error: invalid conversion from 'int' to 'void*' [-fpermissive]

Question

Building a program for a lab I have to do that uses threads, I'm kind of lost with it, but I'm close to getting it compiled. I have 2 errors: one mentioned in the title, and the other is the same thing but it says invalid conversion from 'void*' to 'int'.

The errors occur on lines 98 and 124 within the producer and consumer threads, and I've marked them in the code. It's quite a bit of code, but I didn't know how to really shrink it down for this, sorry.

// syncA.cpp for lab2

#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <iostream>

using namespace std;

#define BSIZE 10
#define NUM_ITEMS 10
#define NUM_THREADS 2


int buf[BSIZE];
int nextin=0, nextout=0;


void * producer(void *);    // function for producer thread
void * consumer(void *);    // function for consumer thread
pthread_mutex_t lock;

pthread_t tid[NUM_THREADS];      // array of thread IDs


int main( int argc, char *argv[] ) 
{
    int i;

    cout << "Creating threads" << endl;

    pthread_create(&tid[1], NULL, consumer, (void *) buf[BSIZE]);
    pthread_create(&tid[0], NULL, producer, (void *) buf[BSIZE]);

    for (i = 0; i < NUM_THREADS; i++){
        pthread_join(tid[i], NULL);
    }

    cout << "All threads have been terminated" << endl << endl;

    // Finding minimum
    int minimum = buf[1];

    for (i = 1; i <= BSIZE; i ++){
        if (minimum > buf[i + 1])
            minimum = buf[i + 1];
    }

    // Finding maximum
    int maximum = buf[1];

    for (i = 1; i <= BSIZE; i++){
        if (maximum < buf[i + 1])
            maximum = buf[i + 1];
    }

    // Finding average
    int average;
    int sum = 0;

    for (i = 1; i <= BSIZE; i++){
        sum = sum + buf[i];
    }

    average = sum / BSIZE;

    // Outputting claculated data
    cout << "Minimum value: " << minimum << endl;
    cout << "Maximum value: " << maximum << endl;
    cout << "Average value: " << average << endl;

    return 0;

}  /* main */



void * producer(void * buf[])
{
    int product; // For multiplying inside the for loop for the "wait"
    int num;

    cout << "Producer started" << endl;

    // Locking thread
    pthread_mutex_lock(&lock);

    // Producing 10 items and putting them in the buffer
    for (int i = 0; i < BSIZE; i++){
        num = rand() % 1000;

        // Using a for loop 1000 times to act as the wait
        for (int k = 0; k < 1000; k++){
            product = 8 * 9;
        }

        // Putting the num in the buffer at pos 1, 2, 3, etc
        buf[nextin++] = num; <---------------- ***ERROR***
    }
    // Unlocking thread
    pthread_mutex_unlock(&lock);

    // Exiting the producer
    pthread_exit(0);
}    

void * consumer(void * buf[])
{
    int num;
    int product;

    cout << "Consumer started" << endl << endl;

    // Locking thread
    pthread_mutex_lock(&lock);

    // Waiting before accessing buffer
    for (int k = 0; k < 1000; k++){
        product = 8 * 9;
    }

    //consuming items
    for (int i = 0; i < BSIZE; i++){
        num = buf[nextout++]; <---------------- ***ERROR***
        cout << "Consuming item: " << num << endl;
            // TODO: Consume item
    }

    // Unlocking thread
    pthread_mutex_unlock(&lock);

    // Exiting consumer
    pthread_exit(0);
}

I've done some reading on other threads similar to this, but I can't find the answer I'm looking for. Any help is appreciated, and I'm very new to this type of programming.

boriaz50 boriaz50 · Accepted Answer · 2017-10-02T23:04:18

You declare functions as

void * producer(void *);    // function for producer thread
void * consumer(void *);    // function for consumer thread

But their definitions have another prototypes:

void * producer(void * buf[]);
void * consumer(void * buf[]);

In this case buf is array of pointers to void. And you are trying to assing number to pointer to void:

buf[nextin++] = num;

Function prototypes must be the same in declaration and definition.

It is clear that you want to write numbers to the buffer. But you cannot have array of voids. So cast buf to int *:

static_cast<int*>(buf)[nextin++] = num;

error: invalid conversion from 'int' to 'void*' [-fpermissive]

1 Answers