File operations using POSIX threads

Question

I am learning the concept of multithreading and i encountered a problem using semaphore mutexes.

Here is my code snippet:

void *thread1_funct(void * myfileptr) { static int count; printf("\nThread1 ID:%u\n",(unsigned int) pthread_self()); printf("\nThread1 function, file pointer received:0x%x\n", (FILE*)myfileptr); for (;;) { count++; sem_wait(&mutex); fprintf(myfileptr, "%d\n", count); sem_post(&mutex); } return NULL; } void *thread2_funct(void *myfileptr) { static int count=0; printf("\nThread2 ID:%u\n",(unsigned int) pthread_self()); printf("\nThread2 function, file pointer received:0x%x\n", (FILE*)myfileptr); for (;;) {sem_wait(&mutex); fscanf(myfileptr, "%d\n", &count); printf("\n......%d......\n", count); sem_post(&mutex); } return NULL; }

The two threads i have created. One will write dfata to a file and the other will read the latest data.

Here is my main method:

int main(int argc, char **argv) { FILE *fileptr = fopen(*(argv+1), "a+"); sem_init(&mutex, 0x00, 0x01); if ( (thread1_ret = pthread_create(&thread1, NULL, thread1_funct, (void*)fileptr)) == 0) printf("\nThread1 created successfully....\n"); else printf("\nFailed to create Thread1\n"); if ( (thread2_ret = pthread_create(&thread2, NULL, thread2_funct, (void*)fileptr)) == 0) printf("\nThread2 created successfully....\n"); else printf("\nFailed to create Thread2\n"); pthread_join(thread1, NULL); pthread_join(thread2, NULL); fclose(fileptr); sem_destroy(&mutex); pthread_exit(NULL); return 0; }

The expected output is : ........1........ ........2........ ........3........

and so on...... till the program is interrupted manually.

But my output is all 0s: ........0....... ........0....... ........0....... ........0.......

and so on....

Please help me. Where am i going wrong?

I would suggest that using the name mutex for a semaphore is likely to be confusing down the road - and in this case you should probably be using a mutex anyway, not a semaphore (not because the semaphore won't work, though - it's just a bigger hammer than you need for that job). In addition @Greycon's answer below about the file pointer position, it's quite possible due to buffering that the data never gets written out anyway - at least for some time... — twalberg
– twalberg, Commented Jul 24, 2014 at 14:51
Either flush() after each write or use unbuffer I/O, that is open(), write(), read() and close() and two file descriptors. — alk
– alk, Commented Aug 2, 2014 at 16:32

Greycon · Accepted Answer · 2014-07-24 14:21:09Z

1

Thread 1 writes to the file, and advances the file pointer - to the end of the file. Thread 2 reads from the file pointer, which is pointing at the end of the file, and so you get nothing.

You could use fseek, or rewind in thread 2 to get your data.

answered Jul 24, 2014 at 14:21

Greycon

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3 Comments

twalberg Over a year ago

... or pread() into a buffer and then sscanf(), strtol(), etc. for the parsing... Although you'd have to query the FILE * for the underlying file descriptor first..

shre3841279 Over a year ago

Whether i use rewind or fseek won't i would still be looping around the same data again and again?

Greycon Over a year ago

you would have to calculate the size of the file, then seek to EOF - sizeof(your record). Why not use a different file pointer for the writer and reader threads?

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