pthread_detach未达预期？关于线程内存回收及join限制的疑问

Let me break this down for you—pthread_detach() is one of those pthread functions that trips up a lot of folks, so your confusion is totally understandable!

First, let's unpack that official definition you quoted:

当线程终止时，该线程的存储空间可被回收

What this actually means is that the OS will automatically clean up the thread's kernel resources (like its stack, thread control block, etc.) as soon as the thread terminates, without requiring another thread to call pthread_join() to trigger that cleanup.

Your initial understanding had a couple of key missteps:

• Threads can never be "run again" once they terminate, regardless of whether you called pthread_detach() or not. A thread's lifecycle is one-and-done: create → execute → terminate. Once it's finished, that thread object is gone for good.
• Calling pthread_detach() doesn't make pthread_join() impossible outright—instead, if you try to join a detached thread, the call will fail with the EINVAL error code. This is because the detached thread's resources are meant to be auto-recycled, so there's no state left to retrieve via join.

Why Your Test Code Might Have Confused You #

Chances are, one of these scenarios happened:

1. You called pthread_detach() while the thread was still running: The thread will keep executing until it finishes, and only then will its resources be auto-recycled. If you checked memory usage right after calling detach (before the thread ended), you wouldn't see an immediate drop in resource usage.
2. You accidentally called pthread_join() before pthread_detach(): If join happens first, that call will block until the thread ends, then immediately clean up its resources. Calling pthread_detach() after that has no effect, since the resources are already gone.
3. You misinterpreted the pthread_join() failure: If you tried joining a detached thread and didn't check the return code, you might have missed the EINVAL error that signals the join isn't allowed.

A Quick Example to Demonstrate #

Here's a small code snippet that shows how pthread_detach() interacts with pthread_join():

#include <pthread.h>
#include <stdio.h>
#include <unistd.h>
#include <errno.h>

void* worker_thread(void* arg) {
    sleep(2); // Simulate work
    printf("Worker thread finished execution\n");
    return NULL;
}

int main() {
    pthread_t tid;
    pthread_create(&tid, NULL, worker_thread, NULL);

    // Mark the thread as detached
    pthread_detach(tid);

    // Attempt to join the detached thread
    int join_result = pthread_join(tid, NULL);
    if (join_result == EINVAL) {
        printf("pthread_join() failed: Cannot join a detached thread (EINVAL)\n");
    } else if (join_result == 0) {
        printf("pthread_join() succeeded (this shouldn't happen for a detached thread)\n");
    } else {
        printf("pthread_join() failed with unknown error: %d\n", join_result);
    }

    sleep(3); // Wait for worker to finish and resources to be cleaned
    return 0;
}

When you run this, you'll see the EINVAL error immediately, confirming that joining a detached thread isn't allowed.

Key Takeaways #

• pthread_detach() is all about resource cleanup automation, not about preventing thread execution or blocking join calls preemptively.
• Detached threads still run to completion—they just don't hang around waiting for a join to clean up after them.
• Once any thread terminates (detached or not), it can't be restarted—this is a fundamental property of POSIX threads, not a side effect of detach.

内容的提问来源于stack exchange，提问作者Ziad