Following scenario:
We are supposed to make x Threads maximum. Our main-function is supposed to make a single new thread with a pointer to the function 'makeThreads'. This function is supposed to make up to 2 threads, depending on how many threads are already there. Race conditions are to avoid.
I'm stuck. I'm not exactly sure how to solve the problem I'm running into, partly because I don't can't identify the problem itself.
Suggestions are greatly appreciated!
#include <stdio.h>
#include <unistd.h>
#include <pthread.h>
#include <time.h>
#define MAX_THR 20
pthread_mutex_t mutex;
int threadCount = 0;
int randomNbr(){
int number = (rand() % 10) + 1;
return number;
}
void *makeThreads(void* number){
int rndnmb = *((int *) number);
pthread_mutex_lock(&mutex);
sleep(rndnmb);
pthread_t threadDummy;
int thread1, i, threadID, rndnbr;
threadID = threadCount;
printf("Hello from Thread %d!\n", threadID);
for(i = 0; i < 2; i++){
if(threadCount < MAX_THR){
rndnbr = randomNbr();
int *rnd = &rndnbr;
threadCount++;
thread1 = pthread_create(&threadDummy, NULL, *makeThreads, (void *) rnd);
pthread_join(threadDummy, NULL);
}
}
pthread_mutex_unlock(&mutex);
printf("Goodbye from Thread %d!\n", threadID);
}
int main(){
int t1, rndnbr;
pthread_t threadOne;
pthread_mutex_init(&mutex, NULL);
srand(time(NULL));
rndnbr = randomNbr();
int *rnd = &rndnbr;
threadCount++;
t1 = pthread_create(&threadOne, NULL, *makeThreads, (void *) rnd);
pthread_join(threadOne, NULL);
}
Related
I am trying to have a multithread program that work with struct and semaphore.
I am not sure how to pass those values. Therefore I decided to pass my struct by reference and put my semaphore as a global variable.
What I am trying to do is to make two threads count to 20
(where one would be t1 = 0, 2 , 4 ,6 ... 20 and t2 = 1, 3 , ... , 19)
Here is what I have so far:
#include <fcntl.h>
#include <semaphore.h>
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <pthread.h>
#define SEM_sem;
#define MaxVal 20;
typedef struct {
int startVal;
int maxVal;
} countStruct;
sem_t *sem;
void *t1(void *param) {
countStruct *counting;
counting = (countStruct *) param;
int i = counting->startVal;
while(i <= counting->maxVal) {
printf("%d\n",i);
i = i + 2;
sem_post(sem);
sem_wait(sem);
}
pthread_exit(0);
}
void *t2(void *param) {
countStruct *counting;
counting = (countStruct *) param;
int i = counting->startVal;
while(i <= counting->maxVal) {
printf("%d\n",i);
i = i + 2;
sem_post(sem);
sem_wait(sem);
}
pthread_exit(0);
}
int main() {
sem = sem_open(SEM_sem, O_CREAT | O_EXCL, 0666, 0);
if (sem == SEM_FAILED) {
printf("sem_open() failed\n");
sem_unlink(SEM_mpoulinl);
return(8);
}
pthread_attr_t attr;
pthread_t tid1, tid2
countStruct array_struct[3];
int i = 0;
for(i ; i < 2 ; i++) {
array_struct[i].startVal = i;
array_struct[i].maxVal = MaxVal;
}
pthread_create(&tid1, NULL, t1, (void*)&array_struct[0]);
sleep(1);
pthread_create(&tid2, NULL, t=2, (void*)&array_struct[1]);
pthread_join(tid1, NULL);
pthread_join(tid2, NULL);
sem_close(sem);
sem_unlink(SEM_mpoulinl);
return(0);
}
Unfortunately my code print t1 then t2.
Why is it not working?
is it because my semaphore is global?
I tested taking out sem_post(sem) sem_wait(sem) and the code still work, therefore those two things does not seem to work or am I not using it appropriately?
