Multithreaded Fibonacci Pair Program - c

I'm trying to write a program which creates two threads: a "front-end" and "back-end" thread. I want to create a "back-end" thread to iterate and compute pairs of terms from the fibonacci sequence and put them in an array, and a "front-end" thread that will print out the pairs of the array at each iteration.
"Front-End" Thread - For displaying result of "Back-End" thread operations in each iterations
"Back-End" Thread - For calculating and setting an array
ie. [5, 8], and after an iteration it will contain [13, 21]
I'm struggling to implement the Fibonacci sequence part in a thread and I've made the following progress:
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <errno.h>
int fib;
void *front_end(void *ptr);
void *back_end(void *ptr);
int main() {
pthread_t thread1, thread2;
int arr[2] = {5,8};
const int *ptrtoarr;
ptrtoarr=arr;
int create1, create2;
int *s=(int *)(ptrtoarr);
printf("%d \n", *s);
ptrtoarr++;
s = (int *)(ptrtoarr);
printf("%d \n", *s);
ptrtoarr--;
create1 = pthread_create(&thread1, NULL, back_end, &arr);
if(create1) {
fprintf(stderr,"Error - pthread_create() return code: %d\n",create1);
exit(EXIT_FAILURE);
}
pthread_join(thread1, NULL);
//pthread_join(thread2, NULL);
}
// front-end thread to be callback for each back-end iteration
void *front_end(void *ptr) {
int *sum = ptr;
int i, upper = atoi(ptr);
if (upper > 0) {
for (i=0; i<upper; i++){
//Print the fib pairs
}
}
pthread_exit(0);
}
void *back_end(void *ptr) {
int i, upper = atoi(ptr);
fib=1;
if(upper > 0) {
int pre1 = 0;
int current;
//calc fib numbers.....
if(fib == 1){
printf("")
}
}
}
Can someone guide me through how I might approach this?

Your skeleton needs work.
Assuming the following:
unsigned n = ...; // How many to generate.
unsigned n_ready = 2; // How many are ready to print.
unsigned *fibs = malloc(sizeof(unsigned)*n);
fibs[0] = 0;
fibs[1] = 1;
At the core of your back end worker, you will have
for (unsigned i=2; i<n; ++i) {
fibs[i] = fibs[i-2] + fibs[i-1];
n_ready = i+1;
}
At the core of your frontend worker, you will have
for (unsigned i=0; i<n; ++i) {
while (i >= n_ready)
/* Nothing */;
printf("%u\n", fibs[i]);
}
Problem #1
You get into problems if a thread tries to read a variable when another is writing to it. Two or more threads reading the same variable at the same time is ok.
The variables used by both threads are n, the elements of fib[] and n_ready.
n:Not changed by either thread, so we don't need to control access to it.
fib[i] for i >= n_ready:Only accessed by the back end worker, so we don't need to control access to these.
fib[i] for i < n_ready:Only accessed by the frontend worker, so we don't need to control access to these.
n_ready:The back end worker could set n_ready at any time, and the frontend work could try to read n_ready at any time, so we do need to control access to n_ready.
Mutex are usually used to ensure that only one thread is accessing a resource (e.g. a variable, group of variables, file handle, etc) at a time.
Our back end worker becomes
for (unsigned i=2; i<n; ++i) {
// The mutex only protects n_ready
// --nothing else is going to touch fib[i-2] or fib[i-1] or fib[i]--
// so we don't need to obtain a lock yet.
fibs[i] = fibs[i-2] + fibs[i-1];
// We need to access n_ready.
pthread_mutex_lock(&mutex);
n_ready = i+1;
pthread_mutex_unlock(&mutex);
}
Our frontend worker becomes
for (unsigned i=0; i<n; ++i) {
// We need to access n_ready.
pthread_mutex_lock(&mutex);
while (i >= n_ready) {
// Allow other thread to gain the lock.
pthread_mutex_unlock(&mutex);
// We need to access n_ready.
pthread_mutex_lock(&mutex);
}
// The mutex only protects n_ready
// --nothing is going to change fib[i]--
// so we can release it now rather than later.
pthread_mutex_unlock(&mutex);
printf("%u\n", fibs[i]);
}
Problem #2
You have a busy loop. In general, this is bad because it means your thread is using 100% doing nothing by waiting. (In this particular case, since i >= n_ready is probably already true, this would actually be a good strategy. But let's ignore that.) A thread can sleep until signaled by another thread using condition vars.
Our back end worker becomes
for (unsigned i=2; i<n; ++i) {
// The mutex only protects n_ready
// --nothing else is going to touch fib[i-2] or fib[i-1] or fib[i]--
// so we don't need to obtain a lock yet.
fibs[i] = fibs[i-2] + fibs[i-1];
// We need to access n_ready.
pthread_mutex_lock(&mutex);
n_ready = i+1;
// Wake up the other thread if it's blocked.
pthread_cond_signal(&cond);
pthread_mutex_unlock(&mutex);
}
Our frontend worker becomes
for (unsigned i=0; i<n; ++i) {
// We need to access n_ready.
pthread_mutex_lock(&mutex);
while (i >= n_ready)
pthread_cond_wait(&cond, &mutex);
// The mutex only protects n_ready
// --nothing is going to change fib[i]--
// so we can release it now rather than later.
pthread_mutex_unlock(&mutex);
printf("%u\n", fibs[i]);
}
Always call pthread_cond_wait on a locked mutex. It will unlock the mutex when it's called, and it will lock it before returning. This allows the other thread to obtain the mutex in order to change n_ready.
