Error 234 "More data is available" with GetComputerNameEx - c

Question

I am getting More data is available error with the GetComputerNameEx function, but no idea how to fix it.
This is my code:
int wmain()
{
COMPUTER_NAME_FORMAT nameType = ComputerNameDnsFullyQualified;
WCHAR computerName[MAX_COMPUTERNAME_LENGTH + 1];
DWORD size = ARRAYSIZE(computerName);
BOOL pcName = GetComputerNameEx(nameType, computerName, &size);
DWORD error = GetLastError();
if (pcName != 0)
{
wprintf("Computer name: %s\n", computerName);
}
else
{
wprintf(L"Error getting the name. Code: %li\n", error);
}
return 0;
}
No idea how to set size variable as output so I can declare the computerName array correctly.

Answer 1

You have to call the function twice; once with a null pointer to get the required size, and again with a buffer of (at least) the specified size. As the docs say:
To ensure that this buffer is large enough, set this parameter to NULL
and use the required buffer size returned in the lpnSize parameter.
This is a common pattern for Win32 functions. And yes, it does lead to a possible race condition, but that's just how it works.
Example
DWORD dwSize = 0;
if (GetComputerNameEx(nameType, nullptr, &dwSize))
{
WCHAR* computerName;
computerName = (WCHAR*)malloc(dwSize * sizeof(WCHAR));
if (GetComputerNameEx(nameType, computerName, &dwSize))
{
// use the name
}
free(computerName); // don't forget to free
}

Answer 2

Per the GetComputerNameEx() documentation:
lpBuffer [out]
A pointer to a buffer that receives the computer name or the cluster virtual server name.
The length of the name may be greater than MAX_COMPUTERNAME_LENGTH characters because DNS allows longer names. To ensure that this buffer is large enough, set this parameter to NULL and use the required buffer size returned in the lpnSize parameter.
lpnSize [in, out]
On input, specifies the size of the buffer, in TCHARs. On output, receives the number of TCHARs copied to the destination buffer, not including the terminating null character.
If the buffer is too small, the function fails and GetLastError returns ERROR_MORE_DATA. This parameter receives the size of the buffer required, including the terminating null character.
If lpBuffer is NULL, this parameter must be zero.
For example:
int wmain()
{
COMPUTER_NAME_FORMAT nameType = ComputerNameDnsFullyQualified;
WCHAR *computerName = NULL, *computerNameNew;
DWORD size = 0;
BOOL pcName;
DWORD error;
do
{
pcName = GetComputerNameExW(nameType, computerName, &size);
if (pcName) break;
error = GetLastError();
if (error != ERROR_MORE_DATA) break;
computerNameNew = (WCHAR*) realloc(computerName, sizeof(WCHAR) * size);
if (!computerNameNew) {
error = ERROR_OUTOFMEMORY;
break;
}
computerName = computerNameNew;
}
while (1);
if (pcName)
{
wprintf("Computer name: %s\n", computerName);
}
else
{
wprintf(L"Error getting the name. Code: %ul\n", error);
}
free(computerName);
return 0;
}

Answer 3

That error means that the GetComputerNameEx function needs a larger buffer to store the returned string.
To avoid the race condition cited in Jonathan Potter's answer, you could do something like this:
LONG error = ERROR_MORE_DATA;
WCHAR* buffer = NULL;
DWORD bufferLength = /* Some initial reasonable length for the string buffer */;
while (error == ERROR_MORE_DATA) {
// Create a buffer with bufferLength size (measured in WCHARs)
buffer = realloc(buffer, bufferLength * sizeof(WCHAR));
if (GetComputerNameEx(nameType, buffer, &bufferLength)) {
error = ERROR_SUCCESS;
} else {
error = GetLastError();
}
}
if (error != ERROR_SUCCESS) {
// Some error occurred
...
}
// Use buffer containing computer name
// Don't forget to free(buffer)