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Tuesday, October 19 CS 375 UNIX System Programming - Lecture 15 1

Lecture 15

 Log into Linux. Copy files from

/home/hwang/cs375/lecture15/*.*

 Reminder: Project 4 due today.  Project 5 posted to course webpage.  Questions?

Tuesday, October 19 CS 375 UNIX System Programming - Lecture 15 2

Outline

 Introduction to processes  Creating new processes  Note: this material is covered in both BLP and

AUP (as indicated in the syllabus) as well as the man pages for the various system calls.

Tuesday, October 19 CS 375 UNIX System Programming - Lecture 15 3

Introduction

 Suppose we are writing an application that

requires the production of a nice graph, or the conversion an image from GIF to PNG formats, etc.

 We could do one of the following to solve this

problem:

 write the required routines from scratch  find and use library routines that do the work  use existing programs such as gnuplot or the PNM

utilities.

Tuesday, October 19 CS 375 UNIX System Programming - Lecture 15 4

Introduction

 The third approach can be very powerful. It is

• ften the easiest method as well. Why not use

an existing utility that does what is needed and is known to be efficient and robust?

 UNIX allows us run other programs from our

program and communicate with these programs using a number of different interprocess communication (IPC) methods.

Tuesday, October 19 CS 375 UNIX System Programming - Lecture 15 5

Introduction

 UNIX allows us to do this in a manner that is

completely transparent to the user of our

• program. That is, we can use gnuplot to

produce our graphs, but the user does not necessarily need to know that.

 In BLP Chapter 11 and AUP Chapter 5, we

learn to run other programs from our program. In BLP Chapters 13-15 and AUP Chapters 6-8, we will learn how to communicate (pass data) with these other programs.

Tuesday, October 19 CS 375 UNIX System Programming - Lecture 15 6

What is a Process?

 A program is a collection of instructions and

data kept in an file.

 A process is a running program. It consists of

segments containing instructions, user data, and system data.

 The instruction and user data segments are

initialized from the program.

 System data includes the current directory,

• pen file descriptors, total CPU time, etc.

Tuesday, October 19 CS 375 UNIX System Programming - Lecture 15 7

Parent and Child Processes

 A new child process is created by the kernel on

behalf of a parent process (via fork()).

 A child inherits most of the system data from

the parent. For example, files that are opened by the parent, also will be opened in the child.

 Each process has a unique process ID (PID - a

positive int). getpid( ) and getppid( ) can be used to find the current and parent process IDs. The init process has PID 1.

Tuesday, October 19 CS 375 UNIX System Programming - Lecture 15 8

Process Groups

 Each process is a member of a unique process

group with its own process group ID (PGID). When a process is created it is a member of its parent's process group.

 One process is the process group leader. The

PGID is the same as the leader PID.

 Only one process group is the foreground

process group. The terminal device driver sends tty signals (interrupt, quit, etc) to each process in the foreground process group.

Tuesday, October 19 CS 375 UNIX System Programming - Lecture 15 9

Additional Process Information

 At the command line use “ps ajx” to list the

PIDs, PPIDs and PGIDs of all processes.

 A parent may wait for a child to terminate by

using the wait( ) or waitpid( ) routines.

 If a child ends before the parent calls wait( ),

then the child lives on as a “zombie” process until the parent calls wait( ) or ends.

 If a parent ends, a child's parent process ID is

set to 1. (The child is adopted by init.)

Tuesday, October 19 CS 375 UNIX System Programming - Lecture 15 10

Process Creation: system( )

 The system( ) function (defined in <cstdlib>)

can be used to run another program (and thereby create a new process) from an existing process:

system("program_name");

 The parent process waits until the program

• completes. (See sys_xmpl.cpp)

 The program is run just as if the following had

been entered at the command prompt:

$ sh -c program_name

Tuesday, October 19 CS 375 UNIX System Programming - Lecture 15 11

Process Creation: fork( )/exec( )

 The system( ) function is built upon the fork( ),

exec( ) (both defined in <unistd.h>) and wait( ) (defined in <sys/wait.h>) routines.

 fork( ) creates a child process that is a clone of

the parent (identical instruction, user-data, and system-data segments).