Complete code:
#include <errno.h>
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#define UNUSED(x) (void)(x)
// To control access to n_ready.
static pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
static pthread_cond_t cond = PTHREAD_COND_INITIALIZER;
static unsigned n_ready = 0; // How many are ready to print.
static unsigned n; // How many to generate.
static unsigned *fibs = NULL;
static void *back_worker(void *unused) {
UNUSED(unused);
fibs[0] = 0;
fibs[1] = 1;
// We need to access n_ready.
pthread_mutex_lock(&mutex);
n_ready = 2;
// Wake up the other thread if it's blocked.
pthread_cond_signal(&cond);
pthread_mutex_unlock(&mutex);
for (unsigned i=2; i<n; ++i) {
// The mutex only protects n_ready
// --nothing is going to touch fib[i]--
// so we don't need to obtain a lock yet.
fibs[i] = fibs[i-2] + fibs[i-1];
// We need to access n_ready.
pthread_mutex_lock(&mutex);
n_ready = i+1;
// Wake up the other thread if it's blocked.
pthread_cond_signal(&cond);
pthread_mutex_unlock(&mutex);
}
return NULL;
}
static void *front_worker(void *unused) {
UNUSED(unused);
for (unsigned i=0; i<n; ++i) {
// We need to access n_ready.
pthread_mutex_lock(&mutex);
while (i >= n_ready)
pthread_cond_wait(&cond, &mutex);
// The mutex only protects n_ready
// --nothing is going to change fib[i]--
// so we can release it now rather than later.
pthread_mutex_unlock(&mutex);
printf("%u\n", fibs[i]);
}
return NULL;
}
int main(void) {
n = 20; // How many to generate.
fibs = malloc(sizeof(unsigned) * n);
pthread_t back_thread;
if (errno = pthread_create(&back_thread, NULL, back_worker, NULL)) {
perror(NULL);
exit(1);
}
pthread_t front_thread;
if (errno = pthread_create(&front_thread, NULL, front_worker, NULL)) {
perror(NULL);
exit(1);
}
pthread_join(back_thread, NULL);
pthread_join(front_thread, NULL);
pthread_cond_destroy(&cond);
pthread_mutex_destroy(&mutex);
free(fibs);
return 0;
}
Output:
$ gcc -Wall -Wextra -pedantic a.c -o a -lpthread && a
0
1
1
2
3
5
8
13
21
34
55
89
144
233
377
610
987
1597
2584
4181
Suggestion for an exercise to apply the above
Create a pool of workers that print out the numbers placed into a queue. The output doesn't need to be in order.
The worker function is already written for you. You may not change the main or worker functions. I've even created the queue for you. You simply have to make it thread safe by modifying Queue_enqueue, Queue_dequeue and Queue_done functions. These are the only functions you may change.
#include <errno.h>
#include <inttypes.h>
#include <pthread.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#define NUM_WORKERS 4
#define QUEUE_SIZE 10
#define NUM_ITEMS 40
typedef struct {
pthread_mutex_t mutex;
pthread_cond_t cond;
int done;
int empty;
int full;
size_t max;
size_t next_insert;
size_t next_read;
unsigned *buf;
} Queue;
static void Queue_init(Queue* q, size_t max) {
pthread_mutex_init(&(q->mutex), NULL);
pthread_cond_init(&(q->cond), NULL);
q->done = 0;
q->empty = 1;
q->full = 0;
q->max = max;
q->next_insert = 0;
q->next_read = 0;
q->buf = malloc(sizeof(unsigned)*max);
}
static void Queue_destroy(Queue *q) {
free(q->buf);
pthread_cond_destroy(&(q->cond));
pthread_mutex_destroy(&(q->mutex));
}
static void Queue_done(Queue *q) {
q->done = 1;
}
// Returns the oldest item from the queue (via a parameter) and returns 1.
// If the queue is empty and done, returns 0.
// If the queue is empty and not done, waits until that changes.
static int Queue_dequeue(Queue *q, unsigned *i) {
while (q->empty && !q->done) {
}
if (q->empty) {
// We are completely done.
return 0;
} else {
*i = q->buf[ q->next_read ];
q->next_read = ( q->next_read + 1 ) % q->max;
q->empty = q->next_read == q->next_insert;
q->full = 0;
return 1;
}
}
// Adds the argument to the queue.
// If the queue is full, waits until that changes.
static void Queue_enqueue(Queue *q, unsigned i) {
while (q->full && !q->done) {
}
if (q->done) {
fprintf(stderr, "Error: Attempted to add item to \"done\" queue.\n");
return;
}
q->buf[q->next_insert] = i;
q->next_insert = ( q->next_insert + 1 ) % q->max;
q->empty = 0;
q->full = q->next_insert == q->next_read;
}
static int msleep(long msec) {
struct timespec ts;
int res;
if (msec < 0) {
errno = EINVAL;
return -1;
}
ts.tv_sec = msec / 1000;
ts.tv_nsec = (msec % 1000) * 1000000;
do {
res = nanosleep(&ts, &ts);
} while (res && errno == EINTR);
return res;
}
// Protects access to stdout.
static pthread_mutex_t stdout_mutex;
static Queue q;
static void *worker(void *worker_id_) {
uintptr_t worker_id = (uintptr_t)worker_id_;
unsigned int seed = worker_id; // Whatever.
unsigned i;
while (Queue_dequeue(&q, &i)) {
pthread_mutex_lock(&stdout_mutex);
printf("[%" PRIuPTR "] Dequeued %u\n", worker_id, i);
pthread_mutex_unlock(&stdout_mutex);
// msleep( rand_r(&seed) % 1000 + 1000 ); // Simulate a 1 to 2s load.
