堆栈内存溢出
  繁体   English   中英

C未初始化的互斥锁工作和初始化的互斥锁失败？

[英]C uninitialized mutex works and initialized mutex fails?

 原文  2015-05-31 05:46:51  3   2    c/ concurrency/ mutex

问题描述

我的C程序创建了一个生产者线程，尽可能快地保存数据。 主线程消耗并打印这些。

经过几天的bug查找后，我注意到如果互斥锁初始化，那么程序会在30秒内停止（死锁？）。

但是，如果互斥锁未初始化，则可以完美运行。

谁能解释一下这个？ 为了避免未定义的行为，我希望尽可能初始化它们。

进一步的信息：特别是如果“pthread_mutex_t signalM”（信令互斥）被初始化它就会锁定

初始化 

#include <stdlib.h>                     // exit_failure, exit_success
#include <stdio.h>                      // stdin, stdout, printf
#include <pthread.h>                    // threads
#include <string.h>                     // string
#include <unistd.h>                     // sleep
#include <stdbool.h>                    // bool
#include <fcntl.h>                      // open



struct event {
    pthread_mutex_t critical;
    pthread_mutex_t signalM;
    pthread_cond_t signalC;
    int eventCount;
};

struct allVars {
    struct event inEvents;
    struct event outEvents;
    int bufferSize;
    char buffer[10][128];
};




/**
 * Advance the EventCount
 */
void advance(struct event *event) {
    // increment the event counter
    pthread_mutex_lock(&event->critical);
    event->eventCount++;
    pthread_mutex_unlock(&event->critical);

    // signal await to continue
    pthread_mutex_lock(&event->signalM);
    pthread_cond_signal(&event->signalC);
    pthread_mutex_unlock(&event->signalM);
}



/**
 * Wait for ticket and buffer availability
 */
void await(struct event *event, int ticket) {

    int eventCount;

    // get the counter
    pthread_mutex_lock(&event->critical);
    eventCount = event->eventCount;
    pthread_mutex_unlock(&event->critical);

    // lock signaling mutex
    pthread_mutex_lock(&event->signalM);

    // loop until the ticket machine shows your number
    while (ticket > eventCount) {
        // wait until a ticket is called
        pthread_cond_wait(&event->signalC, &event->signalM);

        // get the counter
        pthread_mutex_lock(&event->critical);
        eventCount = event->eventCount;
        pthread_mutex_unlock(&event->critical);
    }

    // unlock signaling mutex
    pthread_mutex_unlock(&event->signalM);
}



/**
 * Add to buffer
 */
void putBuffer(struct allVars *allVars, char data[]) {
    // get the current write position
    pthread_mutex_lock(&allVars->inEvents.critical);
    int in = allVars->inEvents.eventCount;
    pthread_mutex_unlock(&allVars->inEvents.critical);

    // wait until theres a space free in the buffer
    await(&allVars->outEvents, in - allVars->bufferSize + 1);   // set to 2 to keep 1 index distance

    // add data to buffer
    strcpy(allVars->buffer[in % allVars->bufferSize], data);

    // increment the eventCounter and signal
    advance(&allVars->inEvents);
}



/**
 * Get from buffer
 */
char *getBuffer(struct allVars *allVars) {
    // get the current read position
    pthread_mutex_lock(&allVars->outEvents.critical);
    int out = allVars->outEvents.eventCount;
    pthread_mutex_unlock(&allVars->outEvents.critical);

    // wait until theres something in the buffer
    await(&allVars->inEvents, out + 1);

    // allocate memory for returned string
    char *str = malloc(128);

    // get the buffer data
    strcpy(str, allVars->buffer[out % allVars->bufferSize]);

    // increment the eventCounter and signal
    advance(&allVars->outEvents);

    return str;
}



/** child thread (producer) */
void *childThread(void *allVars) {
    char str[10];
    int count = 0;

    while (true) {
        sprintf(str, "%d", count++);
        putBuffer(allVars, str);
    }

    pthread_exit(EXIT_SUCCESS);
}



int main(void) {
    // init structs
    struct event inEvents = {
        PTHREAD_MUTEX_INITIALIZER,
        PTHREAD_MUTEX_INITIALIZER,
        PTHREAD_COND_INITIALIZER,
        0
    };
    struct event outEvents = {
        PTHREAD_MUTEX_INITIALIZER,
        PTHREAD_MUTEX_INITIALIZER,
        PTHREAD_COND_INITIALIZER,
        0
    };
    struct allVars allVars = {
        inEvents,       // events
        outEvents,
        10,             // buffersize
        {"", {""}}      // buffer[][]
    };


