Process Management(unix)

CE 466 System Programming

Process Management

Computer Engineering Department Yarmouk University 10/19/2008

Objectives • Understand the UNIX process model • Learn how to create a process • Experiment with fork and exec • Discuss the implications of process inheritance • Use wait for cleanup

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Processes in UNIX • A process is a program in execution • It is the basic active entity in an OS • The compiler: ¾Translates the source file into object modules (OM) ¾Links the different OM with the necessary libraries to produce executable module (EM)

Processes in UNIX • Process creation: ¾OS copies the EM into a program image in memory ¾OS allocates proper resources and adds appropriate info to the kernel data structures. ƒ A process has an ID and a state (parts of a PCB)

• A process has: ¾Address space ¾At least one flow of control ¾A program counter

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Process States state new

meaning being created

running instructions are being executed blocked waiting for an event such as I/O ready

waiting to be assigned to a processor

done

finished

Waiting

Program image

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Process Termination • On termination (normal or abnormal) the OS: ¾ Deallocates process resources (VM, locks, files,..) ¾ Updates appropriate statistics ¾ Notifies other processes (parent)

• A process does not completely releases its resources until the parent waits for it. • If no parent waits for the child process: ¾ Process is zombie (inactive process whose resources are deleted later) ¾ Process is adapted by a special process (init process id=1)

• If a parent process terminates without waiting for a child, the child becomes an orphan.

Process Manipulation • A process is identified by its ID • A new process can be created using the system call fork() • Fork() returns two processes: ¾Parent (original) ¾Child (new)

• fork() makes a copy of the parent address space • Both processes continue execution after fork() statement

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Process Creation #include pid_t fork(void);// pid_t: unsigned int #include <stdio.h> #include int main(void) { int x; x = 0; fork(); x++; Printf("I am process %d and my x value is %d\n“, getpid(), x); return 0; }

Process Creation #include <stdio.h> #include int main(void) { int x; x = 0; PC fork(); PC x++; PC Printf("I am process %d and my x value is %d\n“, getpid(), x); return 0; }

PC PC

#include <stdio.h> #include int main(void) { int x; x = 0; fork(); x++; Printf("I am process %d and my x value is %d\n“, getpid(), x); return 0; }

Time

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Process Creation ??? • Because both may perform different tasks, we should be able to identify them • The return value is: ¾Child’s process ID (in parent process) ¾Zero (in the child process)

• Child and parent processes execute different parts of the same program

Forking a Child Process #include <stdio.h> #include <stdlib.h> main() { int i; printf("before forking.\n"); if (fork()){ for (i=0;i<100;i++) { printf(“ \t \t \t Parent } } else{ for (i=0;i<100;i++) { printf(“Child process %d } } }

process %d \n", i );

\n", i );

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Basic Control (wait(), exit()) • exit(exit_status) ¾Used to terminate calling process. ƒ exit_status = 0 ƒ exit_status !=0

(success) (failure)

• Closes all of the of its opened file descriptors, directory streams,…. • If the parent is waiting for a child, it gets notified and its exit status is made available to the parent.

Basic Control (wait(), exit()) • wait(&status) ¾ Allows the parent process to suspend its activities until one of its child processes either stopped or terminated. ¾ Returns ƒ Child’s exit status to the parent process ƒ Process ID of child whose termination caused the wait() to wake up.

¾ Returns immediately with a value of -1, if the calling process does not have any children associated with it. ¾ Check the man pages for wait(), waitid(), waitpid(), wait3()

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Synchronization between parent and child wait(0)

exit()

• Used by the parent • Used by the child to wait for the child upon completion to die

Concurrent Execution of Parent and Child Shell Parent

fork()

Child exec()

wait() exit continue

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Child Execute a Separate Program • Use exec() system call: ¾Causes the process to replace its execution image with that of a new program. ¾Context of the old program is lost ƒ Code space, data space, stack, heap

¾Never returns except on failure. ¾Retains process ID, parent, children, and open file descriptors.