pthread_mutex_lock(&stdout_mutex);
printf("[%" PRIuPTR "] Finished processing %u\n", worker_id, i);
pthread_mutex_unlock(&stdout_mutex);
}
return NULL;
}
int main(void) {
Queue_init(&q, QUEUE_SIZE);
pthread_t workers[NUM_WORKERS];
for (uintptr_t i=0; i<NUM_WORKERS; ++i) {
if (errno = pthread_create(&(workers[i]), NULL, worker, (void*)i)) {
perror(NULL);
exit(1);
}
}
for (unsigned i=0; i<NUM_ITEMS; ++i) {
pthread_mutex_lock(&stdout_mutex);
printf("[x] Enqueuing %u...\n", i);
pthread_mutex_unlock(&stdout_mutex);
Queue_enqueue(&q, i);
pthread_mutex_lock(&stdout_mutex);
printf("[x] Enqueued %u.\n", i);
pthread_mutex_unlock(&stdout_mutex);
}
Queue_done(&q);
pthread_mutex_lock(&stdout_mutex);
printf("[x] Called done.\n");
pthread_mutex_unlock(&stdout_mutex);
for (unsigned i=0; i<NUM_WORKERS; ++i)
pthread_join(workers[i], NULL);
Queue_destroy(&q);
pthread_mutex_destroy(&stdout_mutex);
return 0;
}
If you have questions about this, feel free to post a link to the question as a comment to this answer.
Solution to suggested excercise:
static void Queue_done(Queue *q) {
pthread_mutex_lock(&(q->mutex));
q->done = 1;
pthread_cond_signal(&(q->cond));
pthread_mutex_unlock(&(q->mutex));
}
// Returns the oldest item from the queue (via a parameter) and returns 1.
// If the queue is empty and done, returns 0.
// If the queue is empty and not done, waits until that changes.
static int Queue_dequeue(Queue *q, unsigned *i) {
pthread_mutex_lock(&(q->mutex));
while (q->empty && !q->done)
pthread_cond_wait(&(q->cond), &(q->mutex));
int dequeued;
if (q->empty) {
// We are completely done.
dequeued = 0;
} else {
*i = q->buf[ q->next_read ];
q->next_read = ( q->next_read + 1 ) % q->max;
q->empty = q->next_read == q->next_insert;
q->full = 0;
dequeued = 1;
}
pthread_cond_signal(&(q->cond));
pthread_mutex_unlock(&(q->mutex));
return dequeued;
}
// Adds the argument to the queue.
// If the queue is full, waits until that changes.
static void Queue_enqueue(Queue *q, unsigned i) {
pthread_mutex_lock(&(q->mutex));
while (q->full && !q->done)
pthread_cond_wait(&(q->cond), &(q->mutex));
if (q->done) {
fprintf(stderr, "Error: Attempted to add item to \"done\" queue.\n");
} else {
q->buf[q->next_insert] = i;
q->next_insert = ( q->next_insert + 1 ) % q->max;
q->empty = 0;
q->full = q->next_insert == q->next_read;
}
pthread_cond_signal(&(q->cond));
pthread_mutex_unlock(&(q->mutex));
}

Related

How to find the number of rounds for the producer to deliver characters to buffer?

I have a producer consumer program that reads a file char by char and puts the content inside a buffer.
I need help with outputting the number of rounds the producer function made to deliver characters to the buffer. A round would mean one or more continuous writes to the buffer without being interrupted by wait (due to a full queue).
#include <pthread.h>
#include <semaphore.h>
#include <stdlib.h>
#include <stdio.h>
/*
This program provides a possible solution for producer-consumer problem using mutex and semaphore.
I have used 5 producers and 5 consumers to demonstrate the solution. You can always play with these values.
*/
#define MaxItems 5 // Maximum items a producer can produce or a consumer can consume
#define BufferSize 5 // Size of the buffer
sem_t empty;
sem_t full;
int in = 0;
int out = 0;
int buffer[BufferSize];
pthread_mutex_t mutex;
void *producer(void *pno)
{
int item;
for(int i = 0; i < MaxItems; i++) {
item = rand(); // Produce an random item
sem_wait(&empty);
pthread_mutex_lock(&mutex);
buffer[in] = item;
printf("Producer %d: Insert Item %d at %d\n", *((int *)pno),buffer[in],in);
in = (in+1)%BufferSize;
pthread_mutex_unlock(&mutex);
sem_post(&full);
}
}
void *consumer(void *cno)
{
for(int i = 0; i < MaxItems; i++) {
sem_wait(&full);
pthread_mutex_lock(&mutex);
int item = buffer[out];
printf("Consumer %d: Remove Item %d from %d\n",*((int *)cno),item, out);
out = (out+1)%BufferSize;
pthread_mutex_unlock(&mutex);
sem_post(&empty);
}
}
int main()
{
pthread_t pro[5],con[5];
pthread_mutex_init(&mutex, NULL);
sem_init(&empty,0,BufferSize);
sem_init(&full,0,0);
FILE *fp = fopen("file.txt", "r");
if (fp != NULL) {
if (fseek(fp, 0L, SEEK_END) == 0) {
/* Get the size of the file. */
p1.BUFFER_SIZE = ftell(fp);
if (p1.BUFFER_SIZE == -1) { /* Error */ }
/* Allocate our buffer to that size. */
p1.item = malloc(sizeof(char) * (p1.BUFFER_SIZE + 1));
/* Go back to the start of the file. */
if (fseek(fp, 0L, SEEK_SET) != 0) { /* Error */ }
/* Read the entire file into memory. */
size_t newLen = fread(p1.item, sizeof(char), p1.BUFFER_SIZE, fp);
if ( ferror( fp ) != 0 ) {
fputs("Error reading file", stderr);
} else {
p1.item[newLen++] = '\0'; /* Just to be safe. */
}
}
int a[5] = {1,2,3,4,5}; //Just used for numbering the producer and consumer
for(int i = 0; i < 5; i++) {
pthread_create(&pro[i], NULL, (void *)producer, (void *)&a[i]);
}
for(int i = 0; i < 5; i++) {
pthread_create(&con[i], NULL, (void *)consumer, (void *)&a[i]);
}
for(int i = 0; i < 5; i++) {
pthread_join(pro[i], NULL);
}
for(int i = 0; i < 5; i++) {
pthread_join(con[i], NULL);
}
pthread_mutex_destroy(&mutex);
sem_destroy(&empty);
sem_destroy(&full);
return 0;
}
In general, there is no way to determine whether the time it takes for a call to sem_wait() to return is extended on account of the semaphore's value initially being zero. The sem_wait() function does not communicate that information.
