Read access violation when attempting strcpy from file buffer to char array

Question

I've been working on an assignment to implement hashing. In it, I read through a text file called "proteins". The problem occurs when I try to copy it to another char array. Visual Studio throws a read access violation.

#include <iostream>
#include <fstream>
using namespace std;
struct arrayelement {
  char protein[30];
  int count;
}; 
arrayelement proteins[40];
int main()
{
  char buffer[30];

  // open source file
  ifstream fin("proteins.txt");
  if (!fin) { cerr << "Input file could not be opened\n"; exit(1); }

  // loop through strings in file
  while (fin >> buffer) {
    int index = ((buffer[0] - 65) + (2 * (buffer[strlen(buffer)-1] - 65)) % 40);
    while (true)
    {
        if (proteins[index].protein == buffer)  // Found
        {
            proteins[index].count++;
            break;
        }
        if (proteins[index].protein[0] == 0)    // Empty
        {
            strcpy(proteins[index].protein, buffer); // <-- The error in question
            proteins[index].count++;
            break;
        }
        index++;                                // Collision
     }
  }

  // close file
  fin.close();


  for (int i = 0; i <= 40; i++)
  {
    cout << proteins[i].protein << "\t" << proteins[i].count << "\n";
  }
}

Did you try debug this? index may be at 40 when the error occurs. — Matthieu Brucher, Nov 13 '18 at 13:40
`proteins[index].protein == buffer` This is not how you want to compare char arrays. — 0x5453, Nov 13 '18 at 13:44
@0x5453 Oh man, I'm too tired. That did it. After changing that to a strcmp it all runs. Thank you! — Noctimor, Nov 13 '18 at 13:50
@Noctimor Because you are trying to do C when you are in C++. Use containers.... — Matthieu Brucher, Nov 13 '18 at 13:51

score 2 · Accepted Answer · answered Nov 13 '18 at 13:47

2

If you get more than 30 chars here:

while (fin >> buffer) {

... or if index >= 40 here:

strcpy(proteins[index].protein, buffer);

... the program will probably crash (Undefined behavior). Also, these char*'s will not be pointing at the same address, so the comparison will fail:

proteins[index].protein == buffer

answered Nov 13 '18 at 13:47

Ted Lyngmo

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Yep, it was the comparison that did me in. After rewriting it, everything runs. Part of the code here is given per the assignment -- the examples we're working with are guaranteed not to be above 30, and the index given is double what's needed, to practice collision resolution. Otherwise I'd absolutely clean that stuff up too. Thanks! – Noctimor Nov 13 '18 at 13:53
Great! Note: The index formula will result in 91 if the input string is `"z"`. It'll become: `index = 57 + 114 % 40` => `57 + (114 % 40)` – Ted Lyngmo Nov 13 '18 at 14:10

Read access violation when attempting strcpy from file buffer to char array

1 Answers1