Pthreads and Mutexes for RaceCondition Control - c

Question

I'm having an issue where my code is not acquiring the required mutex lock in order to manage Race Conditions. The code intentionally causes race conditions by using nanosleep to pause the threads and let them compete for the same data. My issue is that when adding the mutex lock to the code, not all of my threads are executing and thus giving a wrong value.
#include <stdio.h>
#include <unistd.h>
#include <pthread.h>
#include <time.h>
#include <sys/time.h>
#define NUM_THREADS 10
//Global Value
int SHARED_VALUE = 0 ;
pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;
void* add10(void*);
struct timespec ts = {0, 10};
int main() {
//Required for thread creation
pthread_t thread_id; //Thread Identifier
pthread_attr_t attributes; //Default Attributes
pthread_attr_init(&attributes); //Attributes Initialized to Default Values
//Initialize Mutex
if (pthread_mutex_init(&lock, NULL) != 0){
fprintf(stderr,"Error Creating Mutex\n");
return -100;
}
//Spawn 10 Threads
for(int i = 0; i < NUM_THREADS;i++){
thread_id = i+1;
if(pthread_create(&thread_id,&attributes,add10,thread_id) != 0) {
fprintf(stderr,"Error Creating Thread: %d", thread_id);
return -100;
}
}
for(int i = 0; i < NUM_THREADS; i++){
pthread_join (thread_id,NULL);
}
//pthread_mutex_destroy(&lock);
printf("Shared_Value: %d", SHARED_VALUE);
}
void* add10(void* arg) {
int thread_id = (int *) arg;
if ((pthread_mutex_lock(&lock)) == 0) {
fprintf(stderr, "Lock Acquired:%d\n", (int *) arg);
//Access Shared Value
int local_value = SHARED_VALUE;
fprintf(stderr, "\tInitial Local Value: %d\n", local_value);
local_value = local_value + 10;
fprintf(stderr, "\tPost Local Value: %d\n", local_value);
//Stall thread for race conditions
nanosleep(&ts, NULL);
SHARED_VALUE = local_value;
} else {
fprintf(stderr, "No Lock!\n");
pthread_exit;
}
if ((pthread_mutex_unlock(&lock)) < 0) {
fprintf(stderr, "Error on unlock\n");
}
}
Output Looks Like:
Lock Acquired:5
Initial Local Value: 0
Post Local Value: 10
Lock Acquired:6
Initial Local Value: 10
Post Local Value: 20
Lock Acquired:4
Initial Local Value: 20
Post Local Value: 30
Lock Acquired:7
Initial Local Value: 30
Post Local Value: 40
Lock Acquired:9
Initial Local Value: 40
Post Local Value: 50
Lock Acquired:10
Initial Local Value: 50
Post Local Value: 60
Shared_Value: 60Lock Acquired:8
Process finished with exit code 0

Answer 1

for(int i = 0; i < NUM_THREADS;i++){
thread_id = i+1;
if(pthread_create(&thread_id,&attributes,add10,thread_id) != 0)
You are using the same variable thread_id for all threads.You need an array here.
pthread_t thread_id[10];
for(int i = 0; i < NUM_THREADS;i++){
if(pthread_create(&thread_id[i],&attributes,add10,thread_id[i]) != 0)

Answer 2

Well, essentially you are waiting for only one thread to join and ignored remaining 9 threads
for(int i = 0; i < NUM_THREADS; i++){
pthread_join (thread_id,NULL);
}
because of this create statement
if(pthread_create(&thread_id,&attributes,add10,thread_id) != 0)