UNIX环境高级编程之-----信号signal

参考书籍：unxi环境高级编程

信号函数：

typedef void (*sighandler_t)(int);
sighandler_t signal(int signum, sighandler_t handler);

其原型为：

void (*signal(int signum,void (*handler)(int))) (int);

我们一般都是用第一个，也就是通过typedef改写过的。

注意：signal函数我一般认为其是向内核注册当前进程收到信号的处理的方式。
signal(SIGINT,handler);

参数说明:

signum  :  指定信号
handler  :  SIG_IGN忽略该信号，SIG_DFL采用系统默认方式处理信号,自定义的信号处理函数指针。

理解实际的例子是学习的最好方式：
例子一：

通过异步方式，给子进程收尸

注意：子进程在终止时会给父进程发SIGCHLD，该信号的默认处理动作是忽略，父进程可以自定义SIGCHLD信号的处理函数，这样父进程只需要专心处理自己的工作，不必关心子进程了，子进程终止时会通知父进程，父进程在信号处理函数中调用wait清理子进程即可。

#include <stdio.h>
#include <signal.h>
#include <unistd.h>
#include <stdlib.h>

void child_exit_handler(int signum)
{
if(signum == SIGCHLD)
{
printf("Child exit.\n");
wait(NULL);
}
}

int main()
{
int pid;
int i = 0;

//想内核注册，处理 SIGCHLD信号的方式
signal(SIGCHLD,child_exit_handler);

if((pid = fork()) < 0)
{
perror("Fail to fork");
exit(EXIT_FAILURE);

}else if(pid == 0){

for(i = 0;i < 5;i ++)
{
printf("child loop.\n");
sleep(1);
}

}else{

for(i = 0;i < 5;i ++)
{
printf("Father loop.\n");
sleep(2);
}

}

exit(EXIT_SUCCESS);
}

运行结果如下：
[uglychen@chenxun test]$gcc test.c 

[uglychen@chenxun test]$ ls
a.out  test.c

[uglychen@chenxun test]$ ./a.out 
child loop.

Father loop.

child loop.

Father loop.

child loop.

child loop.

Father loop.

child loop.

Child exit.

Father loop.

Father loop.

[uglychen@chenxun test]$ 

例二、综合案例

使用FIFO实现clientA与clientB之间聊天

A.输入quit后，两个进程退出

B.如果在20秒内，没有等到另一端发来的消息，则认为对方已不在，此时终止。

clientA:

#include <stdio.h>
#include <stdlib.h>
#include <signal.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <errno.h>
#include <string.h>
#include <fcntl.h>

#define MAX 100

void signal_handler(int signum)
{
static int flag = 0;

switch(signum)
{
case SIGALRM:
if(flag == 0)
{
printf("The people is leaving,the system is closed in 10 seconds \
and you can input 'ctrl + c' cancel.\n");
alarm(10);
}else{

kill(getppid(),SIGKILL);
usleep(500);
exit(EXIT_SUCCESS);
}

flag = 1;
break;

case SIGINT:
printf("The alarm is cancel.\n");
alarm(0);
break;
}

}

int child_recv_fifo(char *fifo_name)
{
int n,fd;
char buf[MAX];

if((fd = open(fifo_name,O_RDONLY)) < 0)
{
fprintf(stderr,"fail to open %s : %s.\n",fifo_name,strerror(errno));
return -1;
}

signal(SIGALRM,signal_handler);
signal(SIGINT,signal_handler);
alarm(15);//璁剧疆瀹氭椂鍣?
while(1)
{
n = read(fd,buf,sizeof(buf));
buf
= '\0';

printf("Read %d bytes : %s.\n",n,buf);

if(strncmp(buf,"quit",4) == 0 || n == 0)
{
kill(getppid(),SIGKILL);
usleep(500);
exit(EXIT_SUCCESS);
}

alarm(15);
}

return 0;
}

int father_send_fifo(char *fifo_name,int pid)
{
int n,fd;
char buf[MAX];

if((fd = open(fifo_name,O_WRONLY)) < 0)
{
fprintf(stderr,"Fail to open %s : %s.\n",fifo_name,strerror(errno));
return -1;
}