    // create child thread (producer)
    pthread_t thread;
    if (pthread_create(&thread, NULL, childThread, &allVars)) {
        fprintf(stderr, "failed to create child thread");
        exit(EXIT_FAILURE);
    }


    // (consumer)
    while (true) {
        char *out = getBuffer(&allVars);
        printf("buf: %s\n", out);
        free(out);
    }


    return (EXIT_SUCCESS);
}

未初始化 

#include <stdlib.h>                     // exit_failure, exit_success
#include <stdio.h>                      // stdin, stdout, printf
#include <pthread.h>                    // threads
#include <string.h>                     // string
#include <unistd.h>                     // sleep
#include <stdbool.h>                    // bool
#include <fcntl.h>                      // open



struct event {
    pthread_mutex_t critical;
    pthread_mutex_t signalM;
    pthread_cond_t signalC;
    int eventCount;
};

struct allVars {
    struct event inEvents;
    struct event outEvents;
    int bufferSize;
    char buffer[10][128];
};




/**
 * Advance the EventCount
 */
void advance(struct event *event) {
    // increment the event counter
    pthread_mutex_lock(&event->critical);
    event->eventCount++;
    pthread_mutex_unlock(&event->critical);

    // signal await to continue
    pthread_mutex_lock(&event->signalM);
    pthread_cond_signal(&event->signalC);
    pthread_mutex_unlock(&event->signalM);
}



/**
 * Wait for ticket and buffer availability
 */
void await(struct event *event, int ticket) {

    int eventCount;

    // get the counter
    pthread_mutex_lock(&event->critical);
    eventCount = event->eventCount;
    pthread_mutex_unlock(&event->critical);

    // lock signaling mutex
    pthread_mutex_lock(&event->signalM);

    // loop until the ticket machine shows your number
    while (ticket > eventCount) {
        // wait until a ticket is called
        pthread_cond_wait(&event->signalC, &event->signalM);

        // get the counter
        pthread_mutex_lock(&event->critical);
        eventCount = event->eventCount;
        pthread_mutex_unlock(&event->critical);
    }

    // unlock signaling mutex
    pthread_mutex_unlock(&event->signalM);
}



/**
 * Add to buffer
 */
void putBuffer(struct allVars *allVars, char data[]) {
    // get the current write position
    pthread_mutex_lock(&allVars->inEvents.critical);
    int in = allVars->inEvents.eventCount;
    pthread_mutex_unlock(&allVars->inEvents.critical);

    // wait until theres a space free in the buffer
    await(&allVars->outEvents, in - allVars->bufferSize + 1);   // set to 2 to keep 1 index distance

    // add data to buffer
    strcpy(allVars->buffer[in % allVars->bufferSize], data);

    // increment the eventCounter and signal
    advance(&allVars->inEvents);
}



/**
 * Get from buffer
 */
char *getBuffer(struct allVars *allVars) {
    // get the current read position
    pthread_mutex_lock(&allVars->outEvents.critical);
    int out = allVars->outEvents.eventCount;
    pthread_mutex_unlock(&allVars->outEvents.critical);

    // wait until theres something in the buffer
    await(&allVars->inEvents, out + 1);

    // allocate memory for returned string
    char *str = malloc(128);

    // get the buffer data
    strcpy(str, allVars->buffer[out % allVars->bufferSize]);

    // increment the eventCounter and signal
    advance(&allVars->outEvents);

    return str;
}



/** child thread (producer) */
void *childThread(void *allVars) {
    char str[10];
    int count = 0;

    while (true) {
        sprintf(str, "%d", count++);
        putBuffer(allVars, str);
    }

    pthread_exit(EXIT_SUCCESS);
}



int main(void) {
    // init structs
    struct event inEvents; /* = {
        PTHREAD_MUTEX_INITIALIZER,
        PTHREAD_MUTEX_INITIALIZER,
        PTHREAD_COND_INITIALIZER,
        0
    }; */
    struct event outEvents; /* = {
        PTHREAD_MUTEX_INITIALIZER,
        PTHREAD_MUTEX_INITIALIZER,
        PTHREAD_COND_INITIALIZER,
        0
    }; */

    struct allVars allVars = {
        inEvents,       // events
        outEvents,
        10,             // buffersize
        {"", {""}}      // buffer[][]
    };


