Invalid allocation size when calling a function from dll

Question

I just started using dlls, but I haven't had this problem before, so it might not be dll connected. I am have KMP String-match algorithm implemented in c++ and I am calling it from c# using dll.

This is my export:

extern "C" __declspec (dllexport) const char* naive(const char* text, const   char* str);
extern "C" __declspec (dllexport) const char* KMP(const char* text, const char* str);

My import:

 [DllImport(@"dll_path", CallingConvention = CallingConvention.Cdecl)]
 public static extern IntPtr KMP([MarshalAs(UnmanagedType.LPStr)] string text, [MarshalAs(UnmanagedType.LPStr)] string str);

Calling from c#

  string output = Marshal.PtrToStringAnsi(KMP(richTextBox1.Text, richTextBox2.Text));

And the c++ function:

const char* KMP(const char* text, const char* str)
{
    int tL = strlen(text);
    int sL = strlen(str);
    /* Algorithm */
 }

The exception is thrown right after the function is called. So I figured it's not the code implementation. The wired thing is it's only thrown when there is a '\n' new line in the second parameter (str), no matter where exactly. If there are no new lines it runs normally. The thing that confuses me the most is why just the second argument, both are identically declared and used. I also have implemented Naive algorithm, same story.

All the answers I found were only when a negative number was given as size to an array or an undeclared variable, but nothing on pointers. But I doubt it's anything similar, because when my search string (2nd parameter (str)) doesn't contain new line the code executes normally.

Any ideas ?

Thank you in front.

EDIT (body of function):

const char* KMP(const char* text, const char* str)
{
    int tL = strlen(text);
    int sL = strlen(str);
    string match = "";

    if (sL == 0 || tL == 0)
        throw "both text and string must be larger than 0";
    else if (sL > tL)
        throw "the text must be longer than the string";

    int tI = 0;
    int col = 0, row = 0;

    while (tI <= tL - sL)
    {
        int i = 0;
        int tmpCol = -1;
        int next = 1;
        for (; i <= sL && text[i + tI] != '\0'; i++)
        {
            if (text[i + tI] == '\n')
            {
                row++;
                tmpCol++;
            }
            if (text[i + tI] == str[0] && next == 1 && i > 0)
                next = i;

            if (text[i + tI] != str[i])
                break;
        }
        if (i == sL)
        {
            match += to_string(row) + ',' + to_string(col) + ';';
        }

        tI += next;

        col = tmpCol > -1 ? tmpCol : col + next;
    }
    char* c = new char[match.length() - 1];
    c[match.length() - 1] = '\0';
    for (int i = 0; i < match.length() - 1; i++)
        c[i] = match[i];
    return c;
}

Answer 1

Just change your code to handle no matches case, because runtime cannot allocate 0-1 = 0xFFFFFFFFF bytes. And now I have also changed your copy buffer allocation and loop code to avoid overwrite(as pointed by @HenkHoltermann):

...
if (match.length() == 0)
    return "No matches";

// Allocate for all chars + \0 except the last semicolon
char* c = new char[match.length()];
c[match.length() - 1] = '\0';

// Copy all chars except the last semicolon
for (int i = 0; i < match.length() - 1; i++)
    c[i] = match[i];

return c;

!It still does not copy the last semicolon, so if you need it then you will have to add one more symbol to the buffer.

---

P.S.: Also I see a few issues with your code:

1. You use C++ exceptions. While CLR will catch them as SEH (because VC++ uses SEH) it is still not a good idea overall - Throwing C++ exceptions across DLL boundaries
2. You use signed int for length int tL = strlen(text); and strlen returns unsigned size_t. It may not be an actual problem, but it is not a right way either.