signal(SIGINT,SIG_IGN);

while(1)
{
getchar();
printf(">");

fgets(buf,sizeof(buf),stdin);
buf[strlen(buf)-1] = '\0';

write(fd,buf,strlen(buf));

if(strncmp(buf,"quit",4) == 0)
{
kill(pid,SIGKILL);
usleep(500);
exit(EXIT_SUCCESS);
}
}

return 0;
}

int main(int argc,char *argv[])
{
int pid;

if(argc < 3)
{
fprintf(stderr,"usage %s argv[1].\n",argv[0]);
exit(EXIT_FAILURE);
}

if(mkfifo(argv[1],0666) < 0 && errno != EEXIST)
{
perror("Fail to mkfifo");
exit(EXIT_FAILURE);
}

if(mkfifo(argv[2],0666) < 0 && errno != EEXIST)
{
perror("Fail to mkfifo");
exit(EXIT_FAILURE);
}

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

perror("Fail to fork");
exit(EXIT_FAILURE);

}else if(pid == 0){

child_recv_fifo(argv[2]);

}else{

father_send_fifo(argv[1],pid);
}

exit(EXIT_SUCCESS);
}

client B

#include <stdio.h>
#include <stdlib.h>
#include <signal.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <errno.h>
#include <string.h>
#include <fcntl.h>

#define MAX 100

void signal_handler(int signum)
{
static int flag = 0;

switch(signum)
{
case SIGALRM:
if(flag == 0)
{
printf("The people is leaving,the system is closed in 10 seconds \
and you can input 'ctrl + c' cancel.\n");
alarm(10);
}else{

kill(getppid(),SIGKILL);
usleep(500);
exit(EXIT_SUCCESS);
}

flag = 1;
break;

case SIGINT:
printf("The alarm is cancel.\n");
alarm(0);
break;
}

}

int child_recv_fifo(char *fifo_name)
{
int n,fd;
char buf[MAX];

if((fd = open(fifo_name,O_RDONLY)) < 0)
{
fprintf(stderr,"fail to open %s : %s.\n",fifo_name,strerror(errno));
return -1;
}

signal(SIGALRM,signal_handler);
signal(SIGINT,signal_handler);
alarm(15);//璁剧疆瀹氭椂鍣?
while(1)
{
n = read(fd,buf,sizeof(buf));
buf
= '\0';

printf("Read %d bytes : %s.\n",n,buf);

if(strncmp(buf,"quit",4) == 0 || n == 0)
{
kill(getppid(),SIGKILL);
usleep(500);
exit(EXIT_SUCCESS);
}

alarm(15);
}

return 0;
}

int father_send_fifo(char *fifo_name,int pid)
{
int n,fd;
char buf[MAX];

if((fd = open(fifo_name,O_WRONLY)) < 0)
{
fprintf(stderr,"Fail to open %s : %s.\n",fifo_name,strerror(errno));
return -1;
}

signal(SIGINT,SIG_IGN);

while(1)
{
getchar();
printf(">");

fgets(buf,sizeof(buf),stdin);
buf[strlen(buf)-1] = '\0';

write(fd,buf,strlen(buf));

if(strncmp(buf,"quit",4) == 0)
{
kill(pid,SIGKILL);
usleep(500);
exit(EXIT_SUCCESS);
}
}

return 0;
}

int main(int argc,char *argv[])
{
int pid;

if(argc < 3)
{
fprintf(stderr,"usage %s argv[1].\n",argv[0]);
exit(EXIT_FAILURE);
}

if(mkfifo(argv[1],0666) < 0 && errno != EEXIST)
{
perror("Fail to mkfifo");
exit(EXIT_FAILURE);
}

if(mkfifo(argv[2],0666) < 0 && errno != EEXIST)
{
perror("Fail to mkfifo");
exit(EXIT_FAILURE);
}

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

perror("Fail to fork");
exit(EXIT_FAILURE);

}else if(pid == 0){

child_recv_fifo(argv[1]);

}else{

father_send_fifo(argv[2],pid);
}

exit(EXIT_SUCCESS);
}