Thank you!
Here is what I need to do:
Write a pthread program that takes an integer command line argument n,
spawns n threads that will each generate a random numbers between -100
and 100, and then computes and prints out the sum of these random
numbers. Each thread needs to print out the random number it
generates.
Here is what I have:
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <getopt.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <time.h>
int randomNum=0;
int randomSum=0;
void *randomNumberGenerator(void *id){
int *myid = (int *)id;
randomNum = rand()% 201 + (-100);
printf("%d\n", randomNum);
randomSum+=randomNum;
}
int main (int argc , char *argv[]){
int command;
char *strNumThreads = NULL;
int i;
while((command = getopt(argc, argv, "n:"))!=-1){
if(command == 'n'){
strNumThreads = optarg;
break;
}
}
int numThreads = atoi(strNumThreads);
pthread_t thread;
int newThread;
for(i = 0; i<numThreads; i++){
srand(time(NULL));
pthread_create(&thread, NULL, randomNumberGenerator, (void*)i);
}
pthread_exit(NULL);
printf("%d\n" , randomSum);
return 0;
}
For some reason randomSum is not getting printed.
randomNum is a variable that is shared among all threads, so you need a mutex
when you access the variable, because randomSum+=randomNum; is not an atomic
operation. The current process might get interrupted and another process is
scheduled which changes both variables. When the interrupted process resumes, it
will overwrite randomNum and you end up with garbage.
Also you have to wait for all threads to finish until you print the sum. For
that you have to execute pthread_wait.
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <getopt.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <time.h>
// can be a global variable
int randomSum=0;
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
void *randomNumberGenerator(void *id){
int randomNum=0; // does not need to be a global variable
randomNum = rand()% 201 + (-100);
printf("%d\n", randomNum);
pthread_mutex_lock(&mutex);
randomSum+=randomNum;
pthread_mutex_unlock(&mutex);
pthread_exit(0);
}
int main (int argc , char *argv[]){
int command;
char *strNumThreads = NULL;
int i;
while((command = getopt(argc, argv, "n:"))!=-1){
if(command == 'n'){
strNumThreads = optarg;
break;
}
}
// initializing the randomizer
srand(time(NULL));
int numThreads = atoi(strNumThreads);
if(numThreads == 0)
{
fprintf(stderr, "Invalid number of threads\n");
return 1;
}
pthread_t threads[numThreads];
for(i = 0; i<numThreads; i++){
pthread_create(threads + i, NULL, randomNumberGenerator, NULL);
}
for(i = 0; i < numThreads; ++i)
pthread_join(threads[i], NULL);
printf("%d\n" , randomSum);
return 0;
}
You really need to learn how to use the libraries you are using. pthread_exit
must be used by the threads to tell the system "I'm finished", calling it in the
main thread makes no sense.
pthread_create(&thread, NULL, randomNumberGenerator, (void*)i);
I consider this an uggly hack, what you should do is create an array with the
ids of the threads and pass every thread a pointer to its id, like this:
int ids[numThreads];
for(i = 0; i<numThreads; i++){
ids[i] = i;
pthread_create(&thread, NULL, randomNumberGenerator, ids+i);
}
and in the thread you can do
void *randomNumberGenerator(void *idp) {
int *id = idp;
printf("My thread id is %d\n", *id);
...
