Process
A process represents a running program. It’s the abstraction through which memory, processor time, and I/O resources can be managed and monitored.
“On a UNIX/Linux system, everything is a file; if not, it is a process.”
A “thread” is an execution context within a process. Every process has at least one thread, but some processes have many. Each thread has its own stack and CPU context but operates within the address space of its enclosing process.
Components
- virtual address space
- status: RUNNABLE, STOP, SLEEP
- id: pid, ppid
- owner & permission: uid, euid, gid, egid
- execution priority: niceness
- signal mask
- control terminal (nondaemon)
Life Cycle
fork() -> exec() -> wait()
Signals
| signal | number | comment |
|---|---|---|
KILL |
9 |
terminate (destroy) process at kernel level. unblocked, uncaught, unignored |
STOP |
17 |
suspend process until CONT. unblocked, uncaught, unignored |
CONT |
19 |
continue (restore) process from STOP or TSTP, sent by bg and fg. unblocked |
INT |
2 |
Ctrl+C |
TERM |
15 |
default. normal termination |
TSTP |
20 |
Ctrl+Z. can be ignored |
References
- Book: UNIX and Linux System Administration Handbook, Fifth Edition (2018)