Using fork-2: Fork with wait.

In the post "Fork-1 : Introduction" we saw how a child gets created using the fork call.
Every parent process which forks a child has to keep track of its child/children. When a child terminates, the parent has to be informed about the termination and only then the child and its resources can be deleted from the system.
To be able to keep track of the child the system call wait() is used by the parent. The wait series of system calls allows the parent to track the changes in the states of the child process.
If the parent does not call wait on its child, then the child that gets terminated goes into a zombie state, and will be removed from the system only when it is assigned a new parent and that parent calls a wait on it or the system is shutdown.
There are three kinds of wait calls
wait:

pid_t wait(int *status); waits for one of the children to terminate. Status gets filled with an integer value indicating the reason of child's exit.

waitpid:

pid_t waitpid(pid_t pid, int *status, int options); waits for the children with pid as its process id to terminate. other than termination the parent can also be made to wait for other conditions by using the appropriate options.

waitid

int waitid(idtype_t idtype, id_t id, siginfo_t *infop, int options); waits for a process based in the id_type and id. The siginfo is filled up as return values and the options can be used to control which state change of the child to wait for.

The following code creates a child and waits on the same using the wait() system call.
The value filled in the variable status denotes the return status of the child and can be inspected using various macros, one of which is WIEXITSTATUS. The code below uses this macro to print the inspected value of status.

fork_wait.c

#include<stdio.h>#include<stdlib.h>#include<unistd.h>#include<sys/types.h>#include<sys/wait.h> main() { int i; pid_t pid; pid=fork(); int status; printf("Fork returned %d\n",pid); if(pid == 0) { printf("\n In child %d \n",getpid()); } else{ printf("\n In parent %d \n",getpid()); wait(&status); printf("\n Child return status %d \n",WEXITSTATUS(status)); } }

Save it,compile and excute it.

cc fork_wait.c

Output:

Fork returned 2415 In parent 2414 Fork returned 0 In child 2415 Return status 17

The following code uses waitpid and waits on the child using its pid and prints the status returned using WIEXITSTATUS.

fork_waitpid

#include<stdio.h>#include<stdlib.h>#include<unistd.h>#include<sys/wait.h>#include<sys/types.h> main() { int i; pid_t pid; int status; pid=fork(); printf("Fork returned %d\n",pid); if(pid == 0) { printf("\n In child %d \n",getpid()); } else{ printf("\n In parent %d \n",getpid()); waitpid(pid,&status,NULL); printf("\n Child return status %d \n",WEXITSTATUS(status)); } }

Save it,compile and excute it.

cc fork_waitpid.c

output:

Fork returned 2498 In parent 2497 Fork returned 0 In child 2498 The child exited with 17

The following code used waitid to wait on the child. One of the field of siginfo_t structure, which is filled with the information regarding the child by the system call, is the return status of the child.

The same is printed after the wait in the example below.

fork_waitid.c

#include<stdio.h>#include<stdlib.h>#include<unistd.h>#include<sys/wait.h>#include<sys/types.h> main() { int i; pid_t pid; int status; siginfo_t *infop; pid=fork(); printf("Fork returned %d\n",pid); if(pid == 0) { printf("\n In child %d \n",getpid()); } else{ printf("\n In parent %d \n",getpid()); infop=malloc(sizeof(siginfo_t )); waitid(P_PID,pid,infop,WEXITED); printf("The child exited with %d",infop->si_status); } } Save it,compile and excute it.

cc fork_waitpid.c

Output:

Fork returned 2507 In parent 2506 Fork returned 0 In child 2507 The child exited with 17 The other flags and options which can be passed to the wait system calls are mentioned in the man page of wait.