pthread_exit(NULL);
}
And if your worker threads are just calculating a value, you can use
pthread_exit to return that value back to the main thread. For example:
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <getopt.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <time.h>
struct thdata {
int id;
int random;
};
void *randomNumberGenerator(void *data) {
struct thdata *ret = data;
ret->random = rand()% 201 + (-100);
printf("thread with id %d: random %d\n", ret->id, ret->random);
pthread_exit(data);
}
int main (int argc , char *argv[]){
int i;
// initializing the randomizer
srand(time(NULL));
int numThreads = 5;
if(numThreads == 0)
{
fprintf(stderr, "Invalid number of threads\n");
return 1;
}
pthread_t threads[numThreads];
struct thdata data[numThreads];
for(i = 0; i<numThreads; i++){
data[i].id = i;
pthread_create(threads + i, NULL, randomNumberGenerator, data+i);
}
int randomSum = 0;
for(i = 0; i < numThreads; ++i)
{
struct thdata *data;
pthread_join(threads[i], (void**) &data);
randomSum += data->random;
}
printf("The sum of the random values is: %d\n" , randomSum);
return 0;
}
Which gives me the output (for 5 threads):
thread with id 0: random 72
thread with id 4: random -94
thread with id 1: random 1
thread with id 2: random -74
thread with id 3: random 42
The sum of the random values is: -53
You currently have a data race in place, because you have multiple threads accessing randomSum concurrently. Here's a solution, with comments, using Mutexes to solve the problem.
Note how using a struct to hold the sum and it's mutex allows us to get rid of all globals.
As a plus, I replaced your random generator with a proper one on POSIX systems. Note that your multiple calls to srand() are wrong, and cause less randomicity. You should only ever call srand() once, to generate the seed.
#include <stdio.h>
#include <stdbool.h>
#include <stdlib.h>
#include <limits.h>
#include <time.h>
#include <pthread.h>
static bool HAS_URANDOM = true; // Global
unsigned int random_uint() {
unsigned int r_uint;
// Try to open the random generator device
FILE *f = fopen("/dev/urandom", "r");
if (f == NULL) {
if (HAS_URANDOM) {
// Warn that urandom isn't working, but fallthrough to rand()
printf("---- Failed loading random generator device /dev/urandom. Defaulting to rand().\n");
srand((unsigned int) time(NULL));
HAS_URANDOM = false;
}
r_uint = (unsigned int) rand();
} else {
// If we have urandom, just read from it and cast to uint
fread(&r_uint, sizeof(r_uint), 1, f);
fclose(f);
}
return r_uint;
}
// Inclusive range
// https://stackoverflow.com/a/17554531/2080712
unsigned int generate_uint(unsigned int lower, unsigned int upper) {
if (upper - lower == UINT_MAX) {
fprintf(stderr, "Invalid bounds on generate_int().\n");
exit(EXIT_FAILURE);
}
unsigned int r_uint;
const unsigned int range = 1 + (upper - lower);
if (range == 0) {
fprintf(stderr, "Invalid range!\n---- upper=%d\n---- lower=%d\n---- range=%d\n", upper, lower, range);
exit(EXIT_FAILURE);
}
const unsigned int buckets = UINT_MAX / range;
const unsigned int limit = buckets * range;
/* Create equal size buckets all in a row, then fire randomly towards
* the buckets until you land in one of them. All buckets are equally
* likely. If you land off the end of the line of buckets, try again. */
do {
r_uint = random_uint();
} while (r_uint >= limit);
unsigned int res = lower + (r_uint / buckets);
return res;
}
typedef struct {
pthread_mutex_t lock; // Our lock to avoid data races
long sum; // The sum value
} sum_t;
// Thread function
void *do_sum(void *arg) {
sum_t *sum = (sum_t*)(arg); // Reinterpret the argument as sum_t
int val = generate_uint(0, 100) - 100; // Generate an integer in the range we want
pthread_mutex_lock(&sum->lock); // Lock the value
