本篇内容介绍了“Linux网络编程wait()和waitpid()的详细讲解”的有关知识,在实际案例的操作过程中,不少人都会遇到这样的困境,接下来就让小编带领大家学习一下如何处理这些情况吧!希望大家仔细阅读,能够学有所成!
客户端断开连接后,服务器端存在大量僵尸进程。这是由于服务器子进程终止后,发送SIGCHLD信号给父进程,而父进程默认忽略了该信号。为避免僵尸进程的产生,无论我们什么时候创建子进程时,主进程都需要等待子进程返回,以便对子进程进行清理。为此,我们在服务器程序中添加SIGCHLD信号处理函数。
代码如下:
#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
#include <string.h>
#include <unistd.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <sys/types.h>
#include <netdb.h>
#define SERV_PORT 1113
#define LISTENQ 32
#define MAXLINE 1024
void str_echo(int fd);
void
sig_chld(int signo)
{
pid_t pid;
int stat;
pid = wait(&stat);//获取子进程进程号
printf("child %d terminated\n", pid);
return;
}
int
main(int argc, char *argv[]){
int listenfd,connfd;
pid_t childpid;
socklen_t clilen;
struct sockaddr_in servaddr;
struct sockaddr_in cliaddr;
//struct sockaddr_in servaddr;
//struct sockaddr_in cliaddr;
if((listenfd = socket(AF_INET, SOCK_STREAM,0))==-1){
fprintf(stderr,"Socket error:%s\n\a",strerror(errno));
exit(1);
}
bzero(&servaddr, sizeof(servaddr));
servaddr.sin_family = AF_INET;
servaddr.sin_addr.s_addr = htonl (INADDR_ANY);
servaddr.sin_port = htons(SERV_PORT);
signal(SIGCHLD,sig_chld);//处理SIGCHLD信号
if(bind(listenfd,(struct sockaddr*)(&servaddr),sizeof(struct sockaddr))==-1){
fprintf(stderr,"Bind error:%s\n\a",strerror(errno));
exit(1);
}
if(listen(listenfd,5)==-1){
fprintf(stderr,"Listen error:%s\n\a",strerror(errno));
exit(1);
}
for ( ; ; ) {
clilen = sizeof(cliaddr);
if((connfd=accept(listenfd,(struct sockaddr*)(&cliaddr),&clilen))<0){
if(errno==EINTR)
continue;
fprintf(stderr,"Accept error:%s\n\a",strerror(errno));
exit(1);
}
//有客户端建立了连接后
if ( (childpid = fork()) == 0) {
close(listenfd);
str_echo(connfd);
exit (0);
}
close(connfd);
}
}
void str_echo(int sockfd){
ssize_t n;
char buf[MAXLINE];
again:
while ( (n = read(sockfd, buf, MAXLINE)) > 0)
write(sockfd, buf, n);
if (n < 0 && errno == EINTR)//被中断,重入
goto again;
else if (n < 0){//出错
fprintf(stderr,"read error:%s\n\a",strerror(errno));
exit(1);
}
}
修改代码后,当客户端断开连接后,服务器端父进程收到子进程的SIGCHLD信号后,会执行sig_chld函数,对子进程进行了清理,便不会再出现僵尸进程。此时,一个客户端主动断开连接后,服务器端会输出类似如下信息:
child 12306 terminated
wait和waitpid
上述程序中sig_chld函数,我们使用了wait()来清除终止的子进程。还有一个类似的函数wait_pid。我们先来看看这两个函数原型:
pid_t wait(int *status);
pid_t waitpid(pid_t pid, int *status, int options);
官方描述:All of these system calls are used to wait for state changes in a child of the calling process, and obtain information about the child whose state has changed. A state change is considered to be: the child ter minated; the child was stopped by a signal; or the child was resumed by a signal. In the case of a terminated child, performing a wait allows the system to release the resources associated with the child; if a wait is not performed, then the terminated child remains in a "zombie" state (see NOTES below).
关于wait和waitpid两者的区别与联系:
The wait() system call suspends execution of the calling process until one of its children terminates. The call wait(&status) is equivalent to:
waitpid(-1, &status, 0);
The waitpid() system call suspends execution of the calling process until a child specified by pid argument has changed state. By default, waitpid() waits only for terminated children, but this behavior is modifiable via the options argument, as described below.
