Hi I am trying to implement an solution to producer consumer problem with threads and semaphores.
I have the two functions that the threads consumer and producer will call but they aren't executing. I am not sure what I am doing wrong as I am not getting any errors, my program is just not executing the thread functions. I am creating and joining the thread, I am not sure what is causing the issue
#include <stdio.h>
#include <pthread.h>
#include <unistd.h>
#include <semaphore.h>
#include <stdlib.h>
#include <errno.h>
#include "buffer.h"
struct bufferStruct *item; //calls buffer structure from buffer.h
pthread_mutex_t mutex; //lock for synchronizing execution of threads(producer and consumer)
sem_t empty; //indicates buffer is empty
sem_t full; //indicates buffer is full
int input = 0;
int newitem;
pthread_attr_t attr;
void *producer(void *param)
{
for(int i = 0; i < 5; i++)
{
sem_wait(&empty);
pthread_mutex_lock(&mutex);
newitem = rand();
item->content[item->in] = newitem;
printf("Producer prouced item %d at position %d in buffer\n", newitem, item->in);
item->in = (item->in+1) % MAX_SIZE;
pthread_mutex_unlock(&mutex);
sem_post(&full);
sleep(1);
}
}
void *consumer(void * param)
{
for(int i =0; i< 5; i++)
{
sem_wait(&full);
pthread_mutex_lock(&mutex);
newitem = item->content[item->out];
printf("Consumer has consumed item %d at position %d in buffer\n", newitem, item->out);
item->out = (item->out+1) % MAX_SIZE;
pthread_mutex_unlock(&mutex);
sem_post(&empty);
sleep(1);
}
}
int main()
{
int initval = 1;
int initval2 = 2;
sem_init(&full, 1, 0);
sem_init(&empty, 1, MAX_SIZE);
if(pthread_mutex_init(&mutex,NULL)!=0){
printf("Mutex init failed\n");
return 1;
}
pthread_attr_init(&attr);
pthread_t thread_produce, thread_consume;
printf("Starting threads...\n");
pthread_create(&thread_produce, &attr, producer, (void*)&(initval));
pthread_create(&thread_consume, &attr, consumer, (void*)&(initval2));
pthread_join(thread_produce, NULL);
pthread_join(thread_consume, NULL);
printf("Threads done executing...\n");
pthread_mutex_destroy(&mutex);
sem_destroy(&empty);
sem_destroy(&full);
exit(0);
}
You declare
struct bufferStruct *item;
then you use item without ever assigning it to point to anything. Undefined behavior results.
If I fix that (first synthesizing a version struct bufferStruct and choosing a MAX_SIZE) then the program compiles without error and seems to run successfully.
Related
This is my current code for the Producer-Consumer problem. I compiled it and ran it but nothing is printed. The command line takes in 3 arguments: Sleep time, producer threads, consumer threads. I've tried setting the values as 5, 1, 1 respectively, the sleep timer works but I'm unsure about the rest.
Code for buffer.h:
typedef int buffer_item;
#define BUFFER_SIZE 5
Code for buffer.c:
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <pthread.h>
#include <semaphore.h>
#include "buffer.h"
buffer_item buffer[BUFFER_SIZE];
void *producer(void *param);
void *consumer(void *param);
pthread_mutex_t mutex;
sem_t empty;
sem_t full;
int insert_item(buffer_item item)
{
do
{
wait(empty);
wait(mutex);
signal(mutex);
signal(full);
}while(1);
return 0;
}
int remove_item(buffer_item *item)
{
do
{
wait(full);
wait(mutex);
signal(mutex);
signal(empty);
}while(1);
return 0;
}
int main(int argc, char *argv[])
{
int sleepTime;
int producerThreads;
int consumerThreads;
int counter_1;
int counter_2;
if(argc != 4)
{
return -1;
}
sleepTime = atoi(argv[1]);
producerThreads = atoi(argv[2]);
consumerThreads = atoi(argv[3]);
srand((unsigned)time(NULL));
for(counter_1 = 0; counter_1 < producerThreads; counter_1++)
{
pthread_t tid;
pthread_attr_t attr;
pthread_attr_init(&attr);
pthread_create(&tid, &attr, producer, NULL);
}
for(counter_2 = 0; counter_2 < consumerThreads; counter_2++)
{
pthread_t tid;
pthread_attr_t attr;
pthread_attr_init(&attr);
pthread_create(&tid, &attr, consumer, NULL);
}
sleep(sleepTime);
return 0;
}
void *producer(void *param)
{
buffer_item item;
int randomTime;
int counter_1 = 0;
while(1)
{
randomTime = rand() % 1000 + 1;
sleep(randomTime);
item = rand();
if(insert_item(item))
{
fprintf(stderr, "Error.");
}
else
{
printf("Producer ID: %lu, Produced Item: %d\n", pthread_self(), item);
printf("The buffer now contains %d items\n", counter_1);
++counter_1;
}
}
}
void *consumer(void *param)
{
buffer_item item;
int randomTime;
int counter_2 = 0;
while(1)
{
randomTime = rand() % 1000 + 1;
sleep(randomTime);
if(insert_item(item))
{
fprintf(stderr, "Error.");
}
else
{
printf("Consumer ID: %lu, Consumed Item: %d\n", pthread_self(), item);
printf("The buffer now contains %d items\n", counter_2);
++counter_2;
}
}
}
So far I've tried declaring the tid separately, skipping sleep and join the threads, but it still doesn't print.
