cpp dll char pointer assignment, writting memory access violation

Question

I have written a dll in cpp, successfully built but having some issues when trying set a value to string pointer.

My codes are as follow:

my sample usage of this dll in cpp

// ConsoleApplication1.cpp : Defines the entry point for the console application.
//
#include <iostream>
#include "OCRv1Dll.h" 

using namespace std;

int main()
{
    char *strIn = "abcd";
    char *strOu = "";
    int abc = autoOCR(strIn, strOu);
    return 0;
}

My dll's body

// ocrv1dll.cpp : defines the exported functions for the dll application.
//
//#ifdef _MSC_VER
//#define _CRT_SECURE_NO_WARNINGS
//#endif

#include "stdafx.h"


__int32 __stdcall autoOCR(char* strIn, char* strOut)
{
__int32     intRtn = 6; 
printf("Received string %s\n", strIn);
strOut += 17;
string temp = "abcd";
strcpy_s(strOut, 16, temp.c_str());
return intRtn;
}

Error has occured at

strcpy_s(strOut, 16, temp.c_str());

saying access violation memory location ...

Could you please enlighten me on this issue? thanks in advance!!

`strOut` doesn't point to any place you can write to. Nothing to do with DLLs. — juanchopanza, Jan 11 '15 at 08:42
I guess you think `strOut += 17` increases the size of the output buffer, however it doesn't. (There is no way to increase the size of string literals). — M.M, Jan 11 '15 at 09:51
possible duplicate of [difference between string pointer and string array](http://stackoverflow.com/questions/27669811/difference-between-string-pointer-and-string-array) — Ulrich Eckhardt, Jan 11 '15 at 10:03

score 2 · Accepted Answer · answered Jan 11 '15 at 09:50

char *strOu = ""; is a pointer to an empty string (char array of length 1).

When, in the function, you write strOut += 17; , that advances the pointer by 17 characters. Now the pointer is pointing into the wilderness . It's likely that this is in a read-only data area which is why the call to strcpy_s causes an access violation.

To fix this you need to only write to memory that has been correctly allocated. You will need to design a contract between this function and its caller; for example specify that the caller must pass a writable buffer of at least a particular size.

Lovely! Thanks for your clear explanation!! <3. It worked thanks a lot. It is meant to be called from C#. Already have working code in C# but wasn't sure how to implement it in C++ since the C# functions couldn't be found in C++ and hence the confusion (in C# IntPtr pBufferOut = Marshal.AllocHGlobal(iStringBufferSize) , autoOCR(pBufferIn, pBufferOut) ) — FailedMathematician, Jan 11 '15 at 10:19

score 0 · Answer 2 · edited May 23 '17 at 10:25

The problem strOu might point to read only section of your program, so if you try to write to it, it will generate a memory violation error (segfault in Unix systems).

You can read more about it in this question as well: String literals: Where do they go?

What you need to do is to pass as an argument a memory location that can be written into and that has enough space to store the string you want to generate.

Try changing the definition of strOut as follows:

int main()
{
    char *strIn = "abcd";
    char strOu[20]; // just enough to hold the string
    int abc = autoOCR(strIn, strOu);
    return 0;
}

cpp dll char pointer assignment, writting memory access violation

2 Answers2