也就是说,wait()系统调用会挂起调用进程,直到它的任意一个子进程终止。调用wait(&status)的效果跟调用waitpid(-1, &status, 0)的效果是一样一样的。
waitpid()会挂起调用进程,直到参数pid指定的进程状态改变,默认情况下,waitpid() 只等待子进程的终止状态。如果需要,可以通过设置options的值,来处理非终止状态的情况。比如:
The value of options is an OR of zero or more of the following constants:
WNOHANG return immediately if no child has exited.
WUNTRACED also return if a child has stopped (but not traced via ptrace(2)). Status for traced children which have stopped is provided even if this option is not specified.
WCONTINUED (since Linux 2.6.10)also return if a stopped child has been resumed by delivery of SIGCONT.
等等一下非终止状态。
现在来通过实例看看wait()和waitpid()的区别。
通过修改客户端程序,在客户端程序中一次性建立5个套接字连接到服务器,状态如下图所示(附代码):
代码如下:
#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
#include <string.h>
#include <unistd.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <sys/types.h>
#include <netdb.h>
#define SERV_PORT 1113
#define MAXLINE 1024
void str_cli(FILE *fp, int sockfd);
int
main(int argc, char **argv)
{
int i,sockfd[5];
struct sockaddr_in servaddr;
if (argc != 2){
fprintf(stderr,"usage: tcpcli <IPaddress>\n\a");
exit(0);
}
for(i=0;i<5;++i){//与服务器建立五个连接,以使得服务器创建5个子进程
if((sockfd[i]=socket(AF_INET,SOCK_STREAM,0))==-1){
fprintf(stderr,"Socket error:%s\n\a",strerror(errno));
exit(1);
}
bzero(&servaddr,sizeof(servaddr));
servaddr.sin_family=AF_INET;
servaddr.sin_port=htons(SERV_PORT);
if (inet_pton(AF_INET, argv[1], &servaddr.sin_addr) <= 0){
fprintf(stderr,"inet_pton Error:%s\a\n",strerror(errno));
exit(1);
}
if(connect(sockfd[i],(struct sockaddr *)(&servaddr),sizeof(struct sockaddr))==-1){
fprintf(stderr,"connect Error:%s\a\n",strerror(errno));
exit(1);
}
}
str_cli(stdin, sockfd[0]);
exit(0);
}
void
str_cli(FILE *fp, int sockfd)
{
int nbytes=0;
char sendline[MAXLINE],recvline[MAXLINE];
while (fgets(sendline, MAXLINE, fp) != NULL){//从标准输入中读取一行
write(sockfd, sendline, strlen(sendline));//将该行发送给服务器
if ((nbytes=read(sockfd, recvline, MAXLINE)) == 0){//从sockfd读取从服务器发来的数据
fprintf(stderr,"str_cli: server terminated prematurely\n");
exit(1);
}
recvline[nbytes]='\0';
fputs(recvline, stdout);
}
}
当客户终止时,所以打开的描述子均由内核自动关闭,因此5个连接基本在同一时刻发生,相当于同时引发了5个FIN发往服务器,这会导致5个服务器子进程基本在同一时刻终止,从而导致5个SIGCHLD信号几乎同时递送给服务器父进程,示意图如下所示:
也就是说,几乎在同一时刻,递送5个SIGCHLD信号给父进程,这又会僵尸进程进程的出现。因为unix一般不对信号进行排队,这就导致了5个SIGCHLD递交上去,只执行了一次sig_chld函数,剩下四个子进程便成为了僵尸进程。对于这种情况,正确的做法是调用waitpid(),而不是wait()。
因此,我们最后的服务器端代码中的信号处理函数做一点小改动,改成如下:
代码如下:
void
sig_chld(int signo)
{
pid_t pid;
int stat;
while ( (pid = waitpid(-1, &stat, WNOHANG)) > 0)
printf("child %d terminated\n", pid);
return;
}
至此,我们解决了网络编程中可能遇到的三类情况:
1.当派生子进程时,必须捕获SIGCHLD信号。代码片段:signal(SIGCHLD,sig_chld);
2.当捕获信号时,必须处理被中断的系统调用。代码片段:if(errno==EINTR) continue;
3.SIGCHLD信号处理函数必须编写正确,以防出现僵尸进程。代码片段:while ( (pid = waitpid(-1, &stat, WNOHANG)) > 0)
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