Process API - 码农教程

fork

when call fork(), OS create a new process.
the two process is all same, and return from fork() function.

but according the man fork(), when it create success.

the child process receives a return code of 0
the parent process receives the PID of the newly-created child process

wait

call wait() to delay its execution, until target proccess is completed

wait for any child process

wait(NULL) will block parent process until any of its children has finished

wait for all child process completed

POSIX defines a function: wait(NULL)；. It's the shorthand for waitpid(-1, NULL, 0);
while ((wpid = wait(&status)) > 0);

exec

work with fork()

the exec family function will replace the current processs image with new process image.
so the rest codes of original process, will not execution.

execvp

The first argument, by convention, should point to the file name associated with the file being exe-cuted. The list of arguments must be terminated by a NULL pointer.

why

the separation of fork() and exec() is essential in building a UNIX shell

shell work flow

cmd: fork ------------------------ -> wait -> output
\ -> pre-env -> exec -> exit /

redirected

Specifically, UNIX systems start looking for free file descriptors at zero. In this case, STDOUT FILENO will be the first available one and thus get assigned when open() is called. Subsequent writes by the child process to the standard output file descriptor, for example by routines such as printf(), will then be routed transpar- ently to the newly-opened file instead of the screen.

extension

init

The process 1, and PID=1

the very first process (called init) is started by the kernel at booting time and never terminates

Zombie Process

A child that terminates, but has not been waited for becomes a "zombie".

Orphan Process

running process whose parent process has finished or terminated

Re-parenting

In most cases, the new parent is the init process, one with the PID 1.

Daemon Process

is an intintionally orphaned process

A daemon process is usually created by a process forking a child process and then immediately exiting, thus causing init to adopt the child process. In a Unix environment, the parent process of a daemon is often, but not always, the init process.

references

原文地址：https://www.cnblogs.com/Gilfoyle/p/15227380.html