One thing you could do, however, is to start out with a sem_trywait(), which will fail instead of blocking if it cannot immediately decrement the target semaphore. You could increment a counter in that case, and then proceed to perform a regular sem_wait(). Example:
int full_count = 0;
void *producer(void *pno) {
int my_num = *(int *)pno;
for(int i = 0; i < MaxItems; i++) {
int item = rand(); // Produce an random item
int result = sem_trywait(&empty);
if (result == -1) {
if (errno == EAGAIN) {
full_count += 1;
result = sem_wait(&empty);
// handle any error ...
} else {
// handle other error ...
}
}
pthread_mutex_lock(&mutex);
buffer[in] = item;
printf("Producer %d: Insert Item %d at %d\n", my_num, buffer[in], in);
in = (in + 1) % BufferSize;
pthread_mutex_unlock(&mutex);
sem_post(&full);
}
}
Do note that there is no guarantee there that in the event that sem_wait() is called, it will actually block, because the semaphore can be incremented between the trywait and the wait. But it does tell you that if a wait had been performed instead of the trywait, then that wait would have blocked. In that case the producer is indeed delayed on account of a full buffer, even if it doesn't spend any of that delay blocked in sem_wait().

Multi threading Raytracer

I am making a raytracer, im trying to use pthread to divide the rendering. i noticed that isnt helping with the speed because the function pthread_join is to slow, if i use a loop to make the 'await' is way faster and works almost every time fine. But i cant use that because the time of rendering changes with the scene.
Is there a way to check if a thread is finished, on a more efficient way. This is the code.
`
int threats(t_file *c) //this function creates the threads
{
int i;
int err;
pthread_t th[THREADS];
i = 0;
printf("1\n");
c->thread = -1;
mlx_clear_window(c->mlx_ptr, c->win_ptr);
while (i < THREADS)
{
err = pthread_create(&th[i], 0, (void *)paint_scene, (void *)c);
if (err)
return parse_error("Thread Error: CAN NOT CREATE THREAD");
i++;
}
// while (i-- >= 0)
// pthread_join(th[i], 0);
//my await function xd
while (i < 200000000)
i++;
mlx_put_image_to_window(c->mlx_ptr, c->win_ptr, c->img.mlx_img, 0, 0);
c->thread = 0;
return 1;
}
void paint_scene(void *a)
{
int y;
int x;
t_ray ray;
int color;
t_file *c;
c = (t_file *)a;
color = 0;
c->thread++;
y = (c->thread * (c->win_heigth / THREADS));
printf("y:%d,hilo%d\n", y, c->thread);
while (y < (c->thread + 1) * (c->win_heigth / THREADS))
{
x = 0;
while (x < c->win_width)
{
ray = generate_ray(x, y, *c);
color = get_intersections(&ray, c);
if (c->ligth)
color = shading(&ray, color, c);
my_mlx_pixel_put(&c->img, x, y, color);
x++;
}
//ft_printf("\rLoading%d: %d%%", c->thread, y / (c->win_heigth / 100));
y++;
}
pthread_exit(0);
}
`
You have a concurrency problem here in your thread function:
c->thread++;
y = (c->thread * (c->win_heigth / THREADS));
printf("y:%d,hilo%d\n", y, c->thread);
while (y < (c->thread + 1) * (c->win_heigth / THREADS))
{
....
}
c->thread is shared between all threads, and based on likely thread timings and current face of the moon, I can make an educated guess and say that the first thread is calculating the whole image. When starting up, the first thread might see c->thread == -1, but later (if thread startup is faster than the while loop) other thread increase the value until the first thread sees c->thread == THREADS-1
To fix this, each call to create_thread must pass a pointer to a unique parameter object that holds that threads id. So remove the thread member from t_file. It probably serves no purpose there. And create a type of struct that holds the parameters to the thread function:
struct thread_param
{
unsigned int thread;
file_t *c;
}
You use it like this when starting threads:
struct thread_param params[THREADS];
while (i < THREADS)
{
params[i].thread = i;
params[i].c = c;
err = pthread_create(&th[i], 0, (void *)paint_scene, (void *)&(params[i]));
if (err)
return parse_error("Thread Error: CAN NOT CREATE THREAD");
i++;
}
And you access the data in your thread function:
void paint_scene(void *a)
{
struct thread_param *param = (struct thread_param *)a;
unsigned int thread = param->thread;
t_file *c = param->c;
/*
in the rest of the code you remove `c->thread++`
and replace `c->thread` with `thread`
*/
....
}
If you have atomic data types (C11, #ifndef __STDC_NO_ATOMICS__) then implement a global counter and wait until it hits zero (if decreasing) or the amount of threads (if increasing).
e.g.
#include <stdatomic.h>
atomic_int num_jobs;
void* thread_func(void*)
{
//the work to do in the thread function
//before exit decrease counter
--num_jobs;
pthread_exit(0);
}
int main()
{
num_jobs = THREADS; // same as your number of threads
create_threads(THREADS); // number of threads = THREADS
while (num_jobs) { // loop while threads running
//sleep for a while
}
join_threads(); // join threads for cleanup
return 0;
}
Otherwise classic lock mechanics,
e.g.
