vault backup: 2022-09-29 09:36:54

content/notes/17-processes-communication.md

@@ -15,10 +15,32 @@ defn: independent process - one does not affect the other and they do not commun
 
 # creating a child process
 A child process is creating from a parent using fork.
-everything the same as the parent except the child has a differnent process ID.
+
+fork returns a value. 0 for the parent, and the id of the child for the child
+
+everything the same as the parent except the child has a differnent process ID. e.g., when they start they the same heap location etc. but as the child runs it may change.
+
+if you kill the parent before the child the child becomes an orhpan process
+if yo ukill the child while the parent is waiting for the child the parent becomes a zombie (defunct) process. -- but only if the parent doesn't wait for the child.
+
+the child process can also call exexlp (load program) which erases the copy of the parent (itself not the parent)
+- when initally forked the child and parent share the same space
+- when the child calls execlp then the child will fully copy the program to a new memory location so it doesn't overwrite the parent
 
 # communcation between parent and child
 process create a chld process to do a task. but they need to communicate
 
+after a process calls a chld process. it can execute a wait system call.
+
+this moves a process off the ready queue, untill the chld process has terminated
+when the child process has terminated, it can return its status to the parent.
+
+# process termination
+automatically executes exit after the last statement.
+- may return status to parent who is `wait`ing
+- all rsouces are dallocated by the OS
+
+ they can also die more violently
+ 
 
 # signal and pipe for inter process communication