System UNIX Programming Lecture 14: Processes
Lecture 14: Processes Introduction
Outline
Introduction to Processes Creating New Processes
Reference
BLP: Chapter 11 AUP: Chapter 5 man pages: getpid, getppid, system, wait, waitpid, fork, exec
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Suppose we are writing an application that requires several matrix operations, or we want to produce a nice graph, or convert an image from GIF to PNG formats, etc. We could either: (1) write the required routines from scratch, (2) find and use library routines that do the work, or (3) use existing programs such as octave, gnuplot or the PNM utilities.
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Lecture 14: Processes Introduction
Lecture 14: Processes Introduction
The third approach can be very powerful. It is often the easiest method as well. Why not use an existing utility that does what I need and I know 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.
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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 this chapter we learn to run other programs from our program. In Chapters 13-15 we will learn how to communicate (pass data) with these other programs.
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Lecture 14: Processes What Is A Process?
Lecture 14: Processes Parent and Child Processes
A program is a collection of instructions and data kept in an file. A process is a running program. It consists of instruction, user data, and system data segments. The instruction and user data segments are initialized from the program. System data includes the current directory, open file descriptors, total CPU time, etc.
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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 open by the parent, will also be open to the child. Each process has a unique process-ID (a positive int). getpid() and getppid() can be used to find the current and parent process IDs. The init process has ID 1.
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Lecture 14: Processes Process Groups
Lecture 14: Processes Additional Process Info
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.
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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 wait() calls or ends. If a parent ends, a child's parent process ID is set to 1. (The child is adopted by init.)
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Lecture 14: Processes Process Creation: system()
Lecture 14: Processes Process Creation: fork/exec
The system() function 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
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The system() function is built upon the fork(), exec() and wait() routines. fork() creates a child process that is a clone of the parent (identical instruction, userdata, and system-data segments). The exec() routine reinitializes a process from a designated program (file on disk). fork() & exec() are usually used together.
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Lecture 14: Processes The exec() System Calls
Lecture 14: Processes The exec() System Calls
There are six exec() calls. First execl():
int execl(char *path, char *arg0, ..., char *argn, (char *)0);
Here is an example:
execl("/bin/ls", "ls", "-al", "/tmp", 0);
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():
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.
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execv*() routines expect the arguments to be passed in an array. exec*p() routines look for the program in the PATH. exec*e() routines pass a pointer to a new environment array (instead of a copy).
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Lecture 14: Processes The fork() System Call
Lecture 14: Processes ...