How to join a thread that is hanging on blocking IO?

Gracefully Handling Hanging Threads on Blocking I/O in C/Linux

Learn robust techniques to manage and terminate pthreads that are blocked indefinitely on I/O operations in C on Linux systems, ensuring application stability and responsiveness.

In multi-threaded C applications on Linux, a common challenge arises when a thread performs blocking I/O operations (e.g., reading from a socket, file, or pipe) and the I/O source becomes unresponsive. This can lead to the thread hanging indefinitely, consuming resources, and preventing proper application shutdown or state management. Simply calling pthread_cancel() might not be sufficient or safe, as it can leave resources in an inconsistent state. This article explores effective strategies to detect, manage, and safely terminate such hanging threads.

The Problem with Blocking I/O and pthread_cancel()

When a thread executes a blocking I/O call like read(), write(), accept(), or recv(), it enters a kernel state where it waits for data or an event. If that event never occurs, the thread remains blocked. While pthread_cancel() is designed to terminate a thread, its behavior with blocking I/O is nuanced:

1. Cancellation Points: pthread_cancel() doesn't immediately terminate a thread. Instead, it sets a cancellation request. The thread is only terminated when it reaches a cancellation point. Many blocking I/O functions are not cancellation points by default, or they might only become cancellation points if the thread's cancellation type is set to asynchronous (which is generally unsafe).
2. Resource Leaks: If a thread is cancelled while holding locks, allocated memory, or open file descriptors, these resources might not be properly released, leading to leaks or deadlocks.
3. Data Inconsistency: Cancelling a thread mid-operation can leave shared data structures in an inconsistent state, potentially corrupting application data.

flowchart TD
    A[Thread Starts] --> B{Blocking I/O Call}
    B --> C{I/O Event Occurs?}
    C -- No --> D[Thread Hangs Indefinitely]
    C -- Yes --> E[I/O Completes]
    E --> F[Thread Continues]
    D --> G{pthread_cancel() called}
    G --> H{Cancellation Point Reached?}
    H -- No --> D
    H -- Yes --> I[Thread Terminates (Potentially Unsafely)]
    I --> J[Resource Leaks / Data Inconsistency]

The challenge of cancelling a thread stuck on blocking I/O.

Strategies for Robust I/O Thread Management

To safely handle threads blocked on I/O, we need to avoid direct cancellation during blocking calls and instead provide mechanisms for the thread to gracefully exit. Here are the primary approaches:

1. Using Non-Blocking I/O with select()/poll()/epoll()

The most robust solution is to avoid indefinite blocking altogether. By configuring I/O descriptors as non-blocking and using multiplexing I/O functions, a thread can periodically check for data availability or a termination signal. This allows the thread to respond to external requests (like a shutdown signal) without being stuck.

When using select(), poll(), or epoll(), you can specify a timeout. If the timeout expires, the function returns, allowing the thread to check a flag or message queue for a termination request. You can also include a 'self-pipe' or eventfd in your select/poll set to signal the thread to wake up and exit.

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <pthread.h>
#include <fcntl.h>
#include <sys/select.h>
#include <sys/time.h>

volatile int shutdown_flag = 0;
int pipefd[2]; // Used for signaling thread to exit

void *io_thread_func(void *arg) {
    int fd = *(int*)arg; // Assuming fd is a socket or file descriptor
    char buffer[256];
    ssize_t bytes_read;

    // Set the I/O descriptor to non-blocking
    int flags = fcntl(fd, F_GETFL, 0);
    fcntl(fd, F_SETFL, flags | O_NONBLOCK);

    fd_set read_fds;
    struct timeval tv;

    while (!shutdown_flag) {
        FD_ZERO(&read_fds);
        FD_SET(fd, &read_fds);
        FD_SET(pipefd[0], &read_fds); // Add read end of pipe to monitor for shutdown signal

        tv.tv_sec = 1;  // Timeout after 1 second
        tv.tv_usec = 0;

        int retval = select(fd > pipefd[0] ? fd + 1 : pipefd[0] + 1, &read_fds, NULL, NULL, &tv);

