C 中的 Pthreads，第二个线程不会执行

Question

I have this code that reads in two int numbers from a file. And stores it in a buffer[] to be taken in a second function to be used. I'm not sure if my stopping conditions in the first function are correct. It looks fine but when running the code it stops at the second function.

static int count = 0;
int buffer[5];
int requestNum = 1;
FILE* f;

int main(int argc, char** argv)
{
    /*THREAD IDs*/
    pthread_t liftR, lift1;

    /*OPEN FILE*/
    f = fopen("sim_input.txt", "r");

    /*CREATE THREAD*/
    pthread_create(&liftR, NULL, request, NULL);
    pthread_create(&lift1, NULL, lift, NULL);

    /*RUNS TILL THREADS FINISHED*/
    pthread_join(liftR, NULL);
    pthread_join(lift1, NULL);
    /*CLEAN UP*/
    fclose(f);

    return 0;
} 
void* request(void* data1)
{
    int req1, req2, eof;

    /*NOT EOF*/
    while(eof != -1)
    {
        /*READ ONE REQUEST*/
        eof = fscanf(f, "%d %d", &req1, &req2);

        /*CHECK IF BUFFER FULLL*/
        if(count < 5)
        {
            /*ADD REQUEST TO BUFFER*/
            buffer[count] = req1;
            buffer[count + 1] = req2;

            count = count + 2;
            printf("COUNT: %d\n", count);
            /*PRINTING*/
            printf("-------------------------------------------\n");
            printf("From Buffer -> Item1: %d, Item2: %d\n", req1, req2);
            printf("Request No: %d\n", requestNum);
            printf("-------------------------------------------\n");

            requestNum++;
        }
    }

    return NULL;
}  

void* lift(void* data2)
{
    while(count > 0)
    {
        sleep(1);
        printf("================\n");
        printf("COUNT: %d\n", count);
        printf("REMOVE ITEM FROM BUFFER - DO STUFF WITH IT\n");
        printf("================\n");
        count = count - 2;
    }
    return NULL;
}  

OUTPUT: Shows count 2, 4, 6. Only 3 request shown, In file it goes up to 10 request

Answer 1

There are multiple bugs in your program:

1. Reading uninitialized eof variable:

    int int req1, req2, eof;  // What is the value of eof? It could be -1 (or anything else).

    /*NOT EOF*/
    while(eof != -1)

2. You throw away the data you read:

        if(count < 5)
        {
            /*ADD REQUEST TO BUFFER*/
        }

If the first thread runs for a while before the second thread starts, then it will store the first two requests into the buffer , and throw away the rest .

To fix this, you need to wait for the second thread to drain the buffer when it is full.

3. You access count without any locking, which is a data race and undefined behavior. You must protect reading and writing shared (between threads) globals with a mutex.