getting this error in pthread_create code? - c++

Question

I am creating my Operating system project in which this is my code,
I am using a Linux operating system and I when I am compiling my code, the pthread_create() function is showing an error. The error is related to void return type.
#include <pthread.h>
#include <semaphore.h>
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#
define LEFT(PhilNum + 4) % 5# define RIGHT(PhilNum + 1) % 5
enum s {
EATING,
HUNGRY,
THINKING
};
struct Philosopher {
char * name;
int id;
};
struct Philosopher P[5] = {
{
"Professor A",
0
},
{
"Professor B",
1
},
{
"Professor C",
2
},
{
"Professor D",
3
},
{
"Professor E",
4
}
};
int ProcessCurrantState[5];
sem_t MUTEX;
sem_t Chop[5];
void CheckAvailability(int PhilNum) {
if (ProcessCurrantState[LEFT] != EATING && ProcessCurrantState[RIGHT] != EATING) {
// ProcessCurrantState that eating
ProcessCurrantState[PhilNum] = EATING;
sleep(2);
printf("--------------------------------------------------------\n");
printf("| Philosopher %s ,id : %d | Picking up Chopsticks %d and %d .| \n", P[PhilNum].name,
PhilNum + 1, LEFT + 1, PhilNum + 1);
printf("--------------------------------------------------------\n");
printf("--------------------------------------------------------\n");
printf("Philosopher %s , id : %d | is Eating .\n", P[PhilNum].name, PhilNum + 1);
printf("--------------------------------------------------------\n");
sem_post( & Chop[PhilNum]);
}
}
// take up chopsticks
void PickUpChopSticks(int PhilNum) {
sem_wait( & MUTEX);
// ProcessCurrantState that hungry
ProcessCurrantState[PhilNum] = HUNGRY;
printf("--------------------------------------------------------\n");
printf(" | Philosopher %s , id : %d | is Hungry . |\n", P[PhilNum].name, PhilNum + 1);
printf("--------------------------------------------------------\n");
// eat if neighbours are not eating
CheckAvailability(PhilNum);
sem_post( & MUTEX);
// if unable to eat wait to be signalled
sem_wait( & Chop[PhilNum]);
sleep(1);
}
// put down chopsticks
void PutChopsticksDown(int PhilNum) {
sem_wait( & MUTEX);
// ProcessCurrantState that thinking
ProcessCurrantState[PhilNum] = THINKING;
printf("---------------------------------------------------------\n");
printf("| Philosopher %s , id : %d | puting down Chopsticks %d and %d .|\n", P[PhilNum].name,
PhilNum + 1, LEFT + 1, PhilNum + 1);
printf("--------------------------------------------------------\n");
printf("| Philosopher %s , id : %d | is thinking . | \n", P[PhilNum].name, PhilNum + 1);
printf("--------------------------------------------------------\n");
CheckAvailability(LEFT);
CheckAvailability(RIGHT);
sem_post( & MUTEX);
}
void * philospher(void * num) {
while (1) {
int i = (int * ) num;
sleep(1);
PickUpChopSticks(i);
sleep(0);
PutChopsticksDown(i);
}
}
int main() {
int i;
pthread_t Thread[5];
// initialize the semaphores
sem_init( & MUTEX, 0, 1);
for (i = 0; i < 5; i++) {
sem_init( & Chop[i], 0, 0);
}
// create philosopher processes
for (i = 0; i < 5; i++) {
pthread_create( & Thread[i], NULL, philospher, (void * ) P[i].id);
printf("--------------------------------------------------------\n");
printf("| Philosopher %s , id : %d | is thinking\n . |", P[i].name, i + 1);
printf("--------------------------------------------------------\n");
}
for (i = 0; i < 5; i++)
pthread_join(Thread[i], NULL);
}
and I am getting this error while running
main.cpp: In function ‘void* philospher(void*)’:
main.cpp:97:21: error: invalid conversion from ‘int*’ to ‘int’ [-fpermissive]
int i = (int *)num;

Answer 1

In pthread_create it casts int to void*. The reverse conversion you need is void* to int:
int i = (int)num;
In C++ code you may like to use std::thread instead of pthread_create, so that you can pass thread arguments without casting. E.g.:
void philospher(int num);
// ...
std::thread Thread[5];
// ...
Thread[i] = std::thread(philospher, P[i].id);
//
for(auto& t : Thread)
t.join();