#include <pthread.h>
pthread_spinlock_t lock;
int num_jobs;
// called by main
int numJobs()
{
pthread_spin_lock(&lock);
int res = num_jobs;
pthread_spin_unlock(&lock);
return res;
}
// called by thread_func
void decNumJobs()
{
pthread_spin_lock(&lock);
--num_jobs;
pthread_spin_unlock(&lock);
}
int main()
{
pthread_spin_init(&lock, PTHREAD_PROCESS_PRIVATE);
// the other stuff as before
pthread_spin_destroy(&lock);
return 0;
}
Another alternative would be with pthread_cond_wait and pthread_cond_signal (mainly to avoid the sleep in the while loop, continue after receiving the signal and not based on a fixed amount of time).
e.g.
#include <pthread.h>
int num_jobs;
pthread_cond_t cond;
pthread_mutex_t lock;
void decNumJobs()
{
pthread_mutex_lock(&lock);
if (--num_jobs == 0)
pthread_cond_signal(&cond);
pthread_mutex_unlock(&lock);
}
void* thread_func(void*)
{
//the work to do in the thread function
//before exit decrease counter
decNumJobs();
pthread_exit(0);
}
int main()
{
num_jobs = THREADS;
pthread_cond_init(&cond, NULL);
pthread_mutex_init(&lock, NULL);
pthread_mutex_lock(&lock);
create_threads(THREADS);
pthread_cond_wait(&cond, &lock);
pthread_mutex_unlock(&lock);
join_threads();
pthread_cond_destroy(&cond);
pthread_mutex_destroy(&lock);
return 0;
}
Note: For the sake of simplicity, there is no error checking nor handling. Reading the documentation of the pthread_* functions (return values, interrupted wait, etc) is strongly advised.

Multithreading : busy waiting in producer-consumer problem

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <string.h>
#include <errno.h>
#include <signal.h>
#include <wait.h>
#include <pthread.h>
int item_to_produce, curr_buf_size;
int total_items, max_buf_size, num_workers, num_masters;
int consumed_items;
int *buffer;
pthread_mutex_t mutex;
pthread_cond_t has_data;
pthread_cond_t has_space;
void print_produced(int num, int master) {
printf("Produced %d by master %d\n", num, master);
}
void print_consumed(int num, int worker) {
printf("Consumed %d by worker %d\n", num, worker);
}
//consume items in buffer
void *consume_requests_loop(void *data)
{
int thread_id = *((int *)data);
while(1)
{
pthread_mutex_lock(&mutex); // mutex lock for consume
if(consumed_items == total_items) {
pthread_mutex_unlock(&mutex);
break;
}
while(curr_buf_size == 0) {
pthread_cond_wait(&has_data, &mutex);
}
print_consumed(buffer[(curr_buf_size--)-1], thread_id);
consumed_items++;
pthread_cond_signal(&has_space);
pthread_mutex_unlock(&mutex);
}
return 0;
}
//produce items and place in buffer
//modify code below to synchronize correctly
void *generate_requests_loop(void *data)
{
int thread_id = *((int *)data);
while(1) {
pthread_mutex_lock(&mutex); // mutex lock for consume
//all of items are produced
//master threads need to join
if(item_to_produce == total_items) {
pthread_mutex_unlock(&mutex);
break;
}
//there is no item to read
while (curr_buf_size == max_buf_size) {
pthread_cond_wait(&has_space, &mutex);
}
buffer[curr_buf_size++] = item_to_produce;
print_produced(item_to_produce, thread_id);
item_to_produce++;
pthread_cond_signal(&has_data);
pthread_mutex_unlock(&mutex); // mutex_produce unlock
}
return 0;
}
//write function to be run by worker threads
//ensure that the workers call the function print_consumed when they consume an item
int main(int argc, char *argv[])
{
int *master_thread_id; // array of master_thread_id
int *worker_thread_id; // array of worker_thread_id
pthread_t *master_thread; // array of master_thread
pthread_t *worker_thread; // array of worker_thread
item_to_produce = 0; // item will be produced by master_thread at next time
curr_buf_size = 0; // index of item will be saved in
consumed_items = 0;
int i;
if (argc < 5) {
printf("./master-worker #total_items #max_buf_size #num_workers #masters e.g. ./exe 10000 1000 4 3\n");
exit(1);
}
else {
num_masters = atoi(argv[4]);
num_workers = atoi(argv[3]);
total_items = atoi(argv[1]);
max_buf_size = atoi(argv[2]);
}
buffer = (int *)malloc (sizeof(int) * max_buf_size);
pthread_mutex_init(&mutex, NULL);
pthread_cond_init(&has_space, NULL);
pthread_cond_init(&has_data, NULL);
//create master producer threads
master_thread_id = (int *)malloc(sizeof(int) * num_masters);
master_thread = (pthread_t *)malloc(sizeof(pthread_t) * num_masters);
for (i = 0; i < num_masters; i++)
master_thread_id[i] = i;
for (i = 0; i < num_masters; i++)
pthread_create(&master_thread[i], NULL, generate_requests_loop, (void *)&master_thread_id[i]);
//create worker consumer threads
worker_thread_id = (int *)malloc(sizeof(int) * num_workers);
worker_thread = (pthread_t *)malloc(sizeof(pthread_t) * num_workers);
for (i = 0; i < num_workers; i++)
worker_thread_id[i] = i;
for (i = 0 ; i < num_workers; i++)
pthread_create(&worker_thread[i], NULL, consume_requests_loop, (void *)&worker_thread_id[i]);
//wait for all threads to complete
for (i = 0; i < num_masters; i++)
{
pthread_join(master_thread[i], NULL);
printf("master %d joined\n", i);
}
for (i = 0; i < num_workers; i++)
{
pthread_join(worker_thread[i], NULL);
printf("worker %d joined\n", i);
}
/*----Deallocating Buffers---------------------*/
free(buffer);
free(master_thread_id);
free(master_thread);
free(worker_thread_id);
free(worker_thread);
pthread_mutex_destroy(&mutex);
pthread_cond_destroy(&has_data);
pthread_cond_destroy(&has_space);
return 0;
}
This code produces a number in the range of given numbers through the argument and consumes it.
