fork and signal

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int main(void){

printf("Will be executed one \n"); 仅执行一次
fork();
printf("Will be executed twice\n"); 将被执行两次。
return 0;
}

fork：

#include <stdio.h>
#include <sched.h>

int data = 10;

int child_process()
{
printf("Child process %d, data %d\n",getpid(),data);
data = 20;
printf("Child process %d, data %d\n",getpid(),data);
while(1);
}

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

if(pid==0) {
  child_process();
}
else{
  sleep(1);
  printf("Parent process %d, data %d\n",getpid(), data);
  while(1);
}
}

运行结果：

lihacker@lihacker-laptop:~$ ./fork
Child process 18604, data 10
Child process 18604, data 20
Parent process 18603, data 10

父子进程运行改变变量互不影响。

vfork :

int data = 10;

int child_process()
{
printf("Child process %d, data %d\n",getpid(),data);
data = 20;
printf("Child process %d, data %d\n",getpid(),data);
//while(1);
}

int main(int argc, char* argv[])
{
if(vfork()==0) {
  child_process();
}
else{
  sleep(1);
  //这里data 将会和子进程中被改变的一样，变量值将会相互影响。
  printf("Parent process %d, data %d\n",getpid(), data);
  while(1);      ////发信号kill子进程
                    //kill(pid,SIGKILL);
}
}

lihacker@lihacker-laptop:~$ ./vfork
Child process 18843, data 10
Child process 18843, data 20
Parent process 18842, data 20

execlp :

int main(int argc,char* argv[]){
if(argc<2){
printf("Usage: %s path\n",argv[0]);
return 1;
}

execlp("/bin/ls","ls",argv[1],(char*)NULL);//调用之后即不返回
return 0;
}

　char * argv[ ] ={ “ls”,”-al”,”/etc/passwd”,0};

　　execvp(“ls”,argv);

signal :

void handle(int sig)
{
    pid_t pid;
while((pid=waitpid(-1,NULL,0))>0)
  {
  printf("handle signal");
  }
sleep(1);
return ;
}
int main()
{
pid_t pid;
    if(signal(SIGCHLD,handle)==SIG_ERR)
  perror("cannot reset the SIGCHLOD signal handler");
  return 1;
  for(int i=0;i<3;i++)
   {
   pid=fork();
             if(pid==0)
             {
             printf("child %d"\n,(int)getpid());
    sleep(1);
             }
  }
  pause();
}

kill(pid,9); 、、也用于结束进程
sigaction:

定义函数 int sigaction(int signum,const struct sigaction *act ,struct sigaction *oldact);

　　函数说明 sigaction()会依参数signum指定的信号编号来设置该信号的处理函数。参数signum可以指定SIGKILL和SIGSTOP以外的所有信号。

参数结构sigaction定义如下
struct sigaction
{
void (*sa_handler) (int);
sigset_t sa_mask;
int sa_flags;
void (*sa_restorer) (void);
}

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

void signal_set(struct sigaction *act)
{
        switch(act->sa_flags){
  case (int)SIG_DFL:
   printf("using default hander\n");
   break;
  case (int)SIG_IGN:
   printf("ignore the signal\n");
   break;
  default:
   printf("%0x\n",act->sa_handler);
     }
}

int main(int argc,char** argv)
{
        int i;
        struct sigaction act,oldact;

act.sa_handler = signal_set;
        act.sa_flags = SA_NODEFER| SA_RESETHAND;

sigaction (SIGUSR1,&act,&oldact) ;

for (i=1; i<12; i++){
                printf("signal %d handler is : ",i);
                sigaction (i,NULL,&oldact) ;
                signal_set (&oldact);
        }

return 0;
}

int status;

while((wait_pid=wait(&status)) && wait_pid!=-1)
printf("process id:%d exit,exit_code is %0x\n",wait_pid,status);

while((wait_pid=wait(&status)) && wait_pid!=-1){
                if(WIFSIGNALED(status))
       printf("process id:%d Receive SIG :%d exit\n",pid,WTERMSIG(status));

  if(WIFEXITED (status))
   printf("process id:%d exit code %d\n",pid,WEXITSTATUS(status));
}

wait :

int main(void)
{
pid_t pid,wait_pid;
int status;

     pid = fork();

if (pid==-1) {
        perror("Cannot create new process");
  exit(1);
} else if (pid==0) {
               printf("child process id: %ld\n", (long) getpid());

        pause();
               _exit(0);
           } else {
     do {
      wait_pid=waitpid(pid, &status, WUNTRACED | WCONTINUED);

      if (wait_pid == -1) {
       perror("cannot using waitpid function");
       exit(1);
      }
     ///    调用定义的宏来判断进程是何种退出方式示例
      if (WIFEXITED(status))
       printf("child process exites, status=%d\n", WEXITSTATUS(status));

      if(WIFSIGNALED(status))
       printf("child process is killed by signal %d\n", WTERMSIG(status));

      if (WIFSTOPPED(status))
       printf("child process is stopped by signal %d\n", WSTOPSIG(status));

      if (WIFCONTINUED(status))
       printf("child process resume running....\n");

     } while (!WIFEXITED(status) && !WIFSIGNALED(status));

     exit(0);
    }
}

产生僵尸进程：一个进程结束了,但是他的父进程没有等待(调用wait / waitpid)他, 那么他将变成一个僵尸进程. 但是如果该进程的父进程已经先结束了,那么该进程就不会变成僵尸进程, 因为每个进程结束的时候,系统都会扫描当前系统中所运行的所有进程, 看有没有哪个进程是刚刚结束的这个进程的子进程,如果是的话,就由Init 来接管他,成为他的父进程……

int main (void)
{
pid_t pid;

pid = fork();

if(pid<0){
perror("cannot create new process");
exit(1);
}

if (pid>0)
sleep (60);
else
exit (0);
return 0;
}

void main()
{

pid_t pid;
/* char buf[128];

if(write(stout,buf,sizeof(buf))!=sizeof(buf)-1)
printf("write_error");*/
if((pid=fork())<0)// fork +exec :--->swap 运行新的进程
{
printf("fork error");
}
else if(pid==0)
{
//子进程，进程的运行有两种方式，即独立运行和使用父进程运行
//父进程已经终止的进程(孤儿进程)由init (ID=1)领养
//ID==0 调度进程-- 交换进程(swapper) : 系统进程
//ID==2 :页守护进程:支持虚拟存储系统的分页操作
        printf("set up signal handle");
signal(SIGUSR1,sig_usr); //建立signal handle
        printf("kill self");
kill(getpid(),SIGTSTP); //STOP itself
}
else
{
sleep(5);//父进程休息，先运行子进程，没有这个，父子进程先后执行顺序不确定
           //子进程在父地址空间中运行
        printf("father running");
}
   exit(0);
}