Linux 信号处理
前言
最近真是倒霉透了，本来就已经够忙的了，跑个步还把手指摔折了，去医院看，这些实习医生让我拍了四次 X 光，他们技术不行却让病人患者为他们买单，有学校报销我倒是不怎么在意这些费用，但是你不能让我一直拍啊，辐射不说还浪费我时间。。。感觉中国体制很多地方还是不尽人意的。
僵尸进程
在Linux进程的状态中，僵尸进程是非常特殊的一种进程，它已经放弃了几乎所有内存空间，没有任何可执行代码，也不能被调度，仅仅在进程列表中保留一个位置，记载该进程的退出状态等信息供其他进程收集，除此之外，僵尸进程不再占有任何内存空间。它需要它的父进程来为它收尸。
如果他的父进程没安装SIGCHLD信号处理函数调用wait或waitpid()等待子进程结束，又没有显式忽略该信号，那么它就一直保持僵尸状态，如果这时父进程结束了，那么init进程自动会接手这个子进程，为它收尸，它还是能被清除的。
但是如果父进程是一个循环，不会结束，那么子进程就会一直保持僵尸状态，这就是为什么系统中有时会有很多的僵尸进程。系统所能使用的进程号是有限的,如果大量的产生僵死进程,将因为没有可用的进程号而导致系统不能产生新的进程。
僵尸进程的避免
⒈父进程通过wait和waitpid等函数等待子进程结束，这会导致父进程挂起。
⒉ 如果父进程很忙，那么可以用signal函数为SIGCHLD安装handler，因为子进程结束后， 父进程会收到该信号，可以在handler中调用wait回收。
根据实验要求，我们来设计一个并发的多进程服务器，客户端给服务器发送文件名，服务器创建进程返回给客户端其所需文件的内容。
客户端
/* TCPdaytime.c - TCPdaytime, main */
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <stdio.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netdb.h>
#include <stdarg.h>
#ifndef INADDR_NONE
#define INADDR_NONE 0xffffffff
#endif /* INADDR_NONE */
extern int errno;
int TCPecho(const char *host, const char *service);
int errexit(const char *format, ...);
int connectTCP(const char *host, const char *service);
int connectsock(const char *host, const char *service, const char *transport);
#define LINELEN 8102
/*------------------------------------------------------------------------
* main - TCP client for DAYTIME service
*------------------------------------------------------------------------
*/
int main(int argc, char *argv[])
{
char *host = "localhost"; /* host to use if none supplied */
char *service = "echo"; /* default service port */
switch (argc) {
case 1:
host = "localhost";
break;
case 3:
service = argv[2];
/* FALL THROUGH */
case 2:
host = argv[1];
break;
default:
fprintf(stderr, "usage: TCPecho [host [port]]\n");
exit(1);
}
TCPecho(host, service);
exit(0);
}
/*------------------------------------------------------------------------
* TCPdaytime - invoke Daytime on specified host and print results
*------------------------------------------------------------------------
*/
int TCPecho(const char *host, const char *service)
{
char buf[LINELEN+1]; /* buffer for one line of text */
int s; /* socket, read count */
s = connectsock( host, service, "tcp");
printf("file:");
if (fgets(buf, sizeof buf, stdin)) {
//从命令行读入用户输入的字符
buf[LINELEN] = '\0'; /* insure null-terminated */
(void) write(s, buf, sizeof buf);
//向网络中发送用户所输入的字符
memset(buf, 0, sizeof buf);
if (read(s, buf, LINELEN) < 0) {
//从网络中读取服务器所返回的的字符
errexit("socket read failed: %s\n", strerror(errno));
}
printf("%s\n", buf);
}
close(s);
}
/*------------------------------------------------------------------------
* connectsock - allocate & connect a socket using TCP or UDP
*------------------------------------------------------------------------
*/
int connectsock(const char *host, const char *service, const char *transport )
/*
* Arguments:
* host - name of host to which connection is desired
* service - service associated with the desired port
* transport - name of transport protocol to use ("tcp" or "udp")
*/
{
struct hostent *phe; /* pointer to host information entry */
struct servent *pse; /* pointer to service information entry */
struct protoent *ppe; /* pointer to protocol information entry*/
struct sockaddr_in sin; /* an Internet endpoint address */
int s, type; /* socket descriptor and socket type */
memset(&sin, 0, sizeof(sin));
sin.sin_family = AF_INET;
/* Map service name to port number */
if ( pse = getservbyname(service, transport) )
sin.sin_port = pse->s_port;
else if ((sin.sin_port=htons((unsigned short)atoi(service))) == 0)
errexit("can't get \"%s\" service entry\n", service);
/* Map host name to IP address, allowing for dotted decimal */
if ( phe = gethostbyname(host) )
memcpy(&sin.sin_addr, phe->h_addr, phe->h_length);
else if ( (sin.sin_addr.s_addr = inet_addr(host)) == INADDR_NONE )
errexit("can't get \"%s\" host entry\n", host);
/* Map transport protocol name to protocol number */
if ( (ppe = getprotobyname(transport)) == 0)
errexit("can't get \"%s\" protocol entry\n", transport);
/* Use protocol to choose a socket type */
if (strcmp(transport, "udp") == 0)
type = SOCK_DGRAM;
else
type = SOCK_STREAM;
/* Allocate a socket */
s = socket(PF_INET, type, ppe->p_proto);
if (s < 0)
errexit("can't create socket: %s\n", strerror(errno));
/* Connect the socket */
if (connect(s, (struct sockaddr *)&sin, sizeof(sin)) < 0)
errexit("can't connect to %s.%s: %s\n", host, service,
strerror(errno));
return s;
}
int errexit(const char *format, ...)