But producer produces a number outside the range and doesn't join if it matches the condition. The consumer is too.
e.g when I give range of number like 0~39(total_item = 500), buff size 30(max_buf_size), num_workers 5, num_master 3, it doesn't produce and consume number only 0~39.
It produces and consumes numbers over 40.
In that way the thread is in a loop. To put the thread in sleep you can use, for example, the condition variables. (You can read this for more info https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_cond_wait.html)

Printing Buffer contents

I'm working on the producer and consumer problem. The producer is generating a random variable and placing it in the buffer. After this is done I want to print out the contents of the buffer. I also want to print the contents of the buffer after the consumer consumes a variable from the buffer. So just as an example,
Producer Thread 34567834 adds 43 to the buffer, and the current buffer contains 7, 29, 43
I am not sure of a way to print the contents of a buffer in one printf() statement. Thanks for any help.
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <semaphore.h>
//Submit with screen shot of compiling and running code.
#define SIZE 20
#define NUMB_THREADS 10
#define PRODUCER_LOOPS 10
#define CONSUMER_LOOPS 2
#define TRUE 1
#define FALSE 0
typedef int buffer_t;
buffer_t buffer[SIZE];
int buffer_index;
pthread_mutex_t buffer_mutex;
/* initially buffer will be empty. full_sem
will be initialized to buffer SIZE, which means
SIZE number of producer threads can write to it.
And empty_sem will be initialized to 0, so no
consumer can read from buffer until a producer
thread posts to empty_sem */
sem_t full_sem; /* when 0, buffer is full */
sem_t empty_sem; /* when 0, buffer is empty. Kind of
like an index for the buffer */
/* sem_post algorithm:
mutex_lock sem_t->mutex
sem_t->value++
mutex_unlock sem_t->mutex
sem_wait algorithn:
mutex_lock sem_t->mutex
while (sem_t->value > 0) {
mutex_unlock sem_t->mutex
sleep... wake up
mutex_lock sem_t->mutex
}
sem_t->value--
mutex_unlock sem_t->mutex
*/
void insertbuffer(buffer_t value) {
if (buffer_index < SIZE) {
buffer[buffer_index++] = value;
} else {
printf("Buffer overflow\n");
}
}
buffer_t dequeuebuffer() {
if (buffer_index > 0) {
return buffer[--buffer_index]; // buffer_index-- would be error!
} else {
printf("Buffer underflow\n");
}
return 0;
}
int isempty() {
if (buffer_index == 0)
return TRUE;
return FALSE;
}
int isfull() {
if (buffer_index == SIZE)
return TRUE;
return FALSE;
}
void *producer2(void *thread_n) {
int thread_numb = *(int *)thread_n;
buffer_t value;
int i=0;
while (i++ < PRODUCER_LOOPS) {
sleep(rand() % 10);
value = rand() % 100;
pthread_mutex_lock(&buffer_mutex);
do {
// cond variables do the unlock/wait and wakeup/lock atomically,
// which avoids possible race conditions
pthread_mutex_unlock(&buffer_mutex);
// cannot go to slepp holding lock
sem_wait(&full_sem); // sem=0: wait. sem>0: go and decrement it
// there could still be race condition here. another
// thread could wake up and aqcuire lock and fill up
// buffer. that's why we need to check for spurious wakeups
pthread_mutex_lock(&buffer_mutex);
} while (isfull()); // check for spurios wake-ups
insertbuffer(value);
pthread_mutex_unlock(&buffer_mutex);
sem_post(&empty_sem); // post (increment) emptybuffer semaphore
//printf("Producer Thread %d adds %d added %d to buffer\n", pthread_self(), thread_numb, value);
printf("Producer Thread %d adds %d to the buffer, and the current buffer contains %d \n", pthread_self(), value, *buffer);
}
pthread_exit(0);
}
void *consumer2(void *thread_n) {
int thread_numb = *(int *)thread_n;
buffer_t value;
int i=0;
while (i++ < CONSUMER_LOOPS) {
pthread_mutex_lock(&buffer_mutex);
do {
pthread_mutex_unlock(&buffer_mutex);
sem_wait(&empty_sem);
pthread_mutex_lock(&buffer_mutex);
} while (isempty()); //check for spurios wakeups
value = dequeuebuffer(value);
pthread_mutex_unlock(&buffer_mutex);
sem_post(&full_sem); // post (increment) fullbuffer semaphore
printf("Consumer Thread %d dequeue %d from buffer, and the current buffer contains %d \n", pthread_self(), value, *buffer);
}
pthread_exit(0);
}
int main(int argc, int **argv) {
buffer_index = 0;
pthread_mutex_init(&buffer_mutex, NULL);
sem_init(&full_sem, // sem_t *sem
0, // int pshared. 0 = shared between threads of process, 1 = shared between processes
SIZE); // unsigned int value. Initial value
sem_init(&empty_sem,
0,
0);
/* full_sem is initialized to buffer size because SIZE number of
producers can add one element to buffer each. They will wait
semaphore each time, which will decrement semaphore value.
empty_sem is initialized to 0, because buffer starts empty and
consumer cannot take any element from it. They will have to wait
until producer posts to that semaphore (increments semaphore
value) */
pthread_t thread[NUMB_THREADS];
int thread_numb[NUMB_THREADS];
int i;
for (i = 0; i < NUMB_THREADS; ) {
thread_numb[i] = i;
if(i <= 2)
{
pthread_create(thread + i, // pthread_t *t
NULL, // const pthread_attr_t *attr
producer2, // void *(*start_routine) (void *)
thread_numb + i); // void *arg
}
thread_numb[i] = i;
// playing a bit with thread and thread_numb pointers...
