Wait and exit child process

#include<stdlib.h>
void exit(int status);

• Calling exit() performs some basic shutdown steps and then instructs the kernel to terminate the process.

• The status parameter (status) is used to indicate the exit status of the process.

• When a process finishes, the kernel sends a SIGCHLD signal to the parent.

pid_t pid = fork();
if (pid > 0) {
   printPIDs("PARENT");
   wait(&child_status);
}
else if (!pid) {
   printPIDs("CHILD");
   exit(0);
} 
else if (pid == -1) {
   printf("ERROR");
   return EXIT_FAILURE;
}

Before terminating the process, the C library performs the following steps:

• Calls all functions registered with atexit() or on_exit(), in reverse order of their registration.

• Flushes all standard open input-output streams.

• Removes all temporary files created with the tmpfile() function.

These steps complete all the work that the process must do in user space, so exit() calls the system call _exit() to allow the kernel to handle the rest of the termination process:

#include <unistd.h>
void _exit (int status);

The process can be terminated if a signal is sent whose default action is to terminate the process. Such signals are SIGTERM and SIGKILL.

When a process finishes, the kernel sends a SIGCHLD signal to the parent process. By default, this signal is ignored and the parent takes no action. Processes can choose to handle this signal through the signal() or sigaction() system calls.

#include<sys/types.h>
#include<sys/wait.h>
pid_t wait(int status);
int WIFEXITED(status);
int WIFSIGNALED(status);
int WIFSTOPPED(status);
int WIFCONTINUED(status);
int WEXITSTATUS(status);
int WTERMSIG(status);
int WSTOPSIG(status);
int WCOREDUMP(status);

• wait() returns the value of the identifier (pid) of a child process or -1 on error.

• If no child process has terminated, the call blocks until a child process finishes.

• WIFEXITED returns true if the process terminated normally.

• On normal termination, WEXITSTATUS provides the eight bits that are passed to _exit.

• WIFSIGNALED returns true if a signal caused the process termination.

• In case of termination by signal, WTERMSIG returns the number of that signal.

• In case of termination by signal, WCOREDUMP returns true if the process dumped core in response to receiving the signal.

• WIFSTOPPED and WIFCONTINUED return true if the process was stopped or continued.

• If WIFSTOPPED is true, WSTOPSIG provides the number of the signal that stopped the process.

When an error occurs, there are two possible values:

Error	Information
ECHILD	The calling process has no child processes.
EINTR	The call terminated early due to a received signal.