Your code can't possibly run, indeed it doesn't even compile.
Here's a list of issues that need to be addressed:
wait should be sem_wait
signal should be sem_post for semaphores
int sem_wait(sem_t *sem); and int sem_post(sem_t *sem); take the pointer to a semaphore
sem_wait(mutex) and sem_post(mutex) give something like "incompatible type for argument 1 of sem_wait", I guess you want to acquire and release the lock on the mutex like pthread_mutex_lock(&mutex) and pthread_mutex_unlock(&mutex)
in the consumer if(insert_item(item)): item is used uninitialized
still in the consumer you use insert_item instead of remove_item
Coming to the main question "I compiled it and ran it but nothing is printed", it doesn't print anything because producer and consumer call, respectively, insert_item and remove_item and are trapped inside infinite loops (e.g. while(1))
I am writing various code snippets and see what happens. The code below was intended to delay all threads until all reached a certain point in the code and then make each print a distinctive number. Since the threads all do that, the numbers should occur in a random order.
My current problem is that I keep they threads busy waiting. If the number of threads gets big, the program slows down significantly.
I would like to change that by using signals, I found pthread_cond_wait() for that, however I don't see how one would use that to signal all threads that they would please wake up.
#include <pthread.h>
#include <stdio.h>
#include <unistd.h>
#define threads 10
int counter=0;
pthread_mutex_t lock;
void handler(void *v) {
pthread_mutex_lock(&lock);
counter++;
printf("%d\n", counter);
pthread_mutex_unlock(&lock);
while(counter != threads); // busy waiting
printf("I am first! %d\n", v);
}
int main() {
pthread_t t[threads];
for(int i =0; i < threads; i++) {
pthread_create(&t[i], NULL, handler, (void*) i);
}
for(int i =0; i < threads; i++) {
pthread_join(t[i], NULL);
}
return 0;
}
EDIT: I changed the code to the following, however, it still does not work :/
pthread_mutex_t lock;
pthread_cond_t cv;
void handler(void *v) {
pthread_mutex_lock(&lock);
pthread_cond_wait(&cv, &lock);
printf("I am first! %d\n", v);
pthread_mutex_unlock(&lock);
}
int main() {
pthread_t t[threads];
for(int i =0; i < threads; i++)
pthread_create(&t[i], NULL, handler, (void*) i);
sleep(2);
pthread_cond_signal(&cv);
for(int i =0; i < threads; i++)
pthread_join(t[i], NULL);
return 0;
}
use broadcast()?
http://pubs.opengroup.org/onlinepubs/009696699/functions/pthread_cond_broadcast.html
The pthread_cond_broadcast() function shall unblock all threads currently blocked on the specified condition variable cond.
The pthread_cond_signal() function shall unblock at least one of the threads that are blocked on the specified condition variable cond (if any threads are blocked on cond).
An alternative solution to pthread_cond_broadcast() might be the following.
You define a read-write mutex and lock it in write in the main thread before creating the other threads. The other threads will try to acquire a read-lock but since the main thread have a write-lock they will be blocked.
After the creation of all thread the main-thread releases the lock. All the other threads will be waken up and since many read-locks can coexist the will execute simultaneously (i.e. no one will be locked).
Code might be something like:
pthread_rwlock_t lock;
void handler(void *v) {
if ((res = pthread_rwlock_rdlock(&lock)!=0)
{
exit(1);
}
printf("I am first! %d\n", v);
pthread_rwlock_unlock(&lock);
}
int main() {
pthread_t t[threads];
//First acquire the write lock:
if ((res = pthread_rwlock_wrlock(&lock)!=0)
{
exit(1);
}
for(int i =0; i < threads; i++)
{
pthread_create(&t[i], NULL, handler, (void*) i);
//it is not clear if you want sleep inside the loop or not
// You indented it as if to be inside but not put brackets
sleep(2);
}
pthread_rwlock_unlock(&lock);
for(int i =0; i < threads; i++)
pthread_join(t[i], NULL);
pthread_rwlock_destroy(&lock);
return 0;
}
Try posting the matching remove_from_buffer code.
