What is Waitpid used for?
The waitpid() function allows the calling thread to obtain status information for one of its child processes. The calling thread suspends processing until status information is available for the specified child process, if the options argument is 0.
What does Waitpid mean in C?
More precisely, waitpid() suspends the calling process until the system gets status information on the child. If the system already has status information on an appropriate child when waitpid() is called, waitpid() returns immediately.
What is the difference of wait and Waitpid?
Difference between wait and waitpid(): Wait() waits for any child process but waitpid() waits for a specific child equal to pid. By default waitpid() waits for the only terminated child where as wait() waits for both terminated or a signaled child.
How do you terminate a child’s parent process?
When you need to terminate the child process, use the kill(2) function with the process ID returned by fork(), and the signal you wish to deliver (e.g. SIGTERM). Remember to call wait() on the child process to prevent any lingering zombies.
What is Wuntraced?
This flag specifies that waitpid should return immediately instead of waiting, if there is no child process ready to be noticed. WUNTRACED. This flag specifies that waitpid should report the status of any child processes that have been stopped as well as those that have terminated.
How many parameters are there in Waitpid () system call?
Basically you have 3 parameters: pid_t waitpid(pid_t pid, int *status, int options); pid is the process you are waiting for.
What is Wstatus?
WEXITSTATUS(status) returns the exit status of the child. This consists of the least significant 8 bits of the status argument that the child specified in a call to exit(3) or _exit(2) or as the argument for a return statement in main().
What does Wnohang do in Waitpid?
If status information is immediately available on an appropriate child process, waitpid() returns this information. Otherwise, waitpid() returns immediately with an error code indicating that the information was not available. In other words, WNOHANG checks child processes without causing the caller to be suspended.
Why is Waitpid returning?
If waitpid() returns because the status of a child process is available, it returns a value equal to the process ID of the child process for which status is reported. If waitpid() returns due to the delivery of a signal to the calling process, -1 is returned and errno is set to EINTR.
What if parent process exits before the child?
When a parent process dies before a child process, the kernel knows that it’s not going to get a wait call, so instead it makes these processes “orphans” and puts them under the care of init (remember mother of all processes). Init will eventually perform the wait system call for these orphans so they can die.
What happens when child process terminates?
When a child process terminates, some information is returned to the parent process. When a child process terminates before the parent has called wait, the kernel retains some information about the process, such as its exit status, to enable its parent to call wait later.
What is Wifstopped?
WIFSTOPPED, WIFSIGNALED, WIFEXITED, are macros that take an argument status , of type int, as returned by wait() , or wait3() , or wait4() . WIFSTOPPED evaluates to true (1) when the process for which the wait() call was made is stopped, or to false (0) otherwise.
What does Wnohang?
WNOHANG. This flag specifies that waitpid should return immediately instead of waiting, if there is no child process ready to be noticed. WUNTRACED. This flag specifies that waitpid should report the status of any child processes that have been stopped as well as those that have terminated.
What is Waitpid system call?
The waitpid() system call suspends execution of the current process until a child specified by pid argument has changed state. By default, waitpid() waits only for terminated children, but this behaviour is modifiable via the options argument, as described below.
What is waitpid() system call?
The waitpid () system call suspends execution of the current process until a child specified by pid argument has changed state. By default, waitpid () waits only for terminated children, but this behaviour is modifiable via the options argument, as described below. The value of pid can be:
Why does waitpid () return 0?
If WNOHANG was given, and if there is at least one process (usually a child) whose status information is not available, waitpid () returns 0.
What is the difference between PiD and waitpid?
If pid is -1, waitpid () waits for any child process to end. If pid is less than -1, waitpid () waits for the termination of any child whose process group ID is equal to the absolute value of pid. Points to a location where waitpid () can store a status value. This status value is zero if the child process explicitly returns zero status.
When to use wnohang and wuntraced?
You usually use WNOHANG and WUNTRACED in different cases. Case 1: Suppose you have a process which spawns off a bunch of children and needs to do other stuff while the children are running.