@Ben has given a good account of errors that you made. I'd like to show you how you would typically call GetComputerName
. You are making it harder than it needs to be. The key information is the excerpt from the documentation:
The buffer size should be large enough to contain MAX_COMPUTERNAME_LENGTH + 1 characters.
You have a hard upper bound on the size of the buffer. You can therefore use a fixed size buffer, and only make a single call to GetComputerName
.
std::string getComputerName()
{
char buffer[MAX_COMPUTERNAME_LENGTH + 1];
DWORD len = MAX_COMPUTERNAME_LENGTH + 1;
if (GetComputerName(buffer, &len))
return std::string(buffer, len);
return "UNKNOWN";
}
Were you compiling for Unicode it would be:
std::wstring getComputerName()
{
wchar_t buffer[MAX_COMPUTERNAME_LENGTH + 1];
DWORD len = MAX_COMPUTERNAME_LENGTH + 1;
if (GetComputerName(buffer, &len))
return std::wstring(buffer, len);
return L"UNKNOWN";
}
If you want to cater for the possibility of the computer name being longer than MAX_COMPUTERNAME_LENGTH
then you can write it like this:
std::string getComputerName()
{
char buffer[MAX_COMPUTERNAME_LENGTH + 1];
DWORD len = MAX_COMPUTERNAME_LENGTH + 1;
if (GetComputerName(buffer, &len))
{
return std::string(buffer, len);
}
if (GetLastError() == ERROR_BUFFER_OVERFLOW)
{
std::vector<char> name(len);
if (GetComputerName(&name[0], &len))
{
return std::string(&name[0], len);
}
}
return "UNKNOWN";
}
I don't know whether or not this can even happen. The docs hint that it can, although if it does happen then it renders MAX_COMPUTERNAME_LENGTH
somewhat of a misnomer. If we pass a value of len
that is less than MAX_COMPUTERNAME_LENGTH+1
then the function succeeds if the name fits. It does not automatically fail with ERROR_BUFFER_OVERFLOW
. Of course, if the name returned by this function can never exceed MAX_COMPUTERNAME_LENGTH
then the second version is rather paranoid.
FWIW, the code in your updated answer is still badly broken. You simply must not pass NULL
for the first parameter to GetComputerName
. The documentation could not be much clearer.