Better yet, Short, Self Contained, Correct Example
Make a short main() with two threads.
One thread adds to the buffer at random intervals.
The other thread removes from the buffer at random intervals.
Example
CELEBP22
/* CELEBP22 */
#define _OPEN_THREADS
#include <pthread.h>
#include <stdio.h>
#include <time.h>
#include <unistd.h>
pthread_cond_t cond;
pthread_mutex_t mutex;
int footprint = 0;
void *thread(void *arg) {
time_t T;
if (pthread_mutex_lock(&mutex) != 0) {
perror("pthread_mutex_lock() error");
exit(6);
}
time(&T);
printf("starting wait at %s", ctime(&T));
footprint++;
if (pthread_cond_wait(&cond, &mutex) != 0) {
perror("pthread_cond_timedwait() error");
exit(7);
}
time(&T);
printf("wait over at %s", ctime(&T));
}
main() {
pthread_t thid;
time_t T;
struct timespec t;
if (pthread_mutex_init(&mutex, NULL) != 0) {
perror("pthread_mutex_init() error");
exit(1);
}
if (pthread_cond_init(&cond, NULL) != 0) {
perror("pthread_cond_init() error");
exit(2);
}
if (pthread_create(&thid, NULL, thread, NULL) != 0) {
perror("pthread_create() error");
exit(3);
}
while (footprint == 0)
sleep(1);
puts("IPT is about ready to release the thread");
sleep(2);
if (pthread_cond_signal(&cond) != 0) {
perror("pthread_cond_signal() error");
exit(4);
}
if (pthread_join(thid, NULL) != 0) {
perror("pthread_join() error");
exit(5);
}
}
OUTPUT
starting wait at Fri Jun 16 10:54:06 2006 IPT is about ready to
release the thread wait over at Fri Jun 16 10:54:09 2006
I'm having an issue with my sleep cycles in the ProducerConsumer.h file. If I comment out the sleep(rnum) the program will access the line printf("producer produced %d\n", item); If it's not commented out it will never access that line and I cant figure out why.
Here is my code it's in 3 separate files I would be grateful of any help thank you.
buffer.h:
typedef int buffer_item;
#define BUFFER_SIZE 5
int insert_item(buffer_item item);
int remove_item(buffer_item *item);
ProducerConsumer.h:
#include <stdlib.h> /* required for rand() */
#include <stdio.h>
#include <pthread.h>
#include <semaphore.h>
#include <iostream>
#include "buffer.h"
#define RAND_DIVISOR 100000000;
#define TRUE 1
int counter;
buffer_item buffer[BUFFER_SIZE];
pthread_mutex_t mutex;
sem_t full, empty;
/* Producer Thread */
void *producer(void *param) {
buffer_item item;
while(TRUE) {
/* sleep for a random period of time */
int rNum = rand() / RAND_DIVISOR;
sleep(rNum);
printf("producer produced %d\n", item);
/* generate a random number */
item = rand();
/* acquire the empty lock */
sem_wait(&empty);
/* acquire the mutex lock */
pthread_mutex_lock(&mutex);
if(insert_item(item)) {
fprintf(stderr, " Producer report error condition\n");
} else {
printf("producer produced %d\n", item);
}
/* release the mutex lock */
pthread_mutex_unlock(&mutex);
/* signal full */
sem_post(&full);
}
}
/* Consumer Thread */
void *consumer(void *param) {
buffer_item item;
while(TRUE) {
/* sleep for a random period of time */
int rNum = rand() / RAND_DIVISOR;
sleep(rNum);
/* aquire the full lock */
sem_wait(&full);
/* aquire the mutex lock */
pthread_mutex_lock(&mutex);
if(remove_item(&item)) {
fprintf(stderr, "Consumer report error condition\n");
} else {
printf("consumer consumed %d\n", item);`enter code here`
}
/* release the mutex lock */
pthread_mutex_unlock(&mutex);
/* signal empty */
sem_post(&empty);
}
}
int insert_item(buffer_item item) {
if(counter < BUFFER_SIZE) {
buffer[counter] = item;
counter++;
return 0;
} else {
return -1;
}
}
int remove_item(buffer_item *item) {
if(counter > 0) {
*item = buffer[(counter-1)];
counter--;
return 0;
} else {
return -1;
}
}
main.cpp:
#include <stdlib.h>
#include <stdio.h>
#include <pthread.h>
#include <semaphore.h>
#include <unistd.h>
#include "ProducerConsumer.h"
#include "buffer.h"
pthread_t tid;
pthread_attr_t attr;
void initializeData()
{
pthread_mutex_init(&mutex, NULL);
sem_init(&full, 0, 0);
sem_init(&empty, 0, BUFFER_SIZE);
pthread_attr_init(&attr);
counter = 0;
}
int main() {
int i;
int numProd = 5; /* Number of producer threads */
int numCons = 5; /* Number of consumer threads */
initializeData();
/* Create the producer threads */
for(i = 0; i < numProd; i++) {
/* Create the thread */
pthread_create(&tid,&attr,producer,NULL);
}
/* Create the consumer threads */
for(i = 0; i < numCons; i++) {
/* Create the thread */
pthread_create(&tid,&attr,consumer,NULL);
}
void *producer(void *param);
void *consumer(void *param);
/* Exit the program */
printf("Exit the program\n");
exit(0);
}
The RAND_MAX value is likely at least 32767, causing the the following line:
int rNum = rand() / RAND_DIVISOR;// defined as 100000000
to most always set rNum == 0.
