Understanding and using void * and void ** in pthread_exit and pthread_join

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June 12th, 2008, 05:18 PM

Lesley_B

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Understanding and using void * and void ** in pthread_exit and pthread_join

Hi

Debian etch, gcc version 4.1.2

I'm having a bit of a problem returning data from a thread.
This problem probably extends into a 'what is (void **) ?' question as well.

I have one thread retrieving some data off a queue.
Another thread (listening thread) receives data and then kicks off another thread (adding thread) to to add to the queue.

This bit of code is the control for the retrieving thread

Code:

64     char cbuff[RD_BUFF+1];
...

122     while (1) {
123         sleep(1);
124         pthread_mutex_lock(&qu_mutex);
125         if (start != NULL ) {
126             snprintf(str,MAXLINE-1, "MAIN : Calling  collector thread \n");
127             do_log(fd, str);
128             //memset(&event,'\0',sizeof(event));
129             memset(cbuff,'\0',RD_BUFF+1);
130             pthread_create(&collect_tid, attr, eventCollector, (void *)(&cbuff));
131             pthread_join(collect_tid,(void *)cbuff);
132             snprintf(str,MAXLINE-1, "MAIN : Received from collector thread %s\n",cbuff);
133             do_log(fd, str);
134             qu_log();
135         }
136         pthread_mutex_unlock(&qu_mutex);
137     }

Given I intend to wait for the completion of thread execution it hardly seems worth having a thread here. However, I shall for the moment persist.

The retrieving thread removes an element at the start of the queue (FIFO arrangement) and readjusts the pointer to the start of the queue.

Code:

288 void *eventCollector(void *data){
289     // gets an event from the event queue
290     //struct ii_thdata *ptr ;
291     char *ptr ;
292     char str[MAXLINE];
293
294     //ptr = (struct ii_thdata *) data;
295     //memset(ptr,'\0',sizeof(ii_thdata));
296     ptr = ( char *) data;
297     memset(ptr,'\0',RD_BUFF+1);
298     if (start != NULL) {
299         strncpy(ptr,start->buff,RD_BUFF);
300         snprintf(str,MAXLINE-1, "EVCOLL : Queue start address before deletion %x\n",start);
301         do_log(fd, str);
302         qu_del(1);
303         snprintf(str,MAXLINE-1, "EVCOLL : Queue start address after deletion %x\n",start);
304         do_log(fd, str);
305     }
306     pthread_exit((void*)ptr);
307 }
308

I am not getting back the data correctly in L131. L132 prints out

Code:

Thu Jun 12 22:04:59 2008 MAIN : Received from collector thread �F� is test buffer 8

fairly consistently. I have a fixed data set to test this with and the only variation is in the digit.

Here's some log content showing addition to the queue, demonstrating the queue data isn't corrupt on addition. (Code for queue addition isn't shown here.)

Code:

 40 Thu Jun 12 22:04:41 2008 EVADD : In eventAdd
 41 Thu Jun 12 22:04:41 2008 EVADD : received data  "This is test buffer 1"
 42 Thu Jun 12 22:04:41 2008 EVADD : Added "This is test buffer 1" to queue
 43 Thu Jun 12 22:04:41 2008 QULOG : Queue start element address 0x804d7f8
 44 Thu Jun 12 22:04:41 2008 QULOG : Queue element This is test buffer 0 (0x804d7f8)
 45 Thu Jun 12 22:04:41 2008 QULOG : Queue element This is test buffer 1 (0x804dc98)
 46 Thu Jun 12 22:04:41 2008 EVADD : Leaving eventAdd

So ... it appears I get the data into the queue allright.

The prototypes of pthread_create and pthread_join are

Code:

int pthread_create(pthread_t * thread, pthread_attr_t * attr, void * (*start_routine)(void *), void * arg);
int pthread_join(pthread_t th, void **thread_return);

I simply don't understand how to change cbuff to a void * to pass it in, nor how to get the data out again using void ** .

I understand that void * is a pointer to something, the shape of which is unknown at compile time.
I understand that one cannot deference a void *.

I interpret char ** as a pointer to a pointer to a char
I have used char ** for an array of strings and int ** for a matrix of integers, similarly for float ** and double **.

I can therefore interpret void ** as a pointer to a pointer to an object of unknown shape or size.

But I am stumped w.r.t. converting from a void ** to anything 'useful' e.g. char, char * .

I'm not understanding void ** and void * well enough to be able to take a char[] or char * and convert between the two.

Can anyone help here?

Regards

Lesley

June 12th, 2008, 09:43 PM

Lesley_B

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Join Date: May 2008

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Hi

I've realised that malloc returns a void pointer which can be cast as a pointer to any type.

To pass a character string cbuff into a thread, the void *arg in the pthread_create prototype can be replaced with

Code:

 (void *) cbuff

which is simple type casting.

I have also corrected the code so that the pthread_exit call is returning a void *, namely the one it came in with, i.e. L306 has been replaced with

Code:

pthread_exit(data);

For the void ** argument of pthread join, (L 131) I defined a void pointer,

Code:

void *status;

and then used

Code:

pthread_join(collect_tid,&status);

which makes much more sense.
I continued to print out cbuff as before and this time it came out correct.

June 12th, 2008, 11:18 PM

salem

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In main, you'd have something like this

Code:

char cbuff[RD_BUFF+1];
void *result;
char *actualResult;
///
pthread_create(&collect_tid, attr, eventCollector, (void *)cbuff);
pthread_join(collect_tid,&result);
actualResult = result;
// now do stuff with the char* actualResult, like print it with %s

You might not even need the cast in the create thread call either. The compiler should treat it as an assignment to void* of another pointer type.

The thread code itself seems fine as it is, although some of the casts do seem redundant. In C, assigning a pointer type to void* and back again is perfectly legal.

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June 13th, 2008, 12:00 PM

Lesley_B

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Hi salem

The code compiles and runs okay without the cast in the pthread_create call and also without the char *ptr in the thread at all i.e. just operating on the void *data argument. Okay so long as I don't need ever to dereference the pointer.

Not sure how compiler dependent the treating of a void * as a char * is but this code is not likely to be used anywhere anyway.

I changed the pthread-exit command to

Code:

pthread_exit(data);

Earlier on, I was making the mistake of treating that return value as a void ** not a void * - which wasn't helping much.

I'm happy. I have a handle on the void

And thank you for your help.

Regards

Lesley

Understanding and using void * and void ** in pthread_exit and pthread_join

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