学系统编程都会学到进程与及线程，在学习Linux编程过程中，最大的困难花时间最多的莫过于进程那一块，包括进程的环境，进程的控制，进程间通信，进程共享内存等。

fork()是linux生成一个新进程的函数，在《unix环境高级编程》书中P172页是这样描述的：子进程是父进程的副本。例如，子进程获得父进程的数据空间， 堆和栈的副本。注意，这是子进程所拥有的副本。父、子进程并不共享这些存储空间部分。父子进程共享正文段。

既然子进程是父进程的一个副本，获得父进程的数据空间，但又不是共享的，那么是否子进程具有独立的物理内存空间呢？父进程中的一个变量a是否还有另一个副本存在子进程空间物理内存里，也即在父、子进程里分别执行printf(“a address: %lx”, &a)会输出不同的值呢？

从输出的结果来看，变量在物理内存只分配了一次，那么子进程的变量副本又是如何实现的呢?不另外分配一块内存来存储它的变量， 那怎样实现和父进程的变量取不同的值呢？

代码如下：

#include "apue.h"

#include

#define ARRAY_SIZE 40000

#define MALLOC_SIZE 100000

#define SHM_SIZE 100000

#define SHM_MODE 0600 /* user read/write */

char array[ARRAY_SIZE]; /* uninitialized data = bss */

int

main(void)

{

int shmid;

int n = 1;

char *ptr, *shmptr;

pid_t pid;

if ((shmid = shmget(IPC_PRIVATE, SHM_SIZE, SHM_MODE)) < 0)

err_sys("shmget error");

if ((shmptr = shmat(shmid, 0, 0)) == (void *)-1)

err_sys("shmat error");

if ((ptr = malloc(MALLOC_SIZE)) == NULL)

err_sys("malloc error");

TELL_WAIT();

if((pid = fork()) < 0)

err_sys("fork error");

else if(pid == 0)

{

n = 5;

printf("pid = %d ", getpid());

printf("n = %d,stack around %lx ", n, (unsigned long)&n);

printf("shared memory attached from %lx to %lx ",

(unsigned long)shmptr, (unsigned long)shmptr+SHM_SIZE);

printf("malloced from %lx to %lx ", (unsigned long)ptr,

(unsigned long)ptr+MALLOC_SIZE);

printf("array[] from %lx to %lx ", (unsigned long)&array[0],

(unsigned long)&array[ARRAY_SIZE]);

TELL_PARENT(pid);

exit(0);

}

WAIT_CHILD();

printf(" pid=%d ", getpid());

printf("n = %d, stack around %lx ", n, (unsigned long)&n);

printf("shared memory attached from %lx to %lx ",

(unsigned long)shmptr, (unsigned long)shmptr+SHM_SIZE);

printf("malloced from %lx to %lx ", (unsigned long)ptr,

(unsigned long)ptr+MALLOC_SIZE);

printf("array[] from %lx to %lx ", (unsigned long)&array[0],

(unsigned long)&array[ARRAY_SIZE]);

if (shmctl(shmid, IPC_RMID, 0) < 0)

err_sys("shmctl error");

exit(0);

}