原文： https://www.bbsmax.com/A/n2d99QxBdD/
#include <unistd.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <string.h>
#include <fcntl.h>
#include <iostream>
using namespace std;
static int global_val = 0;
int main()
{
int *p = (int*)malloc(sizeof(int));
*p=0;
int m = 2;
pid_t pid ;
int fd = open("mytest", O_RDWR | O_CREAT, 0666);
if ((pid = fork()) < 0)
{
cout << "fork error" << endl;
}
else {
if (pid == 0)
{
char buf[20]=" ";
int res = read(fd,buf,20);
cout<<"ppid is "<<getppid()<<" pid is "<<getpid()<<" res is "<<res<<" fd is "<<fd<<" buf is "<< buf<<endl;
//close(fd);
//sleep(1);
char bufs[8]="shenlei";
lseek(fd, 0, SEEK_SET);
write(fd,bufs,strlen(bufs));
// read
char nbuf[20]=" ";
lseek(fd, 0, SEEK_SET);
int nres = read(fd,nbuf,20);
cout<<"after write file: "<<nbuf<<" nres:"<<nres<<endl;
global_val++;
m++;
(*p)++;
}else{
sleep(2);
char buf[20]=" ";
lseek(fd, 0, SEEK_SET);
int res = read(fd,buf,20);
cout<<"parent ppid is "<<getppid()<<" pid is "<<getpid()<<" res is "<<res<<" fd is "<<fd<<" buf is "<< buf<<endl;
cout << *p << " " << m << " " << global_val<< endl;
}
}
return 0;
}
----------------------------
今天我在阅读《Unix网络编程》时候遇到一个问题：accept返回时的connfd，是父子进程之间共享的？我当时很不理解，难道打开的文件描述符不是应该在父子进程间相互独立的吗？为什么是共享的呢？fork之后父子进程之间共享了什么？堆上的变量是否也共享了呢？
做了如下的代码测试，在fork之前先创建一个文件，在子进程中写入字符串“shenlei”,父进程读取文件内容，发现也是“shenlei”。说明打开的文件描述符在父子进程之间是共享的。另一方面，在父子进程中读取文件描述符fd，发现二者相等，进一步证实了这个观点。
看来在《Unix网络编程》中说的是对的，当close一个文件描述符时候会将文件描述符的引用计数-1。在普通文件io操作时，只有当引用计数为0才能真正关闭该文件描述符；在socket操作时，也只有当引用计数为0时才会发送FIN，四次挥手关闭相应的socket。
- #include <unistd.h>
- #include <stdlib.h>
- #include <sys/types.h>
- #include <sys/stat.h>
- #include <string.h>
- #include <fcntl.h>
- #include <iostream>
- using namespace std;
- static int global_val = 0;
- int main()
- {
- int *p = (int*)malloc(sizeof(int));
- *p=0;
- int m = 2;
- pid_t pid ;
- int fd = open("mytest", O_RDWR | O_CREAT, 0666);
- if ((pid = fork()) < 0)
- {
- cout << "fork error" << endl;
- }
- else {
- if (pid == 0)
- {
- char buf[20]=" ";
- int res = read(fd,buf,20);
- cout<<"pid is "<<getpid()<<" res is "<<res<<" fd is "<<fd<<" buf is "<< buf<<endl;
- close(fd);
- //sleep(1);
- char bufs[8]="shenlei";
- lseek(fd, 0, SEEK_SET);
- write(fd,bufs,strlen(bufs));
- global_val++;
- m++;
- (*p)++;
- }
- else{
- sleep(1);
- char buf[20]=" ";
- lseek(fd, 0, SEEK_SET);
- int res = read(fd,buf,20);
- cout<<"pid is "<<getpid()<<" res is "<<res<<" fd is "<<fd<<" buf is "<< buf<<endl;
- cout << *p << " " << m << " " << global_val<< endl;
- }
- }
- return 0;
- }
然后又测试了下，一个进程中的堆对象能否共享，如上述代码所示，结论是不可以的。全局变量，静态变量，全局静态变量也都是不行的。说明在fork创建多进程之后堆栈信息会完全复制给子进程内存空间，父子进程相互独立。