When a child process stops the parent process must reap it to prevent it from becoming a zombie. The zombie meaning that the process is not taken out from process table but they dont execute and do not utilize the memory or CPU.
When a parent process stops before the child stops, then the child process is called an orphan process. Often some process adapts the child process and becomes its parent. This process often is the init process.
When a parent waits for the child process by some means and reaps it, the zombie processes will not happen.
Waiting for a child process can be performed in the following ways.
- calling
waitand finding the child's pid as its return value. - calling
waitpidon a specific child.
The wait system call suspends the execution of the parent process until one of its child processes terminates. On success returns the process ID of the child and -1 on failure.
The prototype is as follows.
pid_t wait(int *status);The waitpid system call suspends the execution of the parent process until a child process specified by the pid argument has changed state.
The prototype is as follows.
pid_t waitpid(pid_t pid, int *status, int options);By default, the waitpid waits only for the terminated children. With the options argument the behavior is changed.
| pid value | meaning |
|---|---|
| -1 | wait for any child process |
| > 0 | wait for the child with the given process ID |
Most of the time the options argument is either 0 or WNOHANG. With WNOHANG the waitpid returns immediately if there is no child exits.
The status value is kept into the waitpid if the passed argument of status is non NULL. The value can be interpreted with the following macros.
| exit status | description |
|---|---|
WIFEXITED(status) |
returns true if the child is exited normally |
WEXITSTATUS(status) |
returns the exit status of the child. call this only if WIFEXITED(status) returns true |