    // create child thread (producer)
    pthread_t thread;
    if (pthread_create(&thread, NULL, childThread, &allVars)) {
        fprintf(stderr, "failed to create child thread");
        exit(EXIT_FAILURE);
    }


    // (consumer)
    while (true) {
        char *out = getBuffer(&allVars);
        printf("buf: %s\n", out);
        free(out);
    }


    return (EXIT_SUCCESS);
}

2 个解决方案

解决方案1
 3 已采纳  2015-05-31 07:26:49

Jonathan解释了为什么没有初始化互斥锁的代码没有死锁（主要是因为尝试使用未初始化的互斥锁永远不会阻塞，它会立即返回错误）。

在正确初始化互斥锁的程序版本中导致无限等待的问题是您没有正确使用条件变量。 谓词表达式的检查和条件变量的等待必须相对于可能正在修改谓词的其他任何线程以原子方式完成。 您的代码正在检查谓词，该谓词是另一个线程甚至无法访问的局部变量。 您的代码将实际谓词读入临界区内的局部变量，但随后释放用于读取谓词的互斥锁，并获取不同的互斥锁以读取“假”谓词（无论如何都不能由其他线程修改）用条件变量等待。

因此，您可以修改实际谓词event->eventCount ，并在等待线程读取谓词和条件变量上的块之间发出该修改的信号。

我认为以下内容将解决您的僵局，但我没有机会进行大量测试。 更改主要是从struct event删除signalM互斥锁，并将其替换为critical互斥锁的任何使用： 

#include <stdlib.h>                     // exit_failure, exit_success
#include <stdio.h>                      // stdin, stdout, printf
#include <pthread.h>                    // threads
#include <string.h>                     // string
#include <unistd.h>                     // sleep
#include <stdbool.h>                    // bool
#include <fcntl.h>                      // open



struct event {
    pthread_mutex_t critical;
    pthread_cond_t signalC;
    int eventCount;
};

struct allVars {
    struct event inEvents;
    struct event outEvents;
    int bufferSize;
    char buffer[10][128];
};




/**
 * Advance the EventCount
 */
void advance(struct event *event) {
    // increment the event counter
    pthread_mutex_lock(&event->critical);
    event->eventCount++;
    pthread_mutex_unlock(&event->critical);

    // signal await to continue
    pthread_cond_signal(&event->signalC);
}



/**
 * Wait for ticket and buffer availability
 */
void await(struct event *event, int ticket) {


    // get the counter
    pthread_mutex_lock(&event->critical);

    // loop until the ticket machine shows your number
    while (ticket > event->eventCount) {
        // wait until a ticket is called
        pthread_cond_wait(&event->signalC, &event->critical);
    }

    // unlock signaling mutex
    pthread_mutex_unlock(&event->critical);
}



/**
 * Add to buffer
 */
void putBuffer(struct allVars *allVars, char data[]) {
    // get the current write position
    pthread_mutex_lock(&allVars->inEvents.critical);
    int in = allVars->inEvents.eventCount;
    pthread_mutex_unlock(&allVars->inEvents.critical);

    // wait until theres a space free in the buffer
    await(&allVars->outEvents, in - allVars->bufferSize + 1);   // set to 2 to keep 1 index distance

    // add data to buffer
    strcpy(allVars->buffer[in % allVars->bufferSize], data);

    // increment the eventCounter and signal
    advance(&allVars->inEvents);
}



/**
 * Get from buffer
 */
char *getBuffer(struct allVars *allVars) {
    // get the current read position
    pthread_mutex_lock(&allVars->outEvents.critical);
    int out = allVars->outEvents.eventCount;
    pthread_mutex_unlock(&allVars->outEvents.critical);

    // wait until theres something in the buffer
    await(&allVars->inEvents, out + 1);

    // allocate memory for returned string
    char *str = malloc(128);

    // get the buffer data
    strcpy(str, allVars->buffer[out % allVars->bufferSize]);

