fork()函數(shù)詳解

1.fork()簡介

函數(shù)原型：

pid_t fork(void)；//pid_t為int類型，進(jìn)行了重載

pid_t getpid();// 獲取當(dāng)前進(jìn)程的 pid 值。

pid_t getppid(); //獲取當(dāng)前進(jìn)程的父進(jìn)程 pid 值。

用于創(chuàng)建一個進(jìn)程，所創(chuàng)建的進(jìn)程復(fù)制父進(jìn)程的代碼段/數(shù)據(jù)段/BSS段/堆/棧等所有用戶空間信息；在內(nèi)核中操作系統(tǒng)重新為其申請了一個PCB，并使用父進(jìn)程的PCB進(jìn)行初始化；



如圖所示 ：我們將A 進(jìn)程， 也就是調(diào)用 fork 的進(jìn)程稱之為父進(jìn)程， 而新的進(jìn)程（B 進(jìn)程）稱之為子進(jìn)程。

關(guān)于fork 可以命令，查看詳細(xì)說明及用法：

man 3 fork

NAME

fork, wait, waitpid - basic process management

SYNOPSIS
@load "fork"

pid = fork()

ret = waitpid(pid)

ret = wait();

DESCRIPTION
The fork extension adds three functions, as follows.

fork() Thisfunctioncreatesa new process. The return value is the zero in the child and the process-id number of the child in the parent, or -1 upon error. In the latter case, ERRNO indicates
the problem.In the child, PROCINFO["pid"] and PROCINFO["ppid"] are updated to reflect the correct values.

waitpid()
This function takes a numeric argument, which is the process-id to wait for. The return value is that of the waitpid(2) system call.

wait() This function waits for the first child to die.The return value is that of the wait(2) system call.

BUGS
There is no corresponding exec() function.

The interfaces could be enhanced to provide more facilities, including pulling out the various bits of the return status.

EXAMPLE
@load "fork"
...
if ((pid = fork()) == 0)
print "hello from the child"
else
print "hello from the parent"

SEE ALSO
GAWK: Effective AWK Programming, filefuncs(3am), fnmatch(3am), inplace(3am), ordchr(3am), readdir(3am), readfile(3am), revoutput(3am), rwarray(3am), time(3am).

fork(2), wait(2), waitpid(2).

AUTHOR
Arnold Robbins,arnold@skeeve.com.

COPYING PERMISSIONS
Copyright 2012, 2013, Free Software Foundation, Inc.

Permission is granted to make and distribute verbatim copies of this manual page provided the copyright notice and this permission notice are preserved on all copies.


2.fork()特性

fork調(diào)用的一個奇妙之處就是它僅僅被調(diào)用一次，卻能夠返回兩次，它可能有三種不同的返回值：

在父進(jìn)程中，fork返回新創(chuàng)建子進(jìn)程的進(jìn)程ID；

在子進(jìn)程中，fork返回0；

如果出現(xiàn)錯誤，fork返回一個負(fù)值；

因此我們可以通過fork返回的值來判斷當(dāng)前進(jìn)程是子進(jìn)程還是父進(jìn)程。（注：fork 調(diào)用生成的新進(jìn)程與其父進(jìn)程誰先執(zhí)行不一定，哪個進(jìn)程先執(zhí)行要看系統(tǒng)的進(jìn)程調(diào)度策略）

舉個例子來解釋fpid的值為什么在父子進(jìn)程中不同：“相當(dāng)于鏈表，進(jìn)程形成了鏈表，父進(jìn)程的fpid(p 意味point)指向子進(jìn)程的進(jìn)程id, 因?yàn)樽舆M(jìn)程沒有子進(jìn)程，所以其fpid為0.

3, fork()例程

看到這里大家對fork()有個大致了解了，讓我們來看個例題：

#include

#include

#include

int main(int argc, const char *argv[])

{

int num = 10;

pid_t pid = fork();

if(pid==0)

{

while (1)

{

num = 100;

printf("The father pid=%d The child pid=%d num=%d ", getppid(),getpid(), num);

sleep(3);

}

}

else

{

while (1)

{

printf("The father pid=%d num=%d ", getpid(), num);

sleep(5);

}

}

return 0;

}

保存為fork_test.c

gcc-o fork_test.a fork_test.c

運(yùn)行：

./fork_test.a
The father pid=15131 num=10
The father pid=15131 The child pid=15132 num=100
The father pid=15131 The child pid=15132 num=100
The father pid=15131 num=10
The father pid=15131 The child pid=15132 num=100
The father pid=15131 The child pid=15132 num=100
The father pid=15131 num=10
The father pid=15131 The child pid=15132 num=100

可以看到產(chǎn)生兩個pid(進(jìn)程）

審核編輯：劉清