I'm learning about process allocation.
Take this code block for example:
int main(){
pid_t pid = fork();
if(pid == 0){
while(1)
printf("Child ");
} else {
for(int i = 0; i<1000;i++)
printf("Parent ");
kill(pid,SIGTERM);
printf("\n%d \n ", pid);
}
}
pid = 0 is the child process, pid > 0 is the parent process. kill(pid,SIGTERM) is executed by the parent with it's own pid, yet it kills the child and not itself. Why?
As #Siguza mentioned in the comments, you should re-read the documentation of fork. fork returns a positive value of pid to the parent process. That value is the PID of the child process. Therefore, kill(pid, SIGTERM) sends the signal to the child and not the parent.
Related
int main() {
pid_t pid;
printf("Parent: %d\n", getpid());
pid = fork();
if (pid == -1) {
perror("fork");
goto clean_up;
} else if (pid > 0) {
sleep(3);
} else {
printf("Child Parent: %d\n", getppid());
printf("Child: %d\n", getpid());
printf("Exiting...\n");
}
clean_up:
return 0;
}
I wanted to create zombie process on purpose (of course for experimenting/learning). After the child exits, the parent doesn't wait()s for the child. So, I'm expecting zombie to show up, in ps -ef | grep zombie.o. But for some reason it is not showing up. What could be the possible reasons?
When the parent exits, all its children (alive or zombie) are assigned PID 1 as their new parent. See the _exit(2) man page: " any children of the process are inherited by process 1".
PID 1 is normally the init daemon, and if it's working properly then it should always wait() for its children. So zombie children will be reaped immediately, and children that are still running will be reaped as soon as they exit.
If you want to create a long-lived zombie, the parent needs to remain alive but not calling wait().
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
int main ( void ) {
int pid, fpid, ppid;
fpid = fork();
printf ("fpid is is %d\n", fpid);
sleep(5);
if (fpid > 0) {
pid = getpid();
ppid = getppid();
printf ("\nThis is Parent. My pid %d. My parent's pid %d\n", pid, ppid);
} else if (fpid == 0) {
sleep(1);
pid = getpid();
ppid = getppid();
printf ("\nThis is Child. My pid %d. My parent'a pid %d\n", pid, ppid);
}
}
I think when the parent process ID is 1 it means that the parent process has been terminated, so the child process gets re-parented to 1 (init, the first process). Is there any reason why the parent process would be terminated?
Parent process doesn't wait (by means of wait(2)) for the child process to complete. So, if parent exits before the child (it becomes an orphan process), then child process will be re-parented (adopted) to init process whose process ID is usually 1. Thus the child process says its parent process ID is 1.
Note that the init process' ID isn't necessarily 1 on all systems. POSIX doesn't mandate any such thing.
Because the child sleeps, by the time it calls getppid(), its parent will have likely died and the child will have been reparented to the init process (pid == 1).
I am a bit confused about why the child process in the following two programs is showing different parents ids.
First program:
int main ( void ) {
int pid, fpid, ppid;
fpid = fork ();
pid = getpid();
ppid = getppid();
printf ("fpid is %d\n", fpid);
sleep(5);
if (fpid > 0){
printf ("\nThis is Parent. My pid %d. My parent's pid %d\n",pid,ppid);
}
else if (fpid ==0){
sleep(1);
printf ("\nThis is Child. My pid %d. My parent's pid %d\n",pid,ppid);
}
else
printf ("fork failed\n");
return (0);
}
Output:
fpid is 53560
fpid is 0
This is Parent. My pid 53559. My parent's pid 44632
MacBook-Pro:~/Desktop/$
This is Child. My pid 53560. My parent's pid 53559
Second program:
int main ( void ) {
int pid, fpid, ppid;
fpid = fork ();
printf ("fpid is is %d\n", fpid);
sleep(5);
if (fpid > 0){
pid = getpid();
ppid = getppid();
printf ("\nThis is Parent. My pid %d. My parent's pid %d\n",pid,ppid);
}
else if (fpid ==0){
sleep(1);
pid = getpid();
ppid = getppid();
printf ("\nThis is Child. My pid %d. My parent's pid %d\n",pid,ppid);
}
else
printf ("fork failed\n");
return (0);
}
Output:
fpid is is 53635
fpid is is 0
This is Parent. My pid 53634. My parent's pid 44632
MacBook-Pro:~/Desktop$
This is Child. My pid 53635. My parent's pid 1
I understand that process 1 is the process that takes over as a parent once the original parent terminates. I guess what I want to know is: isn't the parent process being finished before the child process can process its printf in both cases? Shouldn't the outputs be the same?
