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73 lines
2.9 KiB
Markdown
73 lines
2.9 KiB
Markdown
---
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title: "17-processes-communication"
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aliases:
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tags:
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- cosc204
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- lecture
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sr-due: 2022-12-10
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sr-interval: 39
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sr-ease: 230
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---
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# cooperating processes
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defn: coop process - execution of one process can affect the execution of another. e.e.g, processes for aprallel computing, threads for multithreading.
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defn: independent process - one does not affect the other and they do not communicate etc
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# creating a child process
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A child process is creating from a parent using fork.
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fork returns a value. 0 for the parent, and the id of the child for the child
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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.
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if you kill the parent before the child the child becomes an orhpan process
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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.
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the child process can also call exexlp (load program) which erases the copy of the parent (itself not the parent)
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- when initally forked the child and parent share the same space
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- 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
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# communcation between parent and child
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process create a chld process to do a task. but they need to communicate
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after a process calls a chld process. it can execute a wait system call.
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this moves a process off the ready queue, untill the chld process has terminated
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when the child process has terminated, it can return its status to the parent.
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# process termination
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automatically executes exit after the last statement.
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- may return status to parent who is `wait`ing
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- all rsouces are dallocated by the OS
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they can also die more violently
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- `kill` command
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- you can only processes you 'own'
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- Orphan process: a process whose parent terminated (inherited by init)
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- Zombie process: one which terminated, but its live parent process is not WAITING for it. Since no process is receiving its exit status, it stays in the process table
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# signal and pipe for inter process communication
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## signal
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communication between OS kernel and processes and between processes
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- process can install a handler (function) for a signal using the system call signaction() except for SIGKILL
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- The default action for a signal is usually to kill/terminate the process if a handler is not installed.
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- Other actions are ignore, core (dump a large file for debugging), stop(stop the process), and continue. The last two actions are used by debuggers like gdb to debug a program.
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## pipe
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connect process output and another process input.
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uni-directional channel
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file descriptors
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- 0 - stdin
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- 1 - stdout
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- 2 - sterr
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you can make file descriptors point to the same file using dup/dup2/dup3 |