exec() • Six different flavors, based on: ¾Program path name (relative or absolute) ¾Environment variables (inherited or explicit) ¾Command line arguments (explicit or vector)

• Easiest : ¾ execlp(char *filename, char *arg0, char arg1,..... , char *argn, (char *) 0); ¾ e.g. execlp( “sort”, “sort”, “-n”, “foo”, 0);

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exec() Example arg[1] arg[2]

execlp(“sort”, ”sort”, ”-n”, “foo”, 0); mark end of list What the file to receive as its arguments The program will receive as arg[0] Name of file to execute, Execute the sort program with a passing arguments ‘-n’, ‘foo’.

exec() Flavors • int execl(char *pathname, char *arg0, char arg1,..... , char *argn, (char *) 0); • int execlp(char *filename, char *arg0, char arg1,..... , char *argn, (char *) 0); • int execle(char *pathname, char *arg0, char arg1,..... , char *argn, (char *) 0, char *envp[]); • int execv(char *pathname, char **argv,); • int execvp(char *filename, char **argv); • int execve(char *pathname, char **argv, char **envp);

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exec() Flavors Library Call Name Argument List

Pass Current Environment Variables

Search PATH automatic

execl

list

yes

no

execv

array

yes

no

execle

list

no

no

execve

array

no

no

execlp

list

yes

yes

execvp

array

yes

yes

exec() Flavors Explicit list of arguments terminated by NULL pointer • int execl(char *pathname, char *arg0, char arg1,..... , char *argn, (char *) 0); Explicit pathname is provided Environment variables are inherited from calling process • int execlp(char *filename, char *arg0, char arg1,..... , char *argn, (char *) 0); Filename has a ‘/’ -> execl Filename does not have a ‘/’ -> PATH is searched Environment variables are inherited from calling process • int execle(char *pathname, char *arg0, char arg1,..... , char *argn, (char *) 0, char *envp[]); Explicit pathname is provided Environment is passed as a parameter

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exec() Flavors Passes the command-line arguments in an argument array • int execv(char *pathname, char **argv,); Explicit pathname is provided Enviornment variables are inherited from calling process • int execvp(char *filename, char **argv); Filename has a ‘/’ -> execl Filename does not have a ‘/’ -> PATH is searched Environment variables are inherited from calling process • int execve(char *pathname, char **argv, char **envp); Explicit pathname is provided Environment is passed as a parameter

exec() Flavors • The execl (execl, execlp and execle) functions pass the command-line arguments in an explicit list and are useful if you know the number of command-line arguments at compile time. • The execv (execv, execvp and execve) functions pass the command-line arguments in an argument array.

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Example /* tinymenue.c*/ #include ,stdio.h. main() { char *cmd[]={“who”, “ls”, “date”}; int i; printf(“0=who, 1=ls, 2=date :) ; scanf(“%d”, &i); execlp(cmd[i], cmd[i], 0); printf(“command not found\n”); /*exec failed*/ }

- execlp() call never returns unless execlp() fails. - If execlp() succeeds, control is transferred to the new program - All context within the old program is lost; there is no way back - This limitation can be solved using the idea of fork() with child parent synchronization.

Example main() { char *cmd[]={“who”, “ls”, “date”}; int i; while(1){ printf(“0=who, 1=ls, 2=date :) ; scanf(“%d”, &i); if (fork() == 0) { execlp(cmd[i], cmd[i],0); printf(“command not found\n”); /*exec failed*/ exit(1); } else { wait(0); } } }

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Example main() { char *cmd[]={“who”, “ls”, “date”}; int i; while(1){ printf(“0=who, 1=ls, 2=date :) ; scanf(“%d”, &i);

Parent else { wait(0); } } }

Child if (fork() == 0) { execlp(cmd[i], cmd[i],0); printf(“command not found\n”); /*exec failed*/ exit(1); }

Inheritance of Process Attributes Attribute Process ID Static data Stack Heap Code Open file descriptors Environment Current directory

Inherited by child? No Copied Copied Copied Shared Copied Yes Yes

Retained on exec? Yes No No No No Usually Depends of version Yes

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