C – pthread_self() getting truncated first 4 bytes?

I’m having trouble getting pthread_equal() to match stored pthreads. The pthread_t I get from pthread_self() seems to be truncated by 4 bytes compared to the one I get from pthread_create(). Comparisons are in the last code block!

#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 failedn", 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");
}

As a result, an attempt to loop through the stored pthreads and compare with pthread_equals fail. Some comparisons (all ten, every time, get truncated the first 4 bytes):

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

Any idea as to why this is happening?

Answer

this function body is the root of the problem:

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

the pthread_t does not have the same size as an ‘size_t’ and the code is using sizeof(i) to print the bytes of a pthread_t. suggest:

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

although a better method, not using a loop, would be:

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

caveat: the following code does not compile cleanly. It outputs a warning message about the unused parameter abc

So, with a ‘improved’ function: printThreadID() the program that produced the output is:

#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 failedn", 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, "@@@@ %ldn", (long int)id );
    }
    else
    {
        fprintf( stderr, "thread created: %ldn", (long int)id );
    }
    //fprintf(stderr, "n");
}

Here is the output of a typical run of the program, note the exact order of the outputs is depending on which thread wins the race.

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