pthread_create(&thread[i], // pthread_t *t
NULL, // const pthread_attr_t *attr
consumer2, // void *(*start_routine) (void *)
&thread_numb[i]); // void *arg
i++;
}
for (i = 0; i < NUMB_THREADS; i++)
pthread_join(thread[i], NULL);
pthread_mutex_destroy(&buffer_mutex);
sem_destroy(&full_sem);
sem_destroy(&empty_sem);
return 0;
}
You can't print an array of unknown length in one line, but you can modify your insertbuffer to show the details every time you insert a value. Or, implement the printing as a separate function. Then calling it is a one-liner, obviously.
void insertbuffer(int threadnum, buffer_t value) {
int i;
if (buffer_index < SIZE) {
buffer[buffer_index++] = value;
printf("Producer Thread %d adds %d to the buffer, and the current buffer contains",
threadnum, (int)value);
for(i=0; i<buffer_index; i++) {
if(i)
printf(",");
printf(" %d", (int)buffer[i]);
}
printf("\n");
} else {
printf("Buffer overflow\n");
}
}

unable to reacquire mutex and pass values correctly between threads

I am trying to implement a code to practice synchronization, so might not be best design or approach but goal is as below
Main thread
Creates a payload of 100 integers and waits for any thread to be available
When it gets signal from a thread its available - it unlocks the payload for copying and proceeds to create another payload
Worker thread
on creation of it makes itself available for data processing and sends signal that its available
Tries to lock the data payload from main thread and copy it to local array
( observing bug here - not able to access data properly)
Turn off the sign of available
( unable to turn off available state to off)
Keep processing data through local copy
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <stdbool.h>
#define WORKERS 2
#define ARRAY_ELEMENTS 100
#define MAX 1000
pthread_mutex_t mutex_bucket1 = PTHREAD_MUTEX_INITIALIZER;
pthread_mutex_t mutex_signal = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t cond_go = PTHREAD_COND_INITIALIZER;
pthread_cond_t cond_busy = PTHREAD_COND_INITIALIZER;
static int value = 0;
bool available = false;
void *worker_thread(void *pbucket)
{
sleep(5);
while(1)
{
unsigned int count = 0;
int local_array[ARRAY_ELEMENTS];
int *ptbucket = (int*)pbucket;
setbuf(stdout, NULL);
pthread_mutex_lock(&mutex_signal);
printf(" -------------- \n chainging state to available \n --------- ");
available = true;
printf(" -------------- \n from thread sending go signal \n --------- ");
pthread_cond_signal(&cond_go);
pthread_mutex_unlock(&mutex_signal);
pthread_mutex_lock(&mutex_bucket1);
printf(" -------------- \n data part locked in thread for copying \n --------- ");
while(count < ARRAY_ELEMENTS)
{
printf(" %d - \n", ptbucket[count]); /***incorrect values***/
local_array[count] = ptbucket[count];
count++;
}
pthread_mutex_unlock(&mutex_bucket1);
/*Never able to acquire mutex_signal and change state to not available*/ **BUG**
pthread_mutex_lock(&mutex_signal);
printf(" -------------- \n chainging state to not available \n --------- ");
available = false;
pthread_mutex_unlock(&mutex_signal);
count = 0;
while(count < ARRAY_ELEMENTS)
{
printf(" %d - \n", local_array[count]);
count++;
}
printf(" -------------- \n about to sleep for 5secs \n --------- ");
sleep(5);
}
}
int main(void)
{
pthread_t thread_id[WORKERS];
unsigned int* pbucket1 = (int*) malloc(sizeof(int) * ARRAY_ELEMENTS);
unsigned int* pbucket;
for(int i = 0; i < WORKERS - 1; i++)
{
pthread_create(&thread_id[i], NULL, worker_thread, (void *) pbucket);
}
for(int i = 0; i < MAX; i++)
{
unsigned int count = 0;
pbucket = pbucket1;
// Make the payload ready
pthread_mutex_lock(&mutex_bucket1);
printf(" -------------- creating data payload --------- \n");
while(count < ARRAY_ELEMENTS)
{
pbucket1[count] = i;
i++;
count++;
}
printf(" -------------- \n waiting for go signal \n --------- ");
while(!available)
{
pthread_cond_wait(&cond_go, &mutex_signal);
}
pthread_mutex_unlock(&mutex_bucket1);
/*I believe after we unlock variable "available" can be mutexed
again by other thread but seems thinking is flawed */
printf(" -------------- \n Main thread sleep for 3 seconds \n --------- ");
sleep(3);
}
for(int i = 0; i < WORKERS; i++)
{
pthread_join(thread_id[i], NULL);
}
return 0;
}
I think some of your idea is backwards; It shouldn't be the main context that is waiting, it should be the worker threads waiting for data ...
The job of the main thread should be to keep populating the payload and waking one thread at a time to process it.
