先决条件:前叉系统调用
对wait()的调用将阻止调用进程,直到其子进程之一退出或接收到信号为止。子进程终止后,父进程在等待系统调用指令后继续执行。
子进程可能由于以下任何原因而终止:
- 它调用exit();
- 它从main返回(int)
- 它从操作系统或其他进程接收信号,其默认操作是终止。
C语言语法:
#include
#include
// take one argument status and returns
// a process ID of dead children.
pid_t wait(int *stat_loc);
如果任何进程有多个子进程,则在调用wait()之后,如果没有子进程终止,则父进程必须处于等待状态。
如果只有一个子进程终止,则返回一个wait()返回终止的子进程的进程ID。
如果终止了多个子进程,则wait()将获得任何任意子进程并返回该子进程的进程ID。
当wait()返回时,它们还通过指针(如果status不为NULL )定义退出状态(告诉我们,进程为何终止)。
如果任何进程没有子进程,则wait()立即返回“ -1”。
注意:“由于环境问题,此代码无法在简单的IDE中运行,因此请使用终端来运行代码”
例子:
CPP
// C program to demonstrate working of wait()
#include
#include
#include
#include
int main()
{
pid_t cpid;
if (fork()== 0)
exit(0); /* terminate child */
else
cpid = wait(NULL); /* reaping parent */
printf("Parent pid = %d\n", getpid());
printf("Child pid = %d\n", cpid);
return 0;
}
C
// C program to demonstrate working of wait()
#include
#include
#include
int main()
{
if (fork()== 0)
printf("HC: hello from child\n");
else
{
printf("HP: hello from parent\n");
wait(NULL);
printf("CT: child has terminated\n");
}
printf("Bye\n");
return 0;
}
C
// C program to demonstrate working of status
// from wait.
#include
#include
#include
#include
void waitexample()
{
int stat;
// This status 1 is reported by WEXITSTATUS
if (fork() == 0)
exit(1);
else
wait(&stat);
if (WIFEXITED(stat))
printf("Exit status: %d\n", WEXITSTATUS(stat));
else if (WIFSIGNALED(stat))
psignal(WTERMSIG(stat), "Exit signal");
}
// Driver code
int main()
{
waitexample();
return 0;
}
C
// C program to demonstrate waitpid()
#include
#include
#include
#include
void waitexample()
{
int i, stat;
pid_t pid[5];
for (i=0; i<5; i++)
{
if ((pid[i] = fork()) == 0)
{
sleep(1);
exit(100 + i);
}
}
// Using waitpid() and printing exit status
// of children.
for (i=0; i<5; i++)
{
pid_t cpid = waitpid(pid[i], &stat, 0);
if (WIFEXITED(stat))
printf("Child %d terminated with status: %d\n",
cpid, WEXITSTATUS(stat));
}
}
// Driver code
int main()
{
waitexample();
return 0;
}
输出:
Parent pid = 12345678
Child pid = 89546848
C
// C program to demonstrate working of wait()
#include
#include
#include
int main()
{
if (fork()== 0)
printf("HC: hello from child\n");
else
{
printf("HP: hello from parent\n");
wait(NULL);
printf("CT: child has terminated\n");
}
printf("Bye\n");
return 0;
}
输出:取决于环境
HC: hello from child
Bye
HP: hello from parent
CT: child has terminated
(or)
HP: hello from parent
HC: hello from child
HC: Bye
CT: child has terminated // this sentence does
// not print before HC
// because of wait.
Bye
子状态信息:
通过等待报告的关于孩子的状态信息不仅是孩子的退出状态,还包括
- 正常/异常终止
- 终止原因
- 退出状态
要查找有关状态的信息,我们使用
WIF ….macros
1. WIFEXITED(状态) :孩子正常退出
• WEXITSTATUS(status) :子项退出时返回代码
2. WIFSIGNALED(状态) :由于没有捕获到信号,孩子退出了
• WTERMSIG(status) :给出终止信号的编号
3. WIFSTOPPED(状态) :孩子已停止
• WSTOPSIG(status) :给出停止信号的编号
/*if we want to prints information about a signal */
void psignal(unsigned sig, const char *s);
例子:
检查以下程序的输出。
C
// C program to demonstrate working of status
// from wait.
#include
#include
#include
#include
void waitexample()
{
int stat;
// This status 1 is reported by WEXITSTATUS
if (fork() == 0)
exit(1);
else
wait(&stat);
if (WIFEXITED(stat))
printf("Exit status: %d\n", WEXITSTATUS(stat));
else if (WIFSIGNALED(stat))
psignal(WTERMSIG(stat), "Exit signal");
}
// Driver code
int main()
{
waitexample();
return 0;
}
输出:
Exit status: 1
我们知道如果终止了多个子进程,那么wait()会获得任意子进程,但是如果我们想获得任何特定的子进程,则可以使用waitpid()函数。
Syntax in c language:
pid_t waitpid (child_pid, &status, options);
选项参数
- 如果为0表示没有选项,则父级不必等待终止子级。
- 如果WNOHANG表示父级不等待,如果子级不终止,则只需检查并返回waitpid()。(不阻止父级进程)
- 如果child_pid为-1,则表示任意子级,这里waitpid()的工作方式与wait()相同。
waitpid()的返回值
- 孩子的pid(如果孩子退出了)
- 0,如果使用WNOHANG并且child尚未退出。
C
// C program to demonstrate waitpid()
#include
#include
#include
#include
void waitexample()
{
int i, stat;
pid_t pid[5];
for (i=0; i<5; i++)
{
if ((pid[i] = fork()) == 0)
{
sleep(1);
exit(100 + i);
}
}
// Using waitpid() and printing exit status
// of children.
for (i=0; i<5; i++)
{
pid_t cpid = waitpid(pid[i], &stat, 0);
if (WIFEXITED(stat))
printf("Child %d terminated with status: %d\n",
cpid, WEXITSTATUS(stat));
}
}
// Driver code
int main()
{
waitexample();
return 0;
}
输出:
Child 50 terminated with status: 100
Child 51 terminated with status: 101
Child 52 terminated with status: 102
Child 53 terminated with status: 103
Child 54 terminated with status: 104