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From: 001 on 12 Nov 2009 09:15
I tried to make a producer of 'items' but my implementation crashes
at
pthread_cond_wait ... I really tried looking at everything and put
print statements throughout my code, but there it just crashes. I did
put the wait within a mutex. This code isn't complete, but the
relevant parts are in there (I assume). Any idea what I did wrong?
This is the first time I used condition variables. Oh yeah... another
question: is it possible to wait on multiple condition variables (and
continue when one of them changes)? So that I can stop a thread
independent of whether or not it is waiting for something?

Sincerely,


Stéphane Thibaud


#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <pthread.h> // needed for pthreads
#include <unistd.h> // for usleep()


#include "prodcons.h"


static unsigned int prodCount = 0; // The number of produces items
static unsigned int bufferHead = 0; // Indicates the first free place
in buffer or undefined when prodCount = BUFFER_SIZE
static unsigned int inBufferCnt = 0; // Number of items in buffer
static pthread_mutex_t bufferLock;
static pthread_cond_t bufferNotFull;
static pthread_cond_t availForCons[NROF_CONSUMERS]; // Conditions
variables for the availability of items on the belt (per consumer)
static const unsigned short int ALL_ONES = 0xFFFF;
static int producing = 1;


static void rsleep (int t);


static void * producer (/*void * arg*/)
{
ITEM item; // a produced item


while (prodCount < NROF_ITEMS) // While not all items are
produced
{
rsleep (PRODUCER_SLEEP_FACTOR);


item = (random() % NROF_CONSUMERS) + (prodCount <<
NROF_BITS_DEST);
prodCount++;
pthread_mutex_lock(&bufferLock);
printf("justlocked\n");
if(inBufferCnt == BUFFER_SIZE)
{
printf("full buffer\n");
pthread_cond_wait(&bufferNotFull, &bufferLock);
printf("not full anymore\n");
}
buffer[bufferHead] = item;
bufferHead = (bufferHead + 1) % BUFFER_SIZE;
inBufferCnt++;
if(inBufferCnt == 1)
{
printf("consumer signaled\n");
pthread_cond_signal(&availForCons[buffer
[bufferHead - 1] & ~
(ALL_ONES << NROF_BITS_DEST)]);
}
printf("%04x\n", item); // write info to stdout,
putting it inside
the CS ensures no two threads write to stdout at the same time
printf("unlockingnext\n");
pthread_mutex_unlock(&bufferLock);
}


producing = 0;


return NULL;
From: Ian Collins on 12 Nov 2009 16:59
001 wrote:
> I tried to make a producer of 'items' but my implementation crashes
> at
> pthread_cond_wait ... I really tried looking at everything and put
> print statements throughout my code, but there it just crashes. I did
> put the wait within a mutex. This code isn't complete, but the
> relevant parts are in there (I assume). Any idea what I did wrong?
> This is the first time I used condition variables. Oh yeah... another
> question: is it possible to wait on multiple condition variables (and
> continue when one of them changes)? So that I can stop a thread
> independent of whether or not it is waiting for something?

> static pthread_mutex_t bufferLock;
> static pthread_cond_t bufferNotFull;
> static pthread_cond_t availForCons[NROF_CONSUMERS]; // Conditions

Where are these initialised? You have to call pthread_[mutex|cond]_init
to initialise them.

--
Ian Collins
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