{
va_list args;
va_start(args, format);
vfprintf(stderr, format, args);
va_end(args);
exit(1);
}
并发的多进程服务器
/* TCPdaytimed.c - main */
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <unistd.h>
#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <netdb.h>
#include <stdarg.h>
#include <stdlib.h>
#include <signal.h>
extern int errno;
int errexit(const char *format, ...);
int passiveTCP(const char *service, int qlen);
int errexit(const char *format, ...);
int passivesock(const char *service, const char *transport, int qlen);
void handler(int num) {
int status;
int pid = waitpid(-1, &status, WNOHANG);
if (WIFEXITED(status)) {
printf("The child %d exit with returing %d\n", pid, WEXITSTATUS(status));
}
}
unsigned short portbase = 0; /* port base, for non-root servers */
#define QLEN 32
#define NAMELEN 32
#define BUFSIZE 8102
/*------------------------------------------------------------------------
* main - Iterative TCP server for DAYTIME service
*------------------------------------------------------------------------
*/
int main(int argc, char *argv[])
{
struct sockaddr_in fsin; /* the from address of a client */
char *service = "echo"; /* service name or port number */
int msock, ssock; /* master & slave sockets */
unsigned int alen; /* from-address length */
char file[NAMELEN+1];
char buf[BUFSIZE];
FILE *fp = NULL;
char *find;
switch (argc) {
case 1:
break;
case 2:
service = argv[1];
break;
default:
errexit("usage: TCPecho [port]\n");
}
msock = passivesock(service, "tcp", QLEN);
while (1) {
alen = sizeof(fsin);
ssock = accept(msock, (struct sockaddr *)&fsin, &alen);
signal(SIGCHLD,handler); //处理子进程的信号
int pid = fork();
if (pid == 0) {
if (ssock < 0) errexit("accept failed: %s\n", strerror(errno));
if (read(ssock, file, sizeof file) > 0) {
if (file[strlen(file)-1] == '\n') file[strlen(file)-1] = '\0';
fp = fopen(file, "r");
if (NULL == fp)
{
strcpy(buf, "file open Fail!");
(void) write(ssock, buf, sizeof buf);
memset(buf, 0, sizeof buf);
return -1;
}
fread(buf, 1, sizeof buf, fp);
fclose(fp);
fp = NULL;
(void) write(ssock, buf, sizeof buf);
memset(buf, 0, sizeof buf);
}
(void) close(ssock);
return 1;
}
}
}
/*------------------------------------------------------------------------
* passivesock - allocate & bind a server socket using TCP or UDP
*------------------------------------------------------------------------
*/
int passivesock(const char *service, const char *transport, int qlen)
/*
* Arguments:
* service - service associated with the desired port
* transport - transport protocol to use ("tcp" or "udp")
* qlen - maximum server request queue length
*/
{
struct servent *pse; /* pointer to service information entry */
struct protoent *ppe; /* pointer to protocol information entry*/
struct sockaddr_in sin; /* an Internet endpoint address */
int s, type; /* socket descriptor and socket type */
memset(&sin, 0, sizeof(sin));
sin.sin_family = PF_INET;
sin.sin_addr.s_addr = INADDR_ANY;
/* Map service name to port number */
if ( pse = getservbyname(service, transport) )
sin.sin_port = htons(ntohs((unsigned short)pse->s_port) + portbase);
else if ((sin.sin_port=htons((unsigned short)atoi(service))) == 0)
errexit("can't get \"%s\" service entry\n", service);
/* Map protocol name to protocol number */
if ( (ppe = getprotobyname(transport)) == 0)
errexit("can't get \"%s\" protocol entry\n", transport);
/* Use protocol to choose a socket type */
if (strcmp(transport, "udp") == 0)
type = SOCK_DGRAM;
else
type = SOCK_STREAM;
/* Allocate a socket */
s = socket(PF_INET, type, ppe->p_proto);
if (s < 0)
errexit("can't create socket: %s\n", strerror(errno));
/* Bind the socket */
if (bind(s, (struct sockaddr *)&sin, sizeof(sin)) < 0)
errexit("can't bind to %s port: %s\n", service,
strerror(errno));
if (type == SOCK_STREAM && listen(s, qlen) < 0)
errexit("can't listen on %s port: %s\n", service,
strerror(errno));
return s;
}
int errexit(const char *format, ...)
{
va_list args;
va_start(args, format);
vfprintf(stderr, format, args);
va_end(args);
exit(1);
}
在服务器中我们用到了这个信号处理的函数给服务器安装了 handler，signal(SIGCHLD,handler);这个函数能够接收到子进程退出的信号，然后将僵尸进程杀死。我们把这行代码注释掉看看运行结果：
很明显，在客户端退出之后，服务器 fork 出来的子进程并没有完全退出。
去掉注释，再运行一次：
客户端退出后，服务器的子进程也就退出了，这样就不会一直占用着空间。随便读取个文件试试吧！
