Access Violation exception before entering into a function - c++

Question

I have this function which simply encrypts a string (this function works fine, and tested).
DWORD SomeObj::Encrypt(string * To_Enc) {
DWORD text_len = (To_Enc->length());
if (!CryptEncrypt(this->hKey,
NULL, // hHash = no hash
1, // Final
0, // dwFlags
(PBYTE)(*To_Enc).c_str(), //*pbData
&text_len, //*pdwDataLen
128)) { //dwBufLen
return SERVER_ERROR;
}
return SERVER_SUCCESS;
}
And I have this piece of code:
string s= "stringTest";
Encrypt(&s);
which simply call the function passing the pointer of a string.
The program is causing an access violation exception right when it calls the function Encrypt(&s), I guess that it's something about the parameter &s being passed but I can't figure this out. Any idea from your experience ?

Answer 1

This answer will reiterate important points already made in the comments, with example code.
Your current code:
DWORD SomeObj::Encrypt(string * To_Enc) {
DWORD text_len = (To_Enc->length());
if (!CryptEncrypt(this->hKey,
NULL, // hHash = no hash
1, // Final
0, // dwFlags
(PBYTE)(*To_Enc).c_str(), //*pbData
&text_len, //*pdwDataLen
128)) { //dwBufLen
return SERVER_ERROR;
}
return SERVER_SUCCESS;
}
On the line:
(PBYTE)(*To_Enc).c_str(), //*pbData
Note that you are casting away const-ness from the pointer value returned from the c_str() method call.
This should immediately be a red flag; there may be times when casting away const-ness is a valid use case, but it is more the exception than the rule.
Untested, but using a temporary, mutable buffer should solve your problem, such as:
#include <cstddef>
#include <vector>
...
DWORD SomeObj::Encrypt(string * To_Enc) {
std::vector<std::string::value_type> vecBuffer(To_Enc->length() * 3, 0); // use the maximum value that could be output, possibly some multiple of the length of 'To_Enc'
std::size_t nIndex = 0;
for (auto it = To_Enc->cbegin(); it != To_End->cend(); ++it)
{
vecBuffer[nIndex++] = *it;
}
DWORD text_len = (To_Enc->length());
if (!CryptEncrypt(this->hKey,
NULL, // hHash = no hash
1, // Final
0, // dwFlags
reinterpret_cast<PBYTE>(&vecBuffer[0]), //*pbData
&text_len, //*pdwDataLen
vecBuffer.size())) { //dwBufLen
return SERVER_ERROR;
}
To_Enc->assign(&vecBuffer[0], text_len); // assumes 'text_len' is returned with the new length of the buffer
return SERVER_SUCCESS;
}