So here's some scribbled code that is a little more sensible, I think:
/**
file: answer.c
compile: gcc -o answer answer.c -pthread
usage: answer [numThreads] [numElements]
**/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <pthread.h>
#define STATE_WAIT 1
#define STATE_READY 2
void *routine(void*);
typedef struct _shared_t {
pthread_mutex_t m;
pthread_cond_t c;
unsigned char state;
int *payload;
size_t numElements;
pthread_t *threads;
size_t numThreads;
} shared_t;
static inline void shared_init(shared_t *shared, size_t numThreads, size_t numElements) {
memset(shared, 0, sizeof(shared_t));
pthread_mutex_init(&shared->m, NULL);
pthread_cond_init(&shared->c, NULL);
shared->state = STATE_WAIT;
shared->numThreads = numThreads;
shared->numElements = numElements;
{
int it = 0;
shared->threads = (pthread_t*) calloc(shared->numThreads, sizeof(pthread_t));
while (it < shared->numThreads) {
if (pthread_create(&shared->threads[it], NULL, routine, shared) != 0) {
break;
}
it++;
}
}
}
static inline void shared_populate(shared_t *shared) {
if (pthread_mutex_lock(&shared->m) != 0) {
return;
}
shared->payload = (int*) calloc(shared->numElements, sizeof(int));
{
int it = 0,
end = shared->numElements;
while (it < end) {
shared->payload[it] = rand();
it++;
}
}
shared->state = STATE_READY;
pthread_cond_signal(&shared->c);
pthread_mutex_unlock(&shared->m);
}
static inline void shared_cleanup(shared_t *shared) {
int it = 0,
end = shared->numThreads;
while (it < end) {
pthread_join(shared->threads[it], NULL);
}
pthread_mutex_destroy(&shared->m);
pthread_cond_destroy(&shared->c);
free(shared->threads);
}
void* routine(void *arg) {
shared_t *shared = (shared_t*) arg;
int *payload;
do {
if (pthread_mutex_lock(&shared->m) != 0) {
break;
}
while (shared->state == STATE_WAIT) {
pthread_cond_wait(&shared->c, &shared->m);
}
payload = shared->payload;
shared->state = STATE_WAIT;
pthread_mutex_unlock(&shared->m);
if (payload) {
int it = 0,
end = shared->numElements;
while (it < end) {
printf("Thread #%ld got payload %p(%d)=%d\n",
pthread_self(), payload, it, payload[it]);
it++;
}
free(payload);
}
} while(1);
pthread_exit(NULL);
}
int main(int argc, char *argv[]) {
shared_t shared;
int numThreads = argc > 1 ? atoi(argv[1]) : 1;
int numElements = argc > 2 ? atoi(argv[2]) : 100;
shared_init(&shared, numThreads, numElements);
do {
shared_populate(&shared);
} while (1);
shared_cleanup(&shared);
return 0;
}
Obviously, the code above is not very tolerant of errors, and is not easy to shutdown cleanly ... it's illustration only.
Let's first look at main so that we know what the flow of the main program is going to be:
int main(int argc, char *argv[]) {
shared_t shared;
int numThreads = argc > 1 ? atoi(argv[1]) : 1;
int numElements = argc > 2 ? atoi(argv[2]) : 100;
shared_init(&shared, numThreads, numElements);
do {
shared_populate(&shared);
} while (1);
shared_cleanup(&shared);
return 0;
}
It keeps a shared_t on the stack:
typedef struct _shared_t {
pthread_mutex_t m;
pthread_cond_t c;
unsigned char state;
int *payload;
size_t numElements;
pthread_t *threads;
size_t numThreads;
} shared_t;
Mostly self explanatory, mutex, condition and state are required for synchronization.
First of all the shared_t must be initialized with mutex, condition, state and threads using the provided options:
static inline void shared_init(shared_t *shared, size_t numThreads, size_t numElements) {
memset(shared, 0, sizeof(shared_t));
pthread_mutex_init(&shared->m, NULL);
pthread_cond_init(&shared->c, NULL);
shared->state = STATE_WAIT;
shared->numThreads = numThreads;
shared->numElements = numElements;
{
int it = 0;
shared->threads = (pthread_t*) calloc(shared->numThreads, sizeof(pthread_t));
while (it < shared->numThreads) {
if (pthread_create(&shared->threads[it], NULL, routine, shared) != 0) {
break;
}
it++;
}
}
}
When the worker threads are created by this routine, they are forced into a waiting state.
The first call to shared_populate in the loop awakens the first thread after setting the payload to some random numbers:
static inline void shared_populate(shared_t *shared) {
if (pthread_mutex_lock(&shared->m) != 0) {
return;
}
shared->payload = (int*) calloc(shared->numElements, sizeof(int));
{
int it = 0,
end = shared->numElements;
while (it < end) {
shared->payload[it] = rand();
it++;
}
}
shared->state = STATE_READY;
pthread_cond_signal(&shared->c);
pthread_mutex_unlock(&shared->m);
}
Note the use of pthread_cond_signal over pthread_cond_broadcast, because we only want to wake the first thread.
void* routine(void *arg) {
shared_t *shared = (shared_t*) arg;
int *payload;
do {
if (pthread_mutex_lock(&shared->m) != 0) {
break;
}
while (shared->state == STATE_WAIT) {
pthread_cond_wait(&shared->c, &shared->m);
}
payload = shared->payload;
shared->state = STATE_WAIT;
pthread_mutex_unlock(&shared->m);
if (payload) {
int it = 0,
end = shared->numElements;
while (it < end) {
printf("Thread #%ld got payload %p(%d)=%d\n",
pthread_self(), payload, it, payload[it]);
it++;
}
free(payload);
}
} while(1);
pthread_exit(NULL);
}
So we wake up in routine at the call to pthread_cond_wait, the state has changed, so we break out of the loop, we save the pointer to the payload, reset the state to WAIT, and release the mutex.
At this point main can repopulate the payload and awaken the next thread, meanwhile the current worker thread can process, and then free the payload.
Some advice:
Always use as few mutex and condition variables as possible (KISS)
Research the atomic nature of condition variables
Always follow the basic rules regarding acquisition and release of mutex and signaling of condition variables:
If you locked it, unlock it.
Only ever wait for something: predicated wait loops are absolutely required, all the time.
If you can't reproduce what I done, then take the code and try to expand upon it; The first thing you need to do is be able to shutdown the process gracefully (enter shared_cleanup), maybe you need a variable sized payload, or some other requirement not mentioned in the original question.
Note about printf ... appending to a stream is not guaranteed to be atomic, it so happens that most of the time on *nix it is ... since we are just doing show and tell, we don't need to care about that ... ordinarily, do not rely on atomicity for any stream operations ...

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