 The exec( ) routine reinitializes a process from

a designated program (file on disk).

 fork( ) and exec( ) are usually used together.

Tuesday, October 19 CS 375 UNIX System Programming - Lecture 15 12

The exec( ) System Calls

 There are six exec( ) calls. First execl( ) -

arguments are a vararg list ending with 0.

int execl(char *path, char *arg0, ..., char *argn, (char *)0);

 Here is an example:

execl(“/bin/ls”, “ls”, “-al”, “/tmp”, (char *)0);

 Recall that a new process is not created by

exec( ). The code and user-data segments are reinitialized from the indicated program. The system data segment is not overwritten.

Tuesday, October 19 CS 375 UNIX System Programming - Lecture 15 13

The exec( ) System Calls

 exec( ) is the only way to execute a program

under UNIX. (See exc_xmpl.cpp)

 Other exec( ) calls are execlp( ), execle( ),

execv( ), execvp( ) and execve( ):

 execv*( ) routines expect the arguments to be

passed in an array (i.e. char *argv[ ]).

 exec*p( ) routines look for the program in the PATH

(vs. in the environment's PATH).

 exec*e( ) routines pass a pointer to a new

environment array (instead of a copy).

Tuesday, October 19 CS 375 UNIX System Programming - Lecture 15 14

The fork( ) System Call

 If we only had exec( ) there would be only one

process running on the system. We could run different programs by having each program call exec( ) to start a new program.

 The fork( ) system call is the only way to create

a new process. The new (child) process's instruction, user-data, and system-data segments are almost exact copies of the old (parent) process segments. (The PIDs and PPIDs will, of course, be different.)

Tuesday, October 19 CS 375 UNIX System Programming - Lecture 15 15

The fork( ) System Call

 The child gets copies of the parent's open file

• descriptors. The file pointer is in the system file

table and is shared by both processes.

 When fork( ) returns, both processes (parent

and child) get different return values (of type pid_t defined in <sys/types.h>). The child gets 0 while the parent gets the PID of the

• child. Usually the child will do an exec( ), while

the parent either waits for the child or goes off to do something else.

Tuesday, October 19 CS 375 UNIX System Programming - Lecture 15 16

The fork( ) System Call

 For example, the following code:

cout << “Start of test.” << endl; pid = fork(); cout << “fork() returned ” << pid << endl;

 might display (exact order depends on which

process executes first):

Start of test. fork() returned 0 fork() returned 17625

 See frk_xmpl1.cpp

Tuesday, October 19 CS 375 UNIX System Programming - Lecture 15 17

The fork( ) System Call

 Examine frk_xmpl2.cpp  How many processes will be created? Can

• utput with count=3 appear before output with

count=1?

 See vssh_xmpl.cpp for a very simple shell

program.

Tuesday, October 19 CS 375 UNIX System Programming - Lecture 15 18

The wait( ) System Call

 wait( ) causes the parent to sleep until any

child terminates:

int status; switch (fork()) { case 0: // the child execvp(program, args); default: // the parent pid = wait(&status); }

 It is not necessary for a parent to wait.  See wait_xmpl.cpp for example code.

Tuesday, October 19 CS 375 UNIX System Programming - Lecture 15 19

Using wait( )

 wait( ) returns the child's PID and passes back

the child's exit status in the status argument. The exit status is encoded and can be decoded using macros in <sys/wait.h>:

WIFEXITED(status) Non-zero if normal termination WEXITSTATUS(status) Exit status (normal termination) WIFSIGNALED(status) Non-zero if signal termination WTERMSIG(status) Signal number (if signal term) WIFSTOPPED(status) Non-zero if stopped

 A stopped process is different than a

terminated one (it can be restarted).

Tuesday, October 19 CS 375 UNIX System Programming - Lecture 15 20

Using waitpid( )

 waitpid( ) allows you to: (1) wait on a specific

process, (2) check status without blocking, and (3) supports job control.

pid_t waitpid(pid_t pid, int *status, int opts)

 If pid == -1, waitpid( ) waits for any child.

If pid > 0, waitpid( ) waits for child with that pid. status is defined just as for wait( ). If opts is WNOHANG, check status and return.