sum->sum += val; // Sum
pthread_mutex_unlock(&sum->lock); // Unlock the value
return NULL;
}
int main(int argc, char *argv[]) {
// Guarantee argument
if(argc != 2) {
printf("Please provide a number of threads.\n");
exit(EXIT_FAILURE);
}
// Get our thread count
long count = strtol(argv[1], NULL, 10);
// Allocate threads
pthread_t threads[count];
// Create & initialize sum structure
sum_t sum;
pthread_mutex_init(&(sum.lock), NULL);
sum.sum = 0;
// Run sum threads
for (long i = 0; i < count; ++i) {
pthread_create(&(threads[i]), NULL, do_sum, &sum);
}
// Wait until they have finished
for (long i = 0; i < count; ++i) {
pthread_join(threads[i], NULL);
}
// Destroy the mutex lock
pthread_mutex_destroy(&(sum.lock));
// Print result
printf("%ld\n", sum.sum);
return 0;
}
#include <stdio.h>
#include <pthread.h>
#include <unistd.h>
#include <stdlib.h>
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
void* thread_even(void* arg);
void* thread_odd(void* arg);
int main(int argc, char** argv) {
pthread_t tid[2];
pthread_create(&tid[0], 0, &thread_even, 0);
pthread_create(&tid[1], 0, &thread_odd, 0);
pthread_join(tid[0], NULL);
pthread_join(tid[1], NULL);
return 0;
}
void* thread_even(void* arg) {
int* thread_id = (int*)arg;
pthread_mutex_lock(&mutex);
for(int i = 1; i <= *thread_id; i++)
{
if(i%2 != 0)
{
printf("Thread 1: %d", i);
}
}
pthread_mutex_unlock(&mutex);
return NULL;
}
void* thread_odd(void* arg) {
int* thread_id = (int*)arg;
pthread_mutex_lock(&mutex);
for(int i = 1; i <= *thread_id; i++)
{
if(i%2 == 0)
{
printf("Thread 2: %d", i);
}
}
pthread_mutex_unlock(&mutex);
return NULL;
}
The above is the code I am working on but I get a segment fault error... What I want to achieve is, for example,
when I compile it and run with an argument 8 (./number 8)
it should print out
thread 1: 1
thread 2: 2
thread 1: 3
... etc till the number, 8.
in which thread 1s should represent the even numbers and the thread 2s stand for the odd numbers.
Please help... I want to develop my knowledge about C but have no one to ask..
Thanks.
Looks like you are passing 0 AKA NULL to the last parameter of pthread_create, and then doing the following:
int* thread_id = (int*)arg;
pthread_mutex_lock(&mutex);
for(int i = 1; i <= *thread_id; i++)
So, thread_id will certainly be NULL, referencing it will be a SEGFAULT.
If you want to pass an upper bound for each thread to run to, do you can do something like this:
int main(int argc, char** argv) {
pthread_t tid[2];
int *ids = malloc(2 * sizeof(int));
ids[0] = 10; /* upper bound for thread 1 */
ids[1] = 10; /* upper bound for thread 2 */
pthread_create(&tid[0], 0, &thread_even, &ids[0]);
pthread_create(&tid[1], 0, &thread_odd, &ids[1]);
pthread_join(tid[0], NULL);
pthread_join(tid[1], NULL);
free(ids);
return 0;
}
There are ways to do this without resorting to heap allocation, but this is the most straight forward.
My program's desired functionality:
Using the command line user inputs N and M. N is the number of new threads that will be created and M is the number of how much every thread increments the global variable A.
This is my code:
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <pthread.h>
static int A = 0;
void *Thread(void* x){
int i;
int n = *((int*)x);
for (i = 0; i<n; i++){
A++;
}
}
int main(int argc, char* argv[]){
int i;
int N = atoi(argv[1]);
int M = atoi(argv[2]);
pthread_t *thread = (pthread_t *) malloc(sizeof(pthread_t)*N);
if(!thread){
printf("No memory\n");
exit(2);
}
for (i = 0; i< N; i++){
if (pthread_create(&thread[i], NULL, Thread, &M)){
printf("Not able to create a thread\n");
exit(1);
}
}
for(i = 0; i< N; i++)
pthread_join(thread[i], NULL);
printf("A = %d\n", A);
return 0;
}
The problem is that every time I run it there's a different output.
