could be found at: http://alcor.concordia.ca/~sa_mokho/comp229/ Serguei A. Mokhov, mokhov@cs.concordia.ca

  2. Overview • System Call Interface • Process Management • fork() • exec() • wait() • exit() Serguei A. Mokhov, mokhov@cs.concordia.ca

  3. User-running App Software syscall - trap exit() Kernel-running Zombie API Sys Software read(), write(), wait(), sleep() Sys Call (OS) Interface Blocked Ready OS Sw-Hw I-face (drivers) Hardware System Call Interface “You are here” Serguei A. Mokhov, mokhov@cs.concordia.ca

  4. The fork() System Call (1) • A process calling fork() spawns a child process • The child is almost an identical clone of the parent: • Text • Stack • PCB • Data Serguei A. Mokhov, mokhov@cs.concordia.ca

  5. PID=28 PID=28 PID=34 p1 p1 c1 The fork() System Call (2) • fork() is one of the those system calls, which is called once, but returns twice! • After fork() both the parent and the child are executing the same program. • On error, fork() returns -1 Consider a piece of program (see fork_pid.c example): ... pid_t pid = fork(); printf(“PID: %d\n”, pid); ... The parent will print: PID: 34 And the child will always print: PID: 0 fork() Serguei A. Mokhov, mokhov@cs.concordia.ca

  6. The fork() System Call (3) • Remember, after fork() the execution order is not guaranteed. • Simple fork() examples: • fork_pid.c prints out return from fork() • fork_child.c distinguishes between the parent and the child Serguei A. Mokhov, mokhov@cs.concordia.ca

  7. The exec()System Call (1) • The exec() call replaces a current process’ image with a new one (i.e. loads a new program within current process). • The new image is either regular executable binary file or a shell script. • There’s no a syscall under the name exec(). By exec() we usually refer to a family of calls: • int execl(char *path, char *arg, ...); • int execv(char *path, char *argv[]); • int execle(char *path, char *arg, ..., char *envp[]); • int execve(char *path, char *argv[], char *envp[]); • int execlp(char *file, char *arg, ...); • int execvp(char *file, char *argv[]); • Here's what c, l, e, and p mean: • l means an argument list, • v means an argument vector, • e means an environment vector, and • p means a search path. Serguei A. Mokhov, mokhov@cs.concordia.ca

  8. PID=28 PID=28 p1 Legend: Old Program p1 New Program The exec()System Call (2) • Upon success, exec()never returns to the caller. If it does return, it means the call failed. Typical reasons are: non-existent file or bad permissions. • Arguments passed via exec() appear in the argv[] of the main() function. • For more info: man -S 3 exec; Example is: exec_pid.c exec() Serguei A. Mokhov, mokhov@cs.concordia.ca

  9. Environment • The e exec calls use the environment when attempt to invoke a new program. • Name = Value • HOME • PATH • SHELL • USER • LOGNAME • ... • set or env - will display current environment, which you can modify with: • the export command in a shell or a shell script; • getenv(), setenv(), putenv(), etc in C Serguei A. Mokhov, mokhov@cs.concordia.ca

  10. PID=28 PID=28 PID=34 PID=34 p1 p1 c1 c1 fork() and exec() Combined • Often after doing fork() we want to load a new program into the child. E.g.: a shell. tcsh tcsh tcsh fork() tcsh ls PID=34 exec(ls) c1 Serguei A. Mokhov, mokhov@cs.concordia.ca

  11. The System wait() Call • Forces the parent to suspend execution, i.e. wait for its children or a specific child to die (terminate is more appropriate terminology, but a bit less common). • Example: fork_exec.c Serguei A. Mokhov, mokhov@cs.concordia.ca

  12. The exit() System Call • This call gracefully terminates process execution. Gracefully means it does clean up and release of resources, and puts the process into the zombie state. • By calling wait(), the parent cleans up all its zombie children. • exit() specifies a return value from it’s program, which a parent process might want to examine as well as status of the dead process. • _exit() call is another possibility of quick death without cleanup. Serguei A. Mokhov, mokhov@cs.concordia.ca

  13. User-running syscall() - trap exit() Kernel-running Zombie wait() Blocked Ready Process Overview INIT PID=1 ? Serguei A. Mokhov, mokhov@cs.concordia.ca

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