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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
vfork
VFORK(2) BSD System Calls Manual VFORK(2) NAME
vfork -- spawn new process in a virtual memory efficient way SYNOPSIS
#include <unistd.h> pid_t vfork(void); DESCRIPTION
Vfork() can be used to create new processes without fully copying the address space of the old process, which is horrendously inefficient in a paged environment. It is useful when the purpose of fork(2) would have been to create a new system context for an execve. Vfork() differs from fork in that the child borrows the parent's memory and thread of control until a call to execve(2) or an exit (either by a call to exit(2) or abnormally.) The parent process is suspended while the child is using its resources. Vfork() returns 0 in the child's context and (later) the pid of the child in the parent's context. Vfork() can normally be used just like fork. It does not work, however, to return while running in the childs context from the procedure that called vfork() since the eventual return from vfork() would then return to a no longer existent stack frame. Be careful, also, to call _exit rather than exit if you can't execve, since exit will flush and close standard I/O channels, and thereby mess up the parent processes standard I/O data structures. (Even with fork it is wrong to call exit since buffered data would then be flushed twice.) SEE ALSO
fork(2), execve(2), sigaction(2), wait(2), DIAGNOSTICS
Same as for fork. BUGS
This system call will be eliminated when proper system sharing mechanisms are implemented. Users should not depend on the memory sharing semantics of vfork as it will, in that case, be made synonymous to fork. To avoid a possible deadlock situation, processes that are children in the middle of a vfork() are never sent SIGTTOU or SIGTTIN signals; rather, output or ioctl(2) calls are allowed and input attempts result in an end-of-file indication. HISTORY
The vfork() function call appeared in 3.0BSD. 4th Berkeley Distribution June 4, 1993 4th Berkeley Distribution