fork,vfork,clone都是linux的系統調用，用來創建子進程的（確切說vfork創造出來的是線程）。
先介紹下進程必須的4要點：
a.要有一段程序供該進程運行，就像一場戲劇要有一個劇本一樣。該程序是可以被多個進程共享的，多場戲劇用一個劇本一樣。
b.有起碼的私有財產，就是進程專用的系統堆棧空間。
c.有“戶口”，既操作系統所說的進程控制塊，在linux中具體實現是task_struct
d.有獨立的存儲空間。
當一個進程缺少d條件時候，我們稱其爲線程。
1.fork 創造的子進程複製了父親進程的資源，包括內存的內容task_struct內容（2個進程的pid不同）。這裏是資源的複製不是指針的複製。下面的例子可以看出
[root@liumengli program]# cat testFork.c
#include"stdio.h"
int main() {
int count = 1;
int child;
if(!(child = fork())) { //開始創建子進程
printf("This is son, his count is: %d. and his pid is: %d\n", ++count, getpid());//子進程的內容
} else {
printf("This is father, his count is: %d, his pid is: %d\n", count, getpid());
}
}
[root@liumengli program]# gcc testFork.c -o testFork
[root@liumengli program]# ./testFork
This is son, his count is: 2. and his pid is: 3019
This is father, his count is: 1, his pid is: 3018
[root@liumengli program]#
從代碼裏面可以看出2者的pid不同，內存資源count是值得複製，子進程改變了count的值，而父進程中的count沒有被改變。有人認爲這樣大批量的複製會導致執行效率過低。其實在複製過程中，子進程複製了父進程的task_struct，系統堆棧空間和頁面表，這意味着上面的程序，我們沒有執行count++前，其實子進程和父進程的count指向的是同一塊內存。而當子進程改變了父進程的變量時候，會通過copy_on_write的手段爲所涉及的頁面建立一個新的副本。所以當我們執行++count後，這時候子進程才新建了一個頁面複製原來頁面的內容，基本資源的複製是必須的，而且是高效的。整體看上去就像是父進程的獨立存儲空間也複製了一遍。
其次，我們看到子進程和父進程直接沒有互相干擾，明顯2者資源都獨立了。我們看下面程序
[root@liumengli program]# cat testFork.c
#include"stdio.h"
int main() {
int count = 1;
int child;
if(!(child = fork())) {
int i;
for(i = 0; i < 200; i++) {
printf("This is son, his count is: %d. and his pid is: %d\n", i, getpid());
}
} else {
printf("This is father, his count is: %d, his pid is: %d\n", count, getpid());
}
}
[root@liumengli program]# gcc testFork.c -o testFork
[root@liumengli program]# ./testFork
...
This is son, his count is: 46. and his pid is: 4092
This is son, his count is: 47. and his pid is: 4092
This is son, his count is: 48. and his pid is: 4092
This is son, his count is: 49. and his pid is: 4092
This is son, his count is: 50. and his pid is: 4092
This is father, his count is: 1, his pid is: 4091
[root@liumengli program]# This is son, his count is: 51. and his pid is: 4092
This is son, his count is: 52. and his pid is: 4092
...
（運氣很衰，非要200多個纔有效果，鬱悶）從結果可以看出父子2個進程是同步運行的。這和下面的vfork有區別。
2.vfork創建出來的不是真正意義上的進程，而是一個線程，因爲它缺少了我們上面提到的進程的四要素的第4項，獨立的內存資源，看下面的程序
[root@liumengli program]# cat testVfork.c
#include "stdio.h"
int main() {
int count = 1;
int child;
printf("Before create son, the father's count is:%d\n", count);
if(!(child = vfork())) {
printf("This is son, his pid is: %d and the count is: %d\n", getpid(), ++count);
exit(1);
} else {
printf("After son, This is father, his pid is: %d and the count is: %d, and the child is: %d\n", getpid(), count, child);
}
}
[root@liumengli program]# gcc testVfork.c -o testVfork
[root@liumengli program]# ./testVfork
Before create son, the father's count is:1
This is son, his pid is: 4185 and the count is: 2
After son, This is father, his pid is: 4184 and the count is: 2, and the child is: 4185
[root@liumengli program]#
從運行結果可以看到vfork創建出的子進程（線程）共享了父進程的count變量，這一次是指針複製，2者的指針指向了同一個內存，所以子進程修改了count變量，父進程的 count變量同樣受到了影響。另外由vfork創造出來的子進程還會導致父進程掛起，除非子進程exit或者execve纔會喚起父進程，看下面程序：
[root@liumengli program]# cat testVfork.c
#include "stdio.h"
int main() {
int count = 1;
int child;
printf("Before create son, the father's count is:%d\n", count);
if(!(child = vfork())) {
int i;
for(i = 0; i < 100; i++) {
printf("This is son, The i is: %d\n", i);
if(i == 70)
exit(1);
}
printf("This is son, his pid is: %d and the count is: %d\n", getpid(), ++count);
exit(1);
} else {
printf("After son, This is father, his pid is: %d and the count is: %d, and the child is: %d\n", getpid(), count, child);
}
}
[root@liumengli program]# gcc testVfork.c -o testVfork
[root@liumengli program]# ./testVfork
...
