Adding an std::string definition causes Access Violation

Question

EDIT: Dear Future Readers, the std::string had nothing to do with the problem. It was an unterminated array.

In a nutshell, the problem is that adding a declaration of a single std::string to a program that otherwise contains only C causes the error "Access violation reading location 0xfffffffffffffffe."

In the code below, if the line where the std::string is declared is commented out, the program runs to completion without error. If the line however is left in the program (uncommented), the program crashes with the above stated Acess Violation error. When I open the running program in the VS2010 debugger, the Access Violation has occurred at the call to ldap_search_sA().

Notice that the declared std::string is never used. It doesn't have to be used for it to cause the access violation. Simply declaring it will cause the Access Violation.

My suspicion is it has nothing to do with the LDAP code, but I could be wrong.

int main() 
{
    try {
        // Uncommenting the next line causes an Access Violation 
        // at the call to ldap_search_sA().
        // std::string s;
        LDAP* pLdapConnection = ldap_initA("eu.scor.local", LDAP_PORT);
        ULONG version = LDAP_VERSION3;
        ldap_set_option(pLdapConnection, LDAP_OPT_PROTOCOL_VERSION, (void*) &version);         
        ldap_connect(pLdapConnection, NULL);
        ldap_bind_sA(pLdapConnection, NULL, NULL, LDAP_AUTH_NTLM);
        LDAPMessage* pSearchResult;
        PCHAR pMyAttributes[2];
        pMyAttributes[0] = "cn";
        pMyAttributes[1] = "description";
        ldap_search_sA(pLdapConnection, "dc=eu,dc=scor,dc=local", LDAP_SCOPE_SUBTREE,  "objectClass=computer)", pMyAttributes, 0, &pSearchResult);    
    } catch (...) {
        printf("exception\n");
    }
    return 0;
}

Answer 1

    PCHAR pMyAttributes[2];
    pMyAttributes[0] = "cn";
    pMyAttributes[1] = "description";

Attribute array should be NULL-terminated:

    PCHAR pMyAttributes[3];
    pMyAttributes[0] = "cn";
    pMyAttributes[1] = "description";
    pMyAttributes[2] = NULL;

Answer 2

I don't know what ldap_search_sA is, but the ldap_search function in OpenLDAP takes a pointer to a null pointer terminated array of char*. The array you are passing isn't correctly terminated, so anything may happen. I'd recommend using std::vector<char*> for this, in general, and wrapping the calls in a C++ function which systematically postfixes the terminator, so you don't forget. Although in such simple cases:

char* attributes[] = { "cn", "description", NULL };

will do the trick. It will probably provoke a warning; it really should be:

char const* attributes[] = { ... };

But the OpenLDAP interface is legacy C, which ignores const, so you'd need a const_cast at the call site. (Another argument for wrapping the function.)

Finally, I'd strongly advise that you drop the obfuscating typedefs like PCHAR; they just make the code less clear.

Answer 3

According to my experience, when weird things like this are observed in C++, what is in fact happening is that some piece of code somewhere corrupts memory, and this corruption may manifest itself in various odd ways, including the possibility that it may not manifest itself at all. These manifestations vary depending on where things are located in memory, so the introduction of a new variable probably causes things to be moved in memory just enough so as to cause a manifestation of the corruption where otherwise it would not be manifested. So, if I were in your shoes I would entirely forget about the string itself and I would concentrate on the rest of the code, trying to figure out exactly what you do in there which corrupts memory.

I notice that you invoke several functions without checking their return values, even though it is not in the spec of these functions to throw exceptions. So, if any of these functions fails, (starting with ldap_initA,) and you proceed assuming that it did not fail, you may get memory corruption. Have you checked this?