Difference between daemon and child thread?

Question

As per my knowledge daemon thread is a thread that runs in background(low priority thread) and also a child thread runs in the background when the parent thread runs foreground so where does the difference lie between a child and a daemon thread?

It's in case of java and can you explain referring to other languages too.

Answer 1

Java does not use the term child thread.

A thread is either a daemon-thread or a non-daemon-thread.

A non-daemon-thread must be stopped manually and keeps the VM running as long as the thread is running.

A daemon-thread cannot keep the VM running. When the VM stops all daemon-threads will stop running.

You can group threads together with a ThreadGroup. Groups can itself be grouped together that means that you can have a parent and a child ThreadGroups.

The priority of a thread has nothing to do with the explained concepts of threads. Any thread can have any priority.

For other languages/OS I cannot tell how the concepts are there.

Answer 2

Daemons (or Demons) and parent | child threading concepts are orthogonal to each other. In other words, the basis of your question is wrong. Likewise, priorities can be assigned to processes and threads, but priority is orthogonal to the definition of a demon, parent thread, or child thread status.

An application is a program that has been written and becomes a process when it is executed. An application is made up of a process and zero, one, or more threads. The Wikipedia entry on Threads does a good job of explaining the difference between a process and a thread.

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• processes are typically independent, while threads exist as subsets of a process
• processes carry considerably more state information than threads, whereas multiple threads within a process share process state as well as memory and other resources
• processes have separate address spaces, whereas threads share their address space
• processes interact only through system-provided inter-process communication mechanisms
• context switching between threads in the same process is typically faster than context switching between processes.

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A couple of notes -

1. threads may or may not share the same address space, it is OS dependent.
2. take the comment about context switching with a grain of salt - this will be OS and chipset dependent.

"Process" and "thread" are sometimes used interchangeably in conversations. It's common to call the process the "parent thread" even though this isn't technically correct since it's a process. Any thread spawning additional threads would be considered parent threads as well.

A daemon is a background process or application, generally without any interactive elements but that's not a hard requirement. A demon can have any priority assigned to it, not just low priority. In addition to the process, a daemon may have some child threads associated with it.

All of these terms are independent of the programming language used to write the application. Some languages provide better support for threads than other languages do.