Change statement to get a non-zero value:
int rNum = 1 + rand() % 10;// yields 1-10
Also, call srand() one time, and before using rand(). eg:
void *producer(void *param) {
buffer_item item;
srand((unsigned) time(&t));
...
I have a program which consists of multiple threads.
These threads have to synchronize at some point, continue together, do their work of different lengths and then synchronize again.
Here's the example of what I mean:
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <pthread.h>
#include <unistd.h>
#define NUM_THREADS 10
void* thread(void* idp);
int threads_running = 0;
pthread_mutex_t* mutex;
pthread_cond_t* cond;
int main(int args, char **argv)
{
pthread_mutex_t mutex_var;
pthread_cond_t cond_var;
mutex = &mutex_var;
cond = &cond_var;
pthread_mutex_init(mutex, NULL);
pthread_cond_init(cond, NULL);
pthread_t threads[NUM_THREADS];
for (int i = 0; i < NUM_THREADS; i++)
{
printf("Creating Thread %d....\n", i);
int* id = malloc(sizeof(int));
*id = i;
if(0 != pthread_create(&threads[i], NULL, thread, (void *) id))
{
perror("Error while creating the threads");
exit(1);
}
}
for (int i = 0; i < NUM_THREADS; i++)
{
pthread_join(threads[i], NULL);
}
return 0;
}
void* thread(void* idp)
{
int* id = (int*) idp;
while (1)
{
sleep(*id+2); // Thread work
pthread_mutex_lock(mutex);
threads_running++;
pthread_mutex_unlock(mutex);
pthread_cond_broadcast(cond);
printf("Thread %d WAIT\n", *id);
fflush(stdout);
// Let threads synchronize
pthread_mutex_lock(mutex);
while(threads_running != NUM_THREADS)
{
pthread_cond_wait(cond, mutex);
}
pthread_mutex_unlock(mutex);
printf("Thread %d WAKEUP\n", *id);
fflush(stdout);
// and continue
pthread_mutex_lock(mutex);
threads_running--;
pthread_mutex_unlock(mutex);
}
}
The thing that happens, is that some (usually #9 and one or two others) reach Thread %d WAKEUP but before the others can wake-up threads_running is already smaller than NUM_THREADS.
How can I synchronize multiple threads, so they continue together?
I am having some trouble wrapping my head around the parallel accesses to variables.
I found the answer seconds later:
Change while(threads_running != NUM_THREADS) to while(threads_running != 0 && threads_running != NUM_THREADS)
Change threads_running--; to threads_running = 0;
Now it works perfectly.
I recommend using pthread_cond_signal() than pthread_cond_broadcast(cond);
So the code flow where while loop is exiting looks like below.
