Process Control (ii)

5 Wait and Waitpid functions

When a process is normal or abnormally terminated, the kernel sends a SIGCHLD signal to its parent process.

5.1 Processes that call wait or waitpid may:

Blocking (if its child processes are still running)

The termination status of the tape process returns immediately

Error returns immediately

The difference between 5.2wait and waitpid:

Before a child process terminates, wait causes its callers to block, and Waitpid has a selection that allows the caller to not block.

Waitpid does not wait for the first terminating child process-it has several choices that can control the process it waits for.

code example:

#include "ourhdr.h" #include <sys/wait.h>void pr_exit (int status) {if (wifexited (status)) printf ("Normal termination, exit status =%d\n ", Wexitstatus (status)), else if (wifsignaled status) printf (" abnormal termination, Signal Number =%d%s\n ", Wtermsig (status), #ifdefWCOREDUMPWCOREDUMP (status)? "(core file generated)": ""), #else ""), #endifelse if (wifstopped) printf ("Child stopped, signal number =%d\n", WST Opsig (status));} int main (void) {pid_tpid;intstatus;if (PID = fork ()) < 0) Err_sys ("fork Error"), else if (PID = = 0)/* Child */exit (7); (Wait (&status)! = PID)/* Wait for child */err_sys ("Wait Error");p r_exit (status);/* and print its status */if (PID = f Ork ()) < 0) Err_sys ("fork Error"), else if (PID = = 0)/* child */abort ();/* Generates SIGABRT */if (Wait (&status)! = P ID)/* Wait for child */err_sys ("Wait Error");p r_exit (status);/* and print its status */if ((PID = fork ()) < 0) Err_sys (" Fork Error "), else if (PID = = 0)/* Child */status/= 0;/* divide by 0 generateS SIGFPE */if (Wait (&status)! = PID)/* Wait for child */err_sys ("Wait Error");p r_exit (status);/* and print its status */exit (0);}

Printing results:

[[email protected] fork]# gcc wait.c wait.c:in function ' main ': Wait.c:48:warning:division by Zero[[email protected] for k]#./a.out normal termination, exit status = 7abnormal termination, signal number = 6abnormal termination, signal number = 8

5.3 If a process wants F o r k a child process, but does not require it to wait for the child process to terminate, nor want the child process to be in a zombie state until the parent process terminates, the trick to implement this requirement is to call F o r k two times.

code example:

#include "ourhdr.h" #include <sys/wait.h> #include <sys/types.h>intmain (void) {pid_tpid;if (PID = fork ())  < 0) {Err_sys ("fork Error"),} else if (PID = = 0) {/* first child */if (PID = fork ()) < 0) Err_sys ("fork Error"); else if (PID > 0) exit (0);/* Parent from second fork = = First child *//* * We ' re the second child; Our parent becomes init as soon * as we real parent calls exit () in the statement above. * Here's where we ' d continue executing, knowing that if * we ' re done, Init would reap our status. */sleep (2);p rintf ("second child, parent PID =%d\n", Getppid ()); exit (0);} if (Waitpid (PID, NULL, 0)! = PID)/* Wait for first child */err_sys ("Waitpid error");/* * We ' re the parent (the original PR ocess); We continue executing, * knowing that we don't have the parent of the second child. */exit (0);}

　　

Printing results:

[Email protected] fork]#/a.out [[email protected] fork]# second child, parent PID = 1

　　

6 Wait3 and WAIT4 functions

Process Control (ii)