This is son, The i is: 68
This is son, The i is: 69
This is son, The i is: 70
After son, This is father, his pid is: 4433 and the count is: 1, and the child is: 4434
[root@liumengli program]#
從這裏就可以看到父進程總是等子進程執行完畢後纔開始繼續執行。
3.clone函數功能強大，帶了衆多參數，因此由他創建的進程要比前面2種方法要複雜。clone可以讓你有選擇性的繼承父進程的資源，你可以選擇想vfork一樣和父進程共享一個虛存空間，從而使創造的是線程，你也可以不和父進程共享，你甚至可以選擇創造出來的進程和父進程不再是父子關係，而是兄弟關係。先有必要說下這個函數的結構
int clone(int (*fn)(void *), void *child_stack, int flags, void *arg);
這裏fn是函數指針，我們知道進程的4要素，這個就是指向程序的指針，就是所謂的“劇本", child_stack明顯是爲子進程分配系統堆棧空間（在linux下系統堆棧空間是2頁面，就是8K的內存，其中在這塊內存中，低地址上放入了值，這個值就是進程控制塊task_struct的值）,flags就是標誌用來描述你需要從父進程繼承那些資源， arg就是傳給子進程的參數）。下面是flags可以取的值
標誌 含義
CLONE_PARENT 創建的子進程的父進程是調用者的父進程，新進程與創建它的進程成了“兄弟”而不是“父子”
CLONE_FS 子進程與父進程共享相同的文件系統，包括root、當前目錄、umask
CLONE_FILES 子進程與父進程共享相同的文件描述符（file descriptor）表
CLONE_NEWNS 在新的namespace啓動子進程，namespace描述了進程的文件hierarchy
CLONE_SIGHAND 子進程與父進程共享相同的信號處理（signal handler）表
CLONE_PTRACE 若父進程被trace，子進程也被trace
CLONE_VFORK 父進程被掛起，直至子進程釋放虛擬內存資源
CLONE_VM 子進程與父進程運行於相同的內存空間
CLONE_PID 子進程在創建時PID與父進程一致
CLONE_THREAD Linux 2.4中增加以支持POSIX線程標準，子進程與父進程共享相同的線程羣
下面的例子是創建一個線程（子進程共享了父進程虛存空間，沒有自己獨立的虛存空間不能稱其爲進程）。父進程被掛起當子線程釋放虛存資源後再繼續執行。
[root@liumengli program]# cat test_clone.c
#include "stdio.h"
#include "sched.h"
#include "signal.h"
#define FIBER_STACK 8192
int a;
void * stack;
int do_something(){
printf("This is son, the pid is:%d, the a is: %d\n", getpid(), ++a);
free(stack); //這裏我也不清楚，如果這裏不釋放，不知道子線程死亡後，該內存是否會釋放，知情者可以告訴下,謝謝
exit(1);
}
int main() {
void * stack;
a = 1;
stack = malloc(FIBER_STACK);//爲子進程申請系統堆棧
if(!stack) {
printf("The stack failed\n");
exit(0);
}
printf("creating son thread!!!\n");
clone(&do_something, (char *)stack + FIBER_STACK, CLONE_VM|CLONE_VFORK, 0);//創建子線程
printf("This is father, my pid is: %d, the a is: %d\n", getpid(), a);
exit(1);
}
[root@liumengli program]# gcc test_clone.c -o test_clone
[root@liumengli program]# ./test_clone
creating son thread!!!
This is son, the pid is:7326, the a is: 2
This is father, my pid is: 7325, the a is: 2
[root@liumengli program]#
讀者可以試試其它的資源繼承方式。