I'm a novice in C and trying to learn Multi Threading.
I played aroud with a C program counting to 100000 with 2 threads and outputing to text.
However, my program seem to have seg. fault.
I cannot figure it out.
Please Help :)
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#define COUNT_TO 100000
#define MAX_CORES 2
static pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
long long i = 0;
void *start_counting(FILE *out)
{
//lock
pthread_mutex_lock(&mutex);
while (i < COUNT_TO)
{
++i;
printf("i = %lld\n", i);
fprintf(out,"i = %lld\n", i);
}
//lock
pthread_mutex_unlock(&mutex);
}
int main(void)
{
int i = 0;
FILE *out;
out = fopen("output.txt","w");
// create a thread group the size of MAX_CORES
pthread_t *thread_group = malloc(sizeof(pthread_t) * MAX_CORES);
// start all threads to begin work
for (i = 0; i < MAX_CORES; ++i)
{
pthread_create(&thread_group[i], NULL, start_counting(out), NULL);
}
// wait for all threads to finish
for (i = 0; i < MAX_CORES; ++i)
{
pthread_join(thread_group[i], NULL);
}
fclose(out);
return EXIT_SUCCESS;
}
You have a error in this call:
pthread_create(&thread_group[i], NULL, start_counting(out), NULL);
It should be:
pthread_create(&thread_group[i], NULL, start_counting, out);
Please refer to the manual at https://man7.org/linux/man-pages/man3/pthread_create.3.html
Related
I have a question about this piece of code I have. It is the classic readers-writers problem. I followed the pseudo-code found on this wikipedia page for the first problem that has writers starving. I would like to know how I would actually notice the starvation of the writers going on.
I tried putting print statements of the shared_variable in various places, but this didn't give me much insight. But maybe I just didn't understand what was going on. Would someone be able to explain to me how I could visually see the starvation happening? Thank you!
The number of attempts that the reader or writer would attempt to read or write is given as a command line argument.
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <semaphore.h>
#include <pthread.h>
// Compile it like so: gcc assignment2.c -lpthread
// Shared variables (semaphore and integer)
static sem_t rw_mutex;
static sem_t mutex;
static int read_count = 0;
// Shared variable
int shared_variable = 0;
static void *writerAction(void *arg){
int number_attempt = *((int *) arg);
int attempt = 0;
do{
sem_wait(&rw_mutex);
shared_variable = shared_variable + 10;
sem_post(&rw_mutex);
attempt++;
}while(attempt < number_attempt);
}
static void *readerAction(void *arg){
int number_attempt = *((int *) arg);
int attempt = 0;
do{
sem_wait(&mutex);
read_count++;
// waiting to be able to read for the possible writer
if (read_count == 1 ){
sem_wait(&rw_mutex); // get the lock so that writter can't write!
}
// Release the read_count variable
sem_post(&mutex);
sem_wait(&mutex);
read_count--;
if (read_count == 0){
sem_post(&rw_mutex); // release the lock so that writter can write
}
sem_post(&mutex);
attempt++;
} while(attempt < number_attempt);
}
int main(int argc, char *argv[]) {
int number_writers = 10;
int number_readers = 500;
int reader_repeat_count = atoi(argv[2]);
int writer_repeat_count = atoi(argv[1]);
// Instantiating the threads for the writters and readers
pthread_t writer_threads[number_writers];
pthread_t reader_threads[number_readers];
// Initation of semaphores
sem_init(&rw_mutex, 0, 1);
sem_init(&mutex, 0, 1);
printf("Start creation of Readers\n");
for(int i = 0; i <number_readers; i++){
pthread_create(&reader_threads[i], NULL, readerAction, &reader_repeat_count);
}
printf("Start creation of Writers\n");
for(int i = 0; i < number_writers; i++){
pthread_create(&writer_threads[i], NULL, writerAction, &writer_repeat_count);
}
// All the actions is hapenning here
printf("Wait for Readers\n");
for(int i = 0; i < number_readers; i++){
printf("Waiting for : %d\n",i);
pthread_join(reader_threads[i], NULL);
}
printf("Wait for Writers\n");
// Collect all the writers
for(int i = 0; i < number_writers; i++){
printf("Waiting for : %d\n",i);
pthread_join(writer_threads[i], NULL);
}
// Results
printf("The shared variable is : %d\n",shared_variable);
}
First of all there is a syntax error, the return type of writerAction and readerAction should be void not void*
In order to see writer starvation you can print "writer trying to write" just before writer is trying to acquire the rw_mutex, the call to sem_wait(&rw_mutex). Add another print when the writer has updated the shared variable, inside the critical section. Add one more printf just after the the entry section of the reader. Which is this code.
// Release the read_count variable
sem_post(&mutex);
printf("reader reading shared value %d\n", shared_variable);
Now when you run the code with large repeat count, You will see "writer trying to write" then you will see a large number of reader printouts instead of the one from writer updating the shared variables, which will prove that the readers are starving the writer by not allowing it to update the variable.
I just started learning about thread today, and wanted to test the race condition of threads by running two codes with/without mutex.
