usa la funzione atoi() per convertire una stringa in intero
codice:
#include <stdio>
#include <stdlib>
#include <string.h>
...

int Esegui_Operazioni(char *str )
{
    int result = 0;
    int right  = 0;
    int left   = atoi( str );
    
    char *op = strpbrk( str, "+-*/" );
    char operator = *op;
    
    if( op == NULL )
        return NULL;
    
    right = atoi( op+1);
    printf("str  = \"%s\"\n", str );
    printf("eval = %d %c %d\n", left, operator, right );
    
    switch( operator )
    {
        case '+':
            result = left + right;
        break;
        case '-':
            result = left - right;
        break;
        case '/':
            result = left / right;
        break;
        case '*':
            result = left * right;
        break;
    }
    return result;
}