Unexpected number output - c++

Question

I'm trying to create a program that solves the problem of dining philosophers using posix threads. However, I got stuck at the very beginning, since the output of std :: cout << id + 1 << "PHILOSOPHER: thinking" << std :: endl; ++ i; is incorrect and id takes too large values. Please point out my mistake.
pthread_mutex_t mutexSpoon[PHILOSOPHERS];
pthread_t createThread(int number){
pthread_t id;
int rc = pthread_create(&id, NULL, philFunc, &number);
if(rc){
abort();
}
return id;
}
void *philFunc(void *arg){
srand(time(0));
int id = *(int*)arg;
int leftSpoon = (id>0) ? id-1 : PHILOSOPHERS;
int rightSpoon = id;
int temp;
int i = 0;
while(i < 10){
usleep((200 - 50) * ( (double)rand() / RAND_MAX ) + 50);
std::cout << id+1 << " PHILOSOPHER: thinking" << std::endl; ++i;
}
return nullptr;
}
main.cpp
using namespace std;
extern pthread_mutex_t mutexSpoon[PHILOSOPHERS];
int main(){
setlocale(LC_ALL, "rus");
for(int i = 0; i < PHILOSOPHERS; ++i)
pthread_mutex_init(&mutexSpoon[i], NULL);
vector<pthread_t> vecID(PHILOSOPHERS);
for(int i = 0; i < PHILOSOPHERS; ++i)
vecID[i] = createThread(i);
for(int i = 0; i < PHILOSOPHERS; ++i)
pthread_join(vecID[i], NULL);
for(int i = 0; i < PHILOSOPHERS; ++i)
pthread_mutex_destroy(&mutexSpoon[i]);
return 0;
}

Answer 1

thread function uses an address for argument, which you pass as an address to a local variable of function createThread - number. The life span of argument should be not shorter than thread, so exactly same as the mutex. Using your snippets as base, I created an example which works around the issue:
#include <iostream>
#include <cstdlib>
#include <vector>
#include <pthread.h>
#include <unistd.h>
void *philFunc(void *arg);
#define PHILOSOPHERS 10
struct Philosopher {
pthread_mutex_t mutexSpoon;
pthread_t id;
int no;
};
Philosopher philosophers[PHILOSOPHERS] = {};
pthread_t createThread(int& number){
pthread_t id;
int rc = pthread_create(&id, NULL, philFunc, &number);
if(rc){
abort();
}
return id;
}
void *philFunc(void *arg){
srand(time(0));
int id = *(int*)arg;
int leftSpoon = (id>0) ? id-1 : PHILOSOPHERS;
int rightSpoon = id;
int temp;
int i = 0;
while(i < 10){
usleep((200 - 50) * ( (double)rand() / RAND_MAX ) + 50);
std::cout << id+1 << " PHILOSOPHER: thinking" << std::endl; ++i;
}
return nullptr;
}
extern pthread_mutex_t mutexSpoon[PHILOSOPHERS];
int main(){
setlocale(LC_ALL, "rus");
for(int i = 0; i < PHILOSOPHERS; ++i) {
pthread_mutex_init(&philosophers[i].mutexSpoon, NULL);
philosophers[i].no = i;
philosophers[i].id = createThread(philosophers[i].no);
}
for(int i = 0; i < PHILOSOPHERS; ++i)
pthread_join(philosophers[i].id, NULL);
for(int i = 0; i < PHILOSOPHERS; ++i)
pthread_mutex_destroy(&philosophers[i].mutexSpoon);
return 0;
}
As you see, there is now own structure Philosopher for each thread, storing its data as it should be. While philosophers here is an array, it can be any other container as long as its elements live long enough and aren't changing their addresses (requirement for some implementations of the pthread mutex).
Note that createThread(int& number) now takes its argument by reference, so the expression &number would get address of the actual object's location, not of local variable.
This code can be simpler, if using C++ thread support and std::future.