Screenshot of my terminal when i run the program multiple times in a row
The problem is that you are creating multiple threads that in parallel are trying to modify your static A global variable at the same time, without any kind of protection.
That means that depending on the scheduling of the threads, the changes on your global variable will not be atomic, producing this effect.
You can solve this with a mutex, declare it with:
pthread_mutex_t mutex;
And initialise / release it with pthread_mutex_init and pthread_mutex_destroy.
Inside of the thread, before doing the change protect the resource to change with pthread_mutex_lock, and release it with pthread_mutex_unlock. So the code will be changed like this:
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <pthread.h>
static int A = 0;
pthread_mutex_t mutex;
void *Thread(void* x){
int i;
int n = *((int*)x);
pthread_mutex_lock(&mutex);
A += n;
pthread_mutex_unlock(&mutex);
}
int main(int argc, char* argv[]){
int i;
int N = atoi(argv[1]);
int M = atoi(argv[2]);
pthread_mutex_init(&mutex, NULL);
pthread_t *thread = (pthread_t *) malloc(sizeof(pthread_t)*N);
if(!thread){
printf("No memory\n");
exit(2);
}
for (i = 0; i< N; i++){
if (pthread_create(&thread[i], NULL, Thread, &M)){
printf("Not able to create a thread\n");
exit(1);
}
}
for(i = 0; i< N; i++)
pthread_join(thread[i], NULL);
printf("A = %d\n", A);
pthread_mutex_destroy(&mutex);
return 0;
}
Why this code give me different outputs every time?
Why it doesnt finish the loop?
What should I do to make it finish the loop? (despite context switches)?
Anything else I'm doing wrong?
Any help would be appreciated!
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <sys/types.h>
#include <unistd.h>
#include <string.h>
#define MAX 10
int buffer[MAX];
int fill = 0;
int use = 0;
int count = 0;
int loops = 15;
void put(int value) {
buffer[fill] = value;
fill = (fill + 1) % MAX;
count++;
printf("putting %d\n", value);
}
int get() {
int tmp = buffer[use];
use = (use + 1) % MAX;
count--;
return tmp;
}
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t c_empty = PTHREAD_COND_INITIALIZER;
pthread_cond_t c_fill = PTHREAD_COND_INITIALIZER;
void *producer(void *arg) {
printf("producer starts\n");
int i;
for (i = 0; i < loops; i++) {
pthread_mutex_lock(&mutex); // p1
while (count == MAX) // p2
pthread_cond_wait(&c_empty, &mutex); // p3
put(i); // p4
pthread_cond_signal(&c_fill); // p5
pthread_mutex_unlock(&mutex); // p6
}
return NULL;
}
void *consumer(void *arg) {
printf("consumer starts\n");
int i;
for (i = 0; i < loops; i++) {
pthread_mutex_lock(&mutex); // c1
while (count == 0) // c2
pthread_cond_wait(&c_fill, &mutex); // c3
int tmp = get(); // c4
pthread_cond_signal(&c_empty); // c5
pthread_mutex_unlock(&mutex); // c6
printf("consuming: %d\n", tmp);
}
return NULL;
}
int main(int argc, char *argv[]) {
printf("parent: begin\n");
pthread_t p, x;
pthread_create(&p, NULL, producer, NULL);
pthread_create(&x, NULL, consumer, NULL);
printf("parent: end\n");
return 0;
}
Makefile:
all: wcountb
wcountb: wcountb.c
gcc -g -Wall -o wcountb wcountb.c -lpthread
At the end of the main, you should call for pthread_join, like this:
...
pthread_join(p, NULL);
pthread_join(x, NULL);
return 0;
}
Without that call, the threads are created and when you reach the end of main(), then your program terminates, thus your threads may be able to finish their job, or not, which explains the fact that sometimes your code works.
The pthread_join() function shall suspend execution of the calling thread until the target thread terminates, unless the target thread has already terminated.
Taken from the manual of pthread_join().
An almost related question lies here.