    // increment the eventCounter and signal
    advance(&allVars->outEvents);

    return str;
}



/** child thread (producer) */
void *childThread(void *allVars) {
    char str[10];
    int count = 0;

    while (true) {
        sprintf(str, "%d", count++);
        putBuffer(allVars, str);
    }

    pthread_exit(EXIT_SUCCESS);
}



int main(void) {
    // init structs
    struct event inEvents = {
        PTHREAD_MUTEX_INITIALIZER,
        PTHREAD_COND_INITIALIZER,
        0
    };
    struct event outEvents = {
        PTHREAD_MUTEX_INITIALIZER,
        PTHREAD_COND_INITIALIZER,
        0
    };
    struct allVars allVars = {
        inEvents,       // events
        outEvents,
        10,             // buffersize
        {"", {""}}      // buffer[][]
    };


    // create child thread (producer)
    pthread_t thread;
    if (pthread_create(&thread, NULL, childThread, &allVars)) {
        fprintf(stderr, "failed to create child thread");
        exit(EXIT_FAILURE);
    }


    // (consumer)
    while (true) {
        char *out = getBuffer(&allVars);
        printf("buf: %s\n", out);
        free(out);
    }


    return (EXIT_SUCCESS);
}

解决方案2
 2  2015-05-31 06:32:10

我修改了getBuffer()和putBuffer()例程，如图所示（在代码的初始化和未初始化版本中）： 

static
void putBuffer(struct allVars *allVars, char data[])
{
    int lock_ok = pthread_mutex_lock(&allVars->inEvents.critical);
    if (lock_ok != 0)
        printf("%s(): lock error %d (%s)\n", __func__, lock_ok, strerror(lock_ok));
    int in = allVars->inEvents.eventCount;
    int unlock_ok = pthread_mutex_unlock(&allVars->inEvents.critical);
    if (unlock_ok != 0)
        printf("%s(): unlock error %d (%s)\n", __func__, unlock_ok, strerror(unlock_ok));

    await(&allVars->outEvents, in - allVars->bufferSize + 1);

    strcpy(allVars->buffer[in % allVars->bufferSize], data);

    advance(&allVars->inEvents);
}

static
char *getBuffer(struct allVars *allVars)
{
    int lock_ok = pthread_mutex_lock(&allVars->outEvents.critical);
    if (lock_ok != 0)
        printf("%s(): lock error %d (%s)\n", __func__, lock_ok, strerror(lock_ok));
    int out = allVars->outEvents.eventCount;
    int unlock_ok = pthread_mutex_unlock(&allVars->outEvents.critical);
    if (unlock_ok != 0)
        printf("%s(): unlock error %d (%s)\n", __func__, unlock_ok, strerror(unlock_ok));

    await(&allVars->inEvents, out + 1);

    char *str = malloc(128);

    strcpy(str, allVars->buffer[out % allVars->bufferSize]);

    advance(&allVars->outEvents);

    return str;
}

然后运行未初始化的代码会产生很多消息，如： 

buf: 46566
putBuffer(): lock error 22 (Invalid argument)
getBuffer(): lock error 22 (Invalid argument)
putBuffer(): unlock error 22 (Invalid argument)
getBuffer(): unlock error 22 (Invalid argument)

基本上，在我看来，你的锁定和解锁被忽略了。 您的代码中还有其他地方也应该检查。

从根本上说，如果忽略报告的错误，则不会注意到锁定和解锁根本不起作用，并且代码没有理由停止运行。

始终检查可能失败的系统调用的返回值。

我没有立即解释为什么初始化代码锁定。 它适用于我，使用GCC 5.1.0在Mac OS X 10.10.3上运行，经过大约100,000到800,000次迭代。

暂无
暂无

声明:本站的技术帖子网页，遵循CC BY-SA 4.0协议，如果您需要转载，请注明本站网址或者原文地址。任何问题请咨询:yoyou2525@163.com.

相关问题 检查 pthread_mutex 是否已初始化 解释互斥锁的工作方式 Mutex的定义和在C中的使用 C Pthreads互斥值？ 在 C 中全局初始化互斥锁 在C中实现互斥锁 C pthread互斥锁 C中的互斥体和线程 无法锁定C中的互斥锁 在C中实现Mutex的问题
相关标签
 
粤ICP备18138465号  © 2020-2024 STACKOOM.COM