进程状态机

最近在看南京大学蒋炎岩老师的 2025 操作系统课程。里面有一句话, 计算机中的一切程序可以视为 state machine。很有启发。进程有初始状态, CPU 这个无情的执行指令的机器执行一条指令后，程序的状态就发生了变化。

linux-process-state-machine-userspace

process state

Running/Runnable(R)

万事俱备，只需要被 scheduler 调度到 CPU 上去运行。

Interruptable Sleep(S)

处于这个状态的不会被 scheduler 调度到 CPU 上去运行。

d_state.c code

#include <unistd.h>

int main(){
    pause();
}
  1. gcc -o d_state d_state.c
  2. ./d_state
  3. ps aux | grep d_state

Uninterruptible Sleep(D)

可能是 Disk Sleep 的原因。状态用 D 表示。

Stopped(T)

如使用 SIGSTOP 信号，暂停的进程。

d_state.c code

#include <unistd.h>

int main(){
    pause();
}
  1. gcc -o d_state d_state.c
  2. ./d_state
  3. ps aux | grep d_state
  4. kill -SIGTSTP $(pidof d_state)
  5. ps aux | grep d_state

Zombie(Z)

进程已经执行了 exit, exit code 还没有被父进程 wait/waitpid 读取。也就是进程的 PCB 还没有从 kernel 的 process table 中清除。

zombie.c code

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sched.h>
int main() {
    pid_t child_pid = fork();

    if (child_pid == 0) {    // Child process
        printf("Child PID: %d\n", getpid());
        exit(0);             // Child exits immediately
    } else {                 // Parent process
        printf("Parent PID: %d\n", getpid());
        printf("Child PID: %d\n", child_pid);
        printf("Parent is sleeping (check for zombie with `ps`).\n");
        sleep(30);           // Parent sleeps (does not call wait())
        printf("Parent exiting.\n");
    }

    return 0;
}
  1. gcc -o zombie zombie.c
  2. ./zombie
  3. ps aux | grep zombie

system calls changing process state on Linux

fork()

• Purpose: Creates a new child process by duplicating the parent process.

• State Change: The child starts in the ready/runnable state (waiting for CPU time).

• Parameters: None.

• Return Value:

• 0 to the child process.

• Child's PID to the parent.

• -1 on error.

• Example: Used in spawning new processes (e.g., shell commands).

execve()

• Purpose: Replaces the current process’s memory space with a new program.

• State Change: The process remains in the running state but executes new code.

• Parameters:

• path: Path to the executable.

• argv: Command-line arguments.

• envp: Environment variables.

• Return Value: Only returns on error (-1).

• Example: Running programs like ls or grep from a shell.

exit() / _exit()

• Purpose: Terminates the current process.

• State Change: Moves the process to zombie (until the parent calls wait()).

• Parameters:

• status: Exit status code.

• Return Value: None (does not return).

• Difference: exit() flushes buffers; _exit() is a raw system call.

• Example: Clean termination after program completion.

wait() / waitpid()

• Purpose: Suspends the parent until a child changes state (exits or stops).

• State Change: Parent enters sleeping state until child exits.

• Parameters:

• status: Stores child’s exit status.

• pid: Specific child to wait for (in waitpid).

• Return Value: Child PID on success; -1 on error.

• Example: Parent process ensuring a child completes first.

kill() / raise()

• Purpose: Sends signals to processes (e.g., SIGTERM, SIGSTOP).

• State Change:

• SIGSTOP: Moves process to stopped state.

• SIGCONT: Resumes a stopped process.

• SIGKILL: Terminates immediately.

• Parameters:

• pid: Target process ID.

• sig: Signal to send.

• Return Value: 0 on success; -1 on error.

• Example: Terminating a frozen process via kill -9 PID.

pause()

• Purpose: Suspends the process until a signal is received.

• State Change: Process enters interruptable sleeping state.

• Parameters: None.

• Return Value: Always -1 (interrupted by signal).

• Example: Waiting indefinitely for user input or signals.

nanosleep()

• Purpose: Pauses execution for a specified time.

• State Change: Process enters interruptible sleep.

• Parameters:

• req: Time to sleep.

• rem: Remaining time if interrupted.

• Return Value: 0 on success; -1 on error.

• Example: High-precision delays in real-time applications.

ptrace()

• Purpose: Allows a process to control another (debugging, tracing).

• State Change: Traced process enters stopped state on signals.

• Parameters:

• request: Action (e.g., PTRACE_ATTACH, PTRACE_CONT).

• pid: Target process ID.

• Return Value: Varies by request; -1 on error.

• Example: Debuggers like gdb using breakpoints.

clone()

• Purpose: Creates a child process or thread with configurable behavior.

• State Change: New process/thread starts in ready state.

• Parameters:

• fn: Function for the child to execute.

• flags: Options (e.g., CLONE_VM for shared memory).

• arg: Arguments for fn.

• Return Value: Child PID on success; -1 on error.

• Example: Implementing threads (used by pthread library).

sched_yield()

• Purpose: Voluntarily yields the CPU to other processes/threads.

• State Change: Process moves from running to ready state.

• Parameters: None.

• Return Value: 0 on success; -1 on error.

• Example: Cooperative multitasking in real-time apps.

exit_group()

• Purpose: Terminates all threads in a process.

• State Change: All threads enter zombie state.

• Parameters:

• status: Exit code.

• Return Value: Does not return.

• Example: Terminating multi-threaded applications.

Key Signals Affecting Process State

• SIGSTOP: Forcefully stops a process.

• SIGCONT: Resumes a stopped process.

• SIGTERM: Requests graceful termination.

• SIGKILL: Forcefully kills a process.

Summary Table

System Call Purpose Key State Change
fork() Create child process Child → Runnable
execve() Replace process image Process runs new code
exit() Terminate process Process → Zombie
wait() Wait for child state change Parent → Sleeping
kill() Send signal (e.g., stop/resume) Target → Stopped/Running
pause() Sleep until signal Process → Sleeping
ptrace() Debug/trace another process Target → Stopped
clone() Create thread/process New entity → Runnable

These system calls and signals form the backbone of process lifecycle management in Linux, enabling creation, termination, synchronization, and debugging of processes.