Since parent and child processes run concurrently, the order of execution depends on runtime. One of them can finish earlier. When parent finishes before child reaches its getppid(), child process would be adopted by init. Hence the parent id 1.
To see child's actual parent process id:
Let the parent wait for its child termination using wait() or waitpid(), or
Let parent sleep for some perceivable amount like sleep(120) after 'This is parent' printf().
isn't the parent process being finished before the child process can process it's printf in both cases?
Very likely so, but not absolutely certain. You cannot ensure that by sleep()ing for any length of time.
Shouldn't the outputs be the same?
They could be the same, but they are unlikely to be. It's important to note when in each program getppid() is called. The parent process ID reported is the one that applied at the time of the call; it is not necessarily still applicable later, when the value is printed.
i have following code
void main()
{
pid_t pid,pid1;
pid = fork();
if(pid==0)
{
pid1= getpid();
printf("\n Child A %d" ,pid);
printf("\n Child B %d",pid1);
}
else
{
pid1 = getpid();
printf("\n Parent C %d:",pid);
printf("\nParent d %d:",pid1);
}
}
i am not understanding why i am getting same Process id for B and C. Can anyone help me here?
pid1 = getpid();
That is run in the child process and hence gives the child process id.
pid = fork();
That is initiated from the parent process but the return value is available to both parent and child. However, it returns a different value to the parent and child processes. Straight from the fork man page:
the PID of the child process is returned in the parent, and 0 is
returned in the child
So in both cases (B and C), it is the pid of the child process.
This code is ran by two processes, one is the parent and the other is children. p process - return id of c process and c process return 0. i think your program maybe a demo of fork(), so it might be like this:
void main()
{
pid_t pid,pid1;
pid = fork();
if(pid==0)
{
pid1= getpid();
printf("\n i am Child A %d" ,pid1);
printf("\n Parent A %d",getppid());
}
else
{
pid1 = getpid();
printf("\n i am Parent B %d:",pid1);
printf("\n Children B %d:",pid);
}
}
I want to write an UNIX program that creates N child processes, so that the first process creates one child process, then this child creates only one process that is its child, then the child of the child creates another child etc.
Here's my code:
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
int main()
{
int N=3;
int i=0;
printf("Creating %d children\n", N);
printf("PARENT PROCESS\nMy pid is:%d \n",getpid() );
for(i=0;i<N;i++)
{
pid_t pid=fork();
if(pid < 0)
{
perror("Fork error\n");
return 1;
}
else if (pid==0) /* child */
{
printf("CHILD My pid is:%d my parent pid is %d\n",getpid(), getppid() );
}
else /* parrent */
{
exit(0);
}
}
return 0;
}
The output that I expect is in the form:
Creating 3 children
PARENT PROCESS
My pid is 1234
CHILD My pid is 4567 my parent pid is 1234
CHILD My pid is 3528 my parent pid is 4567
CHILD My pid is 5735 my parent pid is 3528
The output I get in the terminal is
Creating 3 children
PARENT PROCESS
My pid is:564
CHILD My pid is:5036 my parent pid is 564
User#User-PC ~
$ CHILD My pid is:4804 my parent pid is 1
CHILD My pid is:6412 my parent pid is 4804
The problem is that the program doesn't seem to terminate. I should use Ctrl+C to get out of the terminal, which is not normal. Can you help me to fix this issue?
The children die when the parent dies.
In your case the parent exits before all the children have been created.
Try waiting for the children before exiting:
else /* parrent */
{
int returnStatus;
waitpid(pid, &returnStatus, 0); // Parent process waits for child to terminate.
exit(0);
}
try to wait the process with wait(NULL);
pid_t child = fork();
if (child == -1)
{
puts("error");
exit(0);
}
else if (child == 0)
{
// your action
}
else
{
wait(&child);
exit(0);
}
so your father will wait the child process to exit
The proposed cure is correct, but the reason stated is wrong. Children do not die with the parent. The line
CHILD My pid is:4804 my parent pid is 1
clearly indicates that by the time child called getppid() its parent is already dead, and the child has been reparented to init (pid 1).
The real problem is that after the child prints its message, it continues to execute the loop, producing more children, making your program into a fork bomb.