        if (retval == -1) {
            perror("select");
            break; // Error in select
        } else if (retval == 0) {
            // Timeout occurred, check shutdown_flag again
            printf("Thread: Select timed out, checking shutdown flag.\n");
            continue;
        } else {
            if (FD_ISSET(pipefd[0], &read_fds)) {
                // Shutdown signal received via pipe
                printf("Thread: Shutdown signal received via pipe.\n");
                char dummy;
                read(pipefd[0], &dummy, 1); // Consume the signal byte
                break;
            }
            if (FD_ISSET(fd, &read_fds)) {
                // Data available on I/O descriptor
                bytes_read = read(fd, buffer, sizeof(buffer) - 1);
                if (bytes_read > 0) {
                    buffer[bytes_read] = '\0';
                    printf("Thread: Read %zd bytes: '%s'\n", bytes_read, buffer);
                } else if (bytes_read == 0) {
                    printf("Thread: End of file/stream.\n");
                    break;
                } else if (bytes_read == -1) {
                    if (errno != EWOULDBLOCK && errno != EAGAIN) {
                        perror("read");
                        break;
                    }
                    // EWOULDBLOCK/EAGAIN means no data yet, but select said there was. Should not happen often.
                }
            }
        }
    }

    printf("Thread: Exiting gracefully.\n");
    close(fd); // Close the descriptor managed by this thread
    return NULL;
}

int main() {
    pthread_t io_thread;
    int dummy_fd = STDIN_FILENO; // Example: use stdin as a blocking source

    if (pipe(pipefd) == -1) {
        perror("pipe");
        return 1;
    }

    printf("Main: Starting I/O thread.\n");
    if (pthread_create(&io_thread, NULL, io_thread_func, &dummy_fd) != 0) {
        perror("pthread_create");
        return 1;
    }

    // Simulate main application work
    sleep(5);

    printf("Main: Signaling I/O thread to shut down.\n");
    shutdown_flag = 1; // Set flag for timeout-based check
    write(pipefd[1], "x", 1); // Send signal via pipe

    pthread_join(io_thread, NULL);
    printf("Main: I/O thread joined.\n");

    close(pipefd[0]);
    close(pipefd[1]);

    return 0;
}

Example of using select() with a non-blocking descriptor and a self-pipe for graceful shutdown.

2. Using pthread_cancel() with Caution and Cleanup Handlers

If non-blocking I/O is not feasible or desirable for some reason, and you must use pthread_cancel(), it's crucial to enable cancellation and use cleanup handlers. This approach is more complex and generally less safe than non-blocking I/O.

1. Enable Cancellation: Set the thread's cancellation state to PTHREAD_CANCEL_ENABLE and its type to PTHREAD_CANCEL_DEFERRED (default and safest) or PTHREAD_CANCEL_ASYNCHRONOUS (highly dangerous, avoid if possible).
2. Cancellation Points: Ensure your blocking I/O calls are wrapped in functions that are cancellation points, or that you periodically introduce cancellation points (e.g., pthread_testcancel()).
3. Cleanup Handlers: Use pthread_cleanup_push() and pthread_cleanup_pop() to register functions that will be called if the thread is cancelled. These handlers should release resources (mutexes, memory, file descriptors) to prevent leaks.

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

// Global resource that needs cleanup
FILE *global_file = NULL;
pthread_mutex_t global_mutex = PTHREAD_MUTEX_INITIALIZER;

// Cleanup handler function
void cleanup_handler(void *arg) {
    printf("Thread: Cleanup handler invoked.\n");
    if (global_file) {
        fclose(global_file);
        global_file = NULL;
        printf("Thread: Closed global_file.\n");
    }
    pthread_mutex_unlock(&global_mutex);
    printf("Thread: Unlocked global_mutex.\n");
}

void *blocking_io_thread_func(void *arg) {
    // Enable cancellation and set type to deferred (default)
    pthread_setcancelstate(PTHREAD_CANCEL_ENABLE, NULL);
    pthread_setcanceltype(PTHREAD_CANCEL_DEFERRED, NULL);

