C - pthread_self() 被截断前 4 个字节？

Question

我无法让 pthread_equal() 匹配存储的 pthread。与从 pthread_create() 获得的相比，我从 pthread_self() 获得的 pthread_t 似乎被截断了 4 个字节。比较在最后一个代码块中!

#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <netdb.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <pthread.h>
#include <errno.h>

#define QUEUE_SIZE 10
pthread_t *threads_array;
int abc;

void server_loop();
void *a_loop(void *abc);
void printThreadID(pthread_t id);

int main(int argc, char **argv) {
    server_loop();
    return 0;
}

void server_loop() {
    pthread_t thread_id;
    threads_array = malloc(QUEUE_SIZE * sizeof(pthread_t));
    int i;
    for (i = 0; i < QUEUE_SIZE; i++) {
        if (pthread_create(&thread_id, NULL, a_loop, (void*)&abc) < 0) {
            perror("Thread creation failed");
            fprintf(stderr, "Thread creation #%d failed\n", i);
            printThreadID(thread_id);
            exit(1);
        }

        //... store threadID in array
        fprintf(stderr, "Thread created: ");
        printThreadID(thread_id);

        threads_array[i] = thread_id;

    }
    sleep(5);
}

void *a_loop(void *abc) {
    fprintf(stderr, "@@@@ ");
    printThreadID(pthread_self());

    //... find index of matching pthread_equal(pthread_self(), array[i])

    while (1) {
    }
    return 0;
}


// HELPER METHOD
void printThreadID(pthread_t id){
    size_t i;
    for (i = sizeof(i); i; --i)
        fprintf(stderr, "%02x", *(((unsigned char*) &id) + i - 1));
    fprintf(stderr, "\n");
}

因此，尝试遍历存储的 pthread 并与 pthread_equals 进行比较的尝试失败了。一些比较(全部 10 个，每次都截断前 4 个字节):

Thread created ID: 00007fda1e402700
self():            000000001e402700
Thread created ID: 00007fda1dc01700
self():            000000001dc01700
Thread created ID: 00007fda1cbff700
self():            000000001cbff700

// Sometimes the zeroes are replaced with ones:
Thread created ID: 00007fda1e402700
self():            ffffffff1e402700
Thread created ID: 00007fda1dc01700
self():            ffffffff1dc01700
Thread created ID: 00007fda1cbff700
self():            ffffffff1cbff700

知道为什么会这样吗？

Answer 1

这个函数体是问题的根源:

size_t i;
for (i = sizeof(i); i; --i)
{
    fprintf(stderr, "%02x", *(((unsigned char*) &id) + i - 1));
}

pthread_t 的大小与“size_t”不同，代码使用 sizeof(i) 打印 pthread_t< 的字节。建议:

for (i = sizeof(pthread_t); i; --i)
{
    fprintf(stderr, "%02x", *(((unsigned char*) &id) + i - 1));
}

尽管不使用循环的更好方法是:

fprintf( stderr, "%*x", (int)(2*sizeof(pthread_t)), (int)id );

警告:下面的代码编译不干净。它输出有关未使用参数 abc

的警告消息

因此，使用“改进”函数:printThreadID() 产生输出的程序是:

#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <netdb.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <pthread.h>
#include <errno.h>

#define FROM_CREATE (0)
#define FROM_THREAD (1)

#define QUEUE_SIZE (10)
pthread_t *threads_array;
int abc;

void server_loop( void );
void *a_loop(void *abc);
void printThreadID( int, pthread_t );

int main( void )
{
    server_loop();
    return 0;
}

void server_loop()
{
    pthread_t thread_id;
    if( NULL == (threads_array = malloc(QUEUE_SIZE * sizeof(pthread_t)) ))
    {
        perror( "malloc for thread array failed" );
        exit( EXIT_FAILURE );
    }

    // implied else, malloc successful

    for (int i = 0; i < QUEUE_SIZE; i++)
    {
        if (pthread_create(&thread_id, NULL, a_loop, (void*)&abc) < 0)
        {
            perror("Thread creation failed");
            fprintf(stderr, "Thread creation #%d failed\n", i);
            printThreadID(FROM_CREATE, thread_id);
            exit( EXIT_FAILURE );
        }

        // implied else, pthread_create successful

        //... store threadID in array
        //fprintf(stderr, "Thread created: ");
        printThreadID(FROM_CREATE, thread_id);

        threads_array[i] = thread_id;

    }
    sleep(5);
}

void *a_loop(void *abc)
{
    //fprintf(stderr, "@@@@ ");
    printThreadID( FROM_THREAD, pthread_self());

    //... find index of matching pthread_equal(pthread_self(), array[i])

    while (1)
    {
    }
    return 0;
}


// HELPER METHOD
void printThreadID(int fromThread, pthread_t id)
{
    //size_t i;

    //for (i = sizeof(i); i; --i)
    //{
    //    fprintf(stderr, "%02x", *(((unsigned char*) &id) + i - 1));
    //}
    if( FROM_THREAD == fromThread )
    {
        fprintf( stderr, "@@@@ %ld\n", (long int)id );
    }
    else
    {
        fprintf( stderr, "thread created: %ld\n", (long int)id );
    }
    //fprintf(stderr, "\n");
}

这是程序典型运行的输出，请注意输出的确切顺序取决于赢得比赛的线程。

thread created: 140506339571456
@@@@ 140506339571456
thread created: 140506331178752
thread created: 140506322786048
thread created: 140506314393344
thread created: 140506306000640
thread created: 140506297607936
thread created: 140506289215232
thread created: 140506280822528
thread created: 140506272429824
thread created: 140506264037120
@@@@ 140506331178752
@@@@ 140506297607936
@@@@ 140506322786048
@@@@ 140506289215232
@@@@ 140506280822528
@@@@ 140506314393344
@@@@ 140506272429824
@@@@ 140506264037120
@@@@ 140506306000640