Question

I have a program and in there, I dynamically allocate memory by using malloc; here is the important part:

unsigned int *hours;
unsigned int *minutes;
hours = (unsigned int *) malloc(n * sizeof(*hours));
minutes = (unsigned int *) malloc(n * sizeof(*minutes));

After this, I have a for loop, that loops n times; each time it reads time and stores hours at hours[i] and minutes at minutes[i]; for (i = 0; i < n; i++)

I read n before allocation using :

scanf(%d, &n);

Next, I have function, that takes 3 parameters, it looks like this:

void convertToMinutes(unsigned int *hours[],unsigned int *minutes[],int n)
{
     unsigned int i;
     for (i = 0; i < n; i++)
     {
         *minutes[i] =*hours[i]*60 + *minutes[i];
     }
}

This is part that causes program to crash. I’m using Dr. Memory to see what happens, here is dump:

Error #1: UNADDRESSABLE ACCESS: reading 0x00000005-0x00000009 4 byte(s)
# 0 convertToMinutes    
# 1 main                
Note: @0:00:11.408 in thread 1740
Note: instruction: mov    (%eax) -> %ecx

As you can see, it tries to read address at 0x00000005, which causes Windows to shut it down.

So before calling this function, I looked at addresses of hours and minutes arrays and here is what I found:

for n = 3
hours base 10948584
hours+1    10948588
hours+2    19848592
minutes base 10948608
minutes+1    10948612
minutes+2    10948616

(Addresses are converted to int)

I’m lost. I have no idea what is wrong. Pointers are new to me (I’ve been using them for few days) and I have no idea why it suddenly tries to read at forbidden address. What am I doing wrong?

Answer 1

This function signature takes arguments that are arrays of pointer to unsigned int

void convertToMinutes(unsigned int *hours[],unsigned int *minutes[],int n)

but your arrays are arrays of unsigned int not unsigned int *. Try changing it to

 void convertToMinutes(unsigned int hours[],unsigned int minutes[],int n)

or (since array arguments will decay to a pointer anyway) even

void convertToMinutes(unsigned int* hours,unsigned int* minutes,int n)

The inside the function remove the dereferencing and change to:

 for (i = 0; i < n; i++)
 {
     minutes[i] = hours[i]*60 + minutes[i];
 }