    // Push cleanup handler onto the stack
    pthread_cleanup_push(cleanup_handler, NULL);

    printf("Thread: Attempting to acquire mutex and open file.\n");
    pthread_mutex_lock(&global_mutex);
    global_file = fopen("test.txt", "w");
    if (!global_file) {
        perror("fopen");
        pthread_mutex_unlock(&global_mutex);
        pthread_cleanup_pop(0); // Pop without executing
        return NULL;
    }
    fprintf(global_file, "This is a test.\n");
    fflush(global_file);
    printf("Thread: Mutex acquired, file opened and written.\n");

    // Simulate a blocking I/O call that might hang
    // For demonstration, we'll use sleep, but imagine this is read() on a slow pipe
    printf("Thread: Entering blocking operation (simulated with sleep).\n");
    sleep(10); // This is a cancellation point if cancellation is enabled
    // If sleep() wasn't a cancellation point, you'd need pthread_testcancel() periodically

    printf("Thread: Blocking operation completed.\n");

    // Pop cleanup handler and execute it (0 means don't execute, 1 means execute)
    // We execute it here because we reached the end normally.
    pthread_cleanup_pop(1);

    return NULL;
}

int main() {
    pthread_t io_thread;

    printf("Main: Creating blocking I/O thread.\n");
    if (pthread_create(&io_thread, NULL, blocking_io_thread_func, NULL) != 0) {
        perror("pthread_create");
        return 1;
    }

    // Wait for a short period, then cancel the thread
    sleep(2);

    printf("Main: Cancelling I/O thread.\n");
    pthread_cancel(io_thread);

    printf("Main: Joining I/O thread.\n");
    if (pthread_join(io_thread, NULL) != 0) {
        perror("pthread_join");
        return 1;
    }
    printf("Main: I/O thread joined.\n");

    // Verify cleanup (e.g., global_file should be NULL, mutex unlocked)
    if (global_file == NULL) {
        printf("Main: Global file was successfully closed by cleanup handler.\n");
    }

    return 0;
}

Using pthread_cancel() with cleanup handlers to release resources.

3. Using alarm() and read() with a Timeout (Less Recommended)

For simple read() operations on file descriptors that don't support select() (like regular files on some systems, though most modern Linux systems allow select on regular files), you might consider using alarm() to set a timeout for the read() call. This involves signal handling, which adds complexity and can be error-prone.

When alarm() expires, it sends a SIGALRM signal. You would need a signal handler to catch this signal and potentially set a flag or use longjmp() to exit the blocking read(). However, read() is not guaranteed to be interrupted by SIGALRM on all systems, and signal handling within multi-threaded applications requires careful design (e.g., using sigwaitinfo() or pthread_sigmask()). This method is generally less portable and harder to get right than select()/poll().

sequenceDiagram
    participant App as Main Application
    participant IOT as I/O Thread
    participant OS as Operating System

    App->IOT: Create I/O Thread
    IOT->OS: `fcntl(fd, O_NONBLOCK)`
    loop While not shutdown_flag
        IOT->OS: `select(fd, pipefd[0], timeout)`
        alt Timeout
            OS-->IOT: Timeout (0)
            IOT->IOT: Check `shutdown_flag`
        else Signal on pipe
            OS-->IOT: `pipefd[0]` ready
            IOT->IOT: Read signal, set `shutdown_flag`
            break
        else Data on fd
            OS-->IOT: `fd` ready
            IOT->OS: `read(fd)`
            OS-->IOT: Data / EOF / Error
        end
    end
    App->IOT: Set `shutdown_flag = 1`
    App->OS: `write(pipefd[1], 'x', 1)`
    OS-->IOT: `pipefd[0]` ready
    IOT->IOT: Exit loop
    IOT->App: `pthread_exit()`
    App->IOT: `pthread_join()`
    App->App: Continue shutdown

Sequence diagram for graceful I/O thread shutdown using non-blocking I/O and a self-pipe.

Choosing the right strategy depends on your application's requirements, the type of I/O, and the desired level of robustness. For most scenarios, converting to non-blocking I/O with select(), poll(), or epoll() and using a self-pipe or eventfd for signaling is the safest and most recommended approach.