#define HAVE_STRUCT_TIMESPEC
#include <pthread.h>
#include <stdio.h>
#include <windows.h>
#include <stdlib.h>
#define NTHREADS 3
#define ITERATIONS (long long) 1000000000
//pthread_mutex_t mutex;
static long long counter = 0;
static void * thread_f(void * arg) {
unsigned long long i;
(void)arg;
for (i = 0; i != ITERATIONS; i++) {
// pthread_mutex_lock(&mutex);
counter = counter + 1;
// pthread_mutex_unlock(&mutex);
}
return NULL;
}
int main(void) {
pthread_t threads[NTHREADS];
int i;
for (i = 0; i != NTHREADS; i++)
pthread_create(&threads[i], NULL, thread_f, NULL);
for (i = 0; i != NTHREADS; i++)
pthread_join(threads[i], NULL);
printf("expected = %lld, actual = %lld\n", NTHREADS*ITERATIONS, counter);
printf("experienced %lld race conditions\n", NTHREADS*ITERATIONS - counter);
system("pause");
return 0;
}
So, without mutex, the program prints out these following lines on cmd:
expected = 3000000000, actual = 1174158414
experienced 1825841586 race conditions
However, if I put mutex in the code, and run the program, cmd pops up then shuts down itself without showing any result.
I want to know if I coded anything wrong or is misusing mutex lines as I really don't know much about threads.
p.s this is coded in windows 10, using visual studio
Thanks to EOF from the comment, I found out that I did not initialize mutex in the code.
I simply added:
if (pthread_mutex_init(&mutex, NULL)) {
printf("Something went wrong\n");
return 1;
}
this in the main, and everything works fine now.
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;
}
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.
I'm currently being introduced to the concept of threading programs, and have been given an assignment to simulate a stockmarket using threads and semaphores. Here is the code:
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <sys/ipc.h>
#include <sys/sem.h>
#include <sys/shm.h>
#include <semaphore.h>
#include <pthread.h>
#define NUM_WRITERS 5
#define NUM_READERS 5
#define STOCKLIST_SIZE 10
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
sem_t stop_writers;
typedef struct
{ int readers;
int slots[STOCKLIST_SIZE];
} mem_structure;
int id[NUM_READERS + NUM_WRITERS];
mem_structure *stocklist;
pthread_t thr[NUM_WRITERS + NUM_READERS];
void init(){
sem_init(&stop_writers, 0, 1);
}
void cleanup(int signo) // clean up resources by pressing Ctrl-C
{ sem_destroy(&stop_writers);
printf("Closing...\n");
exit(0);
}
void write_stock(int n_writer)
{
int stock = (int)(rand()%STOCKLIST_SIZE);
int stock_value = 1 + (int)(100.0 * rand()/(RAND_MAX + 1.0));
stocklist->slots[stock] = stock_value;
fprintf(stderr, "Stock %d updated by BROKER %d to %d\n", stock, n_writer, stock_value);
}
void* writer(void* id){
int my_id = *((int*) id);
int i = my_id;
while (1)
{
pthread_mutex_lock(&mutex);
sem_wait(&stop_writers);
write_stock(i);
pthread_mutex_unlock(&mutex);
sem_post(&stop_writers);
sleep(1);
++i;
}
}
void read_stock(int pos, int n_reader){
fprintf(stderr, "Stock %d read by client %d = %d.\n", pos, n_reader, stocklist->slots[pos]);
}
void* reader(void* id){
int my_id = *((int*) id);
int i = my_id;
while (1)
{ sem_wait(&stop_writers);
read_stock((int)(rand()%STOCKLIST_SIZE), i);
sem_post(&stop_writers);
sleep(1 + (int) (3.0 * rand() / (RAND_MAX + 1.0)));
++i;
}
}
void monitor() // main process monitors the reception of Ctrl-C
{
struct sigaction act;
act.sa_handler = cleanup;
act.sa_flags = 0;
if ((sigemptyset(&act.sa_mask) == -1) ||
(sigaction(SIGINT, &act, NULL) == -1))
perror("Failed to set SIGINT to handle Ctrl-C");
while(1){
sleep(5);
printf("Still working...\n");
}
exit(0);
}
int main(void)
{ int i, j;
init();
for (i = 0; i < NUM_WRITERS; ++i){
id[i] = i;
pthread_create(&thr[i], NULL, writer, &id[i]);}
for (j = i; j < NUM_READERS; ++j){
id[j] = j;
pthread_create(&thr[j], NULL, reader, &id[j]);}
stocklist->readers = 0;
pthread_exit(NULL);
monitor();
return 0;
}
This is giving me a segmentation fault pretty much as soon as main() begins, with the creation of a thread... Since it's not code that I've created from root, I'm having trouble backtracing the error and fixing it - it would be great if there was someone who could take a look and maybe give me some hints
Thank you!
EDIT
Fixed the problem, thanks to your help :)
I initialized stocklist on init(),
void init(){
sem_init(&stop_writers, 0, 1);
stocklist = (mem_structure*)malloc(sizeof(mem_structure));
}
And changed the second cicle in main(),
for (j = i; j < NUM_READERS+NUM_WRITERS; ++j){
id[j] = j;
pthread_create(&thr[j], NULL, reader, &id[j]);}
Thanks
Here's what I did to track down the problem:
gcc -g program.c -lpthread
gdb a.out
run
Output:
Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0xb7feeb70 (LWP 23060)]
0x08048834 in write_stock (n_writer=0) at program.c:44
44 stocklist->slots[stock] = stock_value;
Looks like the problem is here:
stocklist->slots[stock] = stock_value;