- LONG ret = 0;
 - HKEY hkey;
 - DWORD size = 0;
 - BOOL usedefault = TRUE;
 
/* snip */
- clear_errors();
 - if(!cbRetBuffer)
 return 0;- if(RetBuffer)
 RetBuffer[0] = 0;- if(!lpszSection || !lpszEntry || !lpszDefault || !RetBuffer || !cbRetBuffer)
 return ret;
You are checking "if (!cbRetBuffer)" twice. Same in the next function.
You don't need to clear RetBuffer. RegGetValue adds a null terminating character if it doesn't already exist.
You also "return 0" and "return ret". They are the same at the moment, so be consistent.
+size = cbRetBuffer; +if(RegGetValueW(hkeysect, NULL, lpszEntry, RRF_RT_REG_SZ, &type, RetBuffer, &size) == ERROR_SUCCESS)
cbRetBuffer is the size in characters of the buffer pointed to by RetBuffer.[1] size needs to be in bytes, so unless RetBuffer uses char, this is wrong.
- ret = SQLGetPrivateProfileStringW(sect, entry, string, retval, cbRetBuffer, file);
 - if(ret)
 - {
 WideCharToMultiByte(CP_ACP, 0, retval, -1, RetBuffer, ret+1, NULL, NULL);- }
 
This doesn't look right to me. "ret+1" should be the size, in bytes, of RetBuffer. [2]
[1] https://msdn.microsoft.com/en-us/library/windows/desktop/ms724868%28v=vs.85%... [2] https://msdn.microsoft.com/en-us/library/windows/desktop/dd374130%28v=vs.85%...