pthread_mutex_lock(mutex);
while(threads_running != 0 && threads_running != NUM_THREADS)
{
pthread_cond_wait(cond, mutex);
}
pthread_cond_signal(cond);
So the final code looks like this:
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <pthread.h>
#include <unistd.h>
#define NUM_THREADS 10
void* thread(void* idp);
int threads_running = 0;
pthread_mutex_t* mutex;
pthread_cond_t* cond;
int main(int args, char **argv)
{
pthread_mutex_t mutex_var;
pthread_cond_t cond_var;
mutex = &mutex_var;
cond = &cond_var;
pthread_mutex_init(mutex, NULL);
pthread_cond_init(cond, NULL);
pthread_t threads[NUM_THREADS];
for (int i = 0; i < NUM_THREADS; i++)
{
printf("Creating Thread %d....\n", i);
int* id = malloc(sizeof(int));
*id = i;
if(0 != pthread_create(&threads[i], NULL, thread, (void *) id))
{
perror("Error while creating the threads");
exit(1);
}
}
for (int i = 0; i < NUM_THREADS; i++)
{
pthread_join(threads[i], NULL);
}
return 0;
}
void* thread(void* idp)
{
int* id = (int*) idp;
while (1)
{
sleep(*id+2); // Thread work
pthread_mutex_lock(mutex);
threads_running++;
pthread_mutex_unlock(mutex);
printf("Thread %d WAIT\n", *id);
fflush(stdout);
// Let threads synchronize
pthread_mutex_lock(mutex);
while(threads_running != 0 && threads_running != NUM_THREADS)
{
pthread_cond_wait(cond, mutex);
}
pthread_cond_signal(cond);
pthread_mutex_unlock(mutex);
printf("Thread %d WAKEUP\n", *id);
fflush(stdout);
// and continue
pthread_mutex_lock(mutex);
threads_running = 0;
pthread_mutex_unlock(mutex);
}
}
I'm trying to synchronize multiple (7) threads. I thought I understood how they work until I was trying it on my code and my threads were still printing out of order. Here is the code:
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <pthread.h>
#include <time.h>
void *text(void *arg);
long code[] = {4,6,3,1,5,0,2}; //Order in which to start threads
int num = 0;
pthread_mutex_t lock; //Mutex variable
int main()
{
int i;
pthread_t tid[7];
//Check if mutex worked
if (pthread_mutex_init(&lock, NULL) != 0){
printf("Mutex init failed\n");
return 1;
}
//Initialize random number generator
time_t seconds;
time(&seconds);
srand((unsigned int) seconds);
//Create our threads
for (i=0; i<7; i++)
pthread_create(&tid[i], NULL, text, (void*)code[i]);
//Wait for threads to finish
for (i=0; i<7; i++){
if(pthread_join(tid[i], NULL)){
printf("A thread failed to join\n");
}
}
//Destroy mutex
pthread_mutex_destroy(&lock);
//Exit main
return 0;
}
void *text (void *arg)
{
//pthread_mutex_lock(&lock); //lock
long n = (long) arg;
int rand_sec = rand() % (3 - 1 + 1) + 1; //Random num seconds to sleep
while (num != n) {} //Busy wait used to wait for our turn
num++; //Let next thread go
sleep(rand_sec); //Sleep for random amount of time
pthread_mutex_lock(&lock); //lock
printf("This is thread %d.\n", n);
pthread_mutex_unlock(&lock); //unlock
//Exit thread
pthread_exit(0);
}
So here I am trying to make threads 0-6 print IN ORDER but right now they are still scrambled. The commented out mutex lock is where I originally had it, but then moved it down to the line above the print statement but I'm having similar results. I am not sure where the error in my mutex's are, could someone give a hint or point me in the right direction? I really appreciate it. Thanks in advance!
You cannot make threads to run in order with only a mutex because they go in execution in an unpredictable order.
In my approach I use a condition variable and a shared integer variable to create a queueing system. Each thread takes a number and when the current_n number is equal to the one of the actual thread, it enters the critical section and prints its number.
#include <pthread.h>
#include <stdio.h>
#define N_THREAD 7
int current_n = 0;
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t number = PTHREAD_COND_INITIALIZER;
void *text (void *arg) {
int i = (int)arg;
pthread_mutex_lock(&mutex);
while ( i > current_n ) {
pthread_cond_wait(&number, &mutex);
}
//i = current_n at this point
/*I use stderr because is not buffered and the output will be printed immediately.
Alternatively you can use printf and then fflush(stdout).
*/
fprintf(stderr, "I'm thread n=%d\n", i);
current_n ++;
pthread_cond_broadcast(&number);
pthread_mutex_unlock(&mutex);
return (void*)0;
}
int main() {
pthread_t tid[N_THREAD];
int i = 0;
for(i = 0; i < N_THREAD; i++) {
pthread_create(&tid[i], NULL, text, (void *)i);
}
for(i = 0; i < N_THREAD; i++) {
if(pthread_join(tid[i], NULL)) {
fprintf(stderr, "A thread failed to join\n");
}
}
return 0;
}
The output is:
I'm thread n=0
I'm thread n=1
I'm thread n=2
I'm thread n=3
I'm thread n=4
I'm thread n=5
I'm thread n=6
Compile with
gcc -Wall -Wextra -O2 test.c -o test -lpthread
Don't worry about the warnings.