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UNIX for Dummies Questions & Answers
UNIX programming problems
Post 1296 by Neo on Thursday 22nd of February 2001 12:55:35 PM
02-22-2001
Administrator
All processes in the UNIX-like environment are assigned process IDs. As I recall, all processes have a parent process ID except for the init process (but you will have to check me on that one). So, there is the 'super parent of all processes' and all subsequent processes are children, grandchildren, gg, ggg, gggg etc. of this process.
When you login, for example, your shell is a child of the process which created the shell. When you are in your shell, the all processes you create (C programs, utilities you run, command you run, etc.) all children of that shell.
Now, you have two main situations possible or questions to consider. When you kill a parent, do the children die? This behavior of the children depend on how the child processes were created. Variations of the <B>exec</B> command are used to create different situations. So, fork and exec are the two areas you need to study to understand process creation and destruction. This is not something you can understand in a quick summary in a class (unless you are very clever and grasp abstract concepts quickly.)
So, if the topic is of interest to you, do not feel constrained that the professor has moved on to further topics. The purpose of school is to point to you different areas; however, mastery of the subject is always based on individual interest and motivation. The best way to understand the relationship between process creation and destruction, parent-child relationship, is to write a simple C program with fork and exec; trying different things. However, <B>never do something like this: </B> while (1) { fork() };
unless you are the owner of the platform and there are no other users. Malicious folks (not true gurus) learn quickly that it is possible to fork unlimited processes and flood the server (sometimes shutting it down). This is often controlled with ULIMIT for individual users. Regardless, this is considered very bad, malicious behavior.
Please, enjoy learning about computing, but always have consideration of other users and be aware of resource issues. Thanks
When you login, for example, your shell is a child of the process which created the shell. When you are in your shell, the all processes you create (C programs, utilities you run, command you run, etc.) all children of that shell.
Now, you have two main situations possible or questions to consider. When you kill a parent, do the children die? This behavior of the children depend on how the child processes were created. Variations of the <B>exec</B> command are used to create different situations. So, fork and exec are the two areas you need to study to understand process creation and destruction. This is not something you can understand in a quick summary in a class (unless you are very clever and grasp abstract concepts quickly.)
So, if the topic is of interest to you, do not feel constrained that the professor has moved on to further topics. The purpose of school is to point to you different areas; however, mastery of the subject is always based on individual interest and motivation. The best way to understand the relationship between process creation and destruction, parent-child relationship, is to write a simple C program with fork and exec; trying different things. However, <B>never do something like this: </B> while (1) { fork() };
unless you are the owner of the platform and there are no other users. Malicious folks (not true gurus) learn quickly that it is possible to fork unlimited processes and flood the server (sometimes shutting it down). This is often controlled with ULIMIT for individual users. Regardless, this is considered very bad, malicious behavior. Please, enjoy learning about computing, but always have consideration of other users and be aware of resource issues. Thanks
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LEARN ABOUT OPENDARWIN
fork
FORK(2) BSD System Calls Manual FORK(2) NAME
fork -- create a new process SYNOPSIS
#include <sys/types.h> #include <unistd.h> pid_t fork(void); DESCRIPTION
Fork() causes creation of a new process. The new process (child process) is an exact copy of the calling process (parent process) except for the following: o The child process has a unique process ID. o The child process has a different parent process ID (i.e., the process ID of the parent process). o The child process has its own copy of the parent's descriptors. These descriptors reference the same underlying objects, so that, for instance, file pointers in file objects are shared between the child and the parent, so that an lseek(2) on a descriptor in the child process can affect a subsequent read or write by the parent. This descriptor copying is also used by the shell to establish standard input and output for newly created processes as well as to set up pipes. o The child processes resource utilizations are set to 0; see setrlimit(2). RETURN VALUES
Upon successful completion, fork() returns a value of 0 to the child process and returns the process ID of the child process to the parent process. Otherwise, a value of -1 is returned to the parent process, no child process is created, and the global variable errno is set to indicate the error. ERRORS
Fork() will fail and no child process will be created if: [EAGAIN] The system-imposed limit on the total number of processes under execution would be exceeded. This limit is configuration- dependent. [EAGAIN] The system-imposed limit MAXUPRC (<sys/param.h>) on the total number of processes under execution by a single user would be exceeded. [ENOMEM] There is insufficient swap space for the new process. SEE ALSO
execve(2), wait(2) HISTORY
A fork() function call appeared in Version 6 AT&T UNIX. 4th Berkeley Distribution June 4, 1993 4th Berkeley Distribution