Visual c++ operation of swprintf

我有一个例程，我称之为swprintf。 从我找到的所有文档中，它应该像sprintf一样工作，只与Unicode字符串一起工作。 但是，代码

swprintf(buffer, wcslen(buffer), L"COM %d", i );

删除由 i 确定的字母数。 如果 i = 0 缓冲区 = "COM0";如果 i = 1，缓冲区 = "COM";如果 i = 2，缓冲区 = "CO" - 依此类推。

有人有解释吗??? 完整代码如下所示

void SelectComPort() //added function to find the present serial 
{
wchar_t lpTargetPath[5000]; // buffer to store the path of the COMPORTS
wchar_t buffer[20];
DWORD test;
bool gotPort=0; // in case the port is not found
size_t  size = 0;
for(int i=0; i<255; i++) // checking ports from COM0 to COM255
{
    swprintf(buffer, wcslen(buffer), L"COM %d", i );
    test = QueryDosDevice(buffer, &lpTargetPath[size], 5000);
        // Test the return value and error if any
    if(test != 0) //QueryDosDevice returns zero if it didn't find an object
    {
        size = wcslen(lpTargetPath);
        gotPort=1; // found port
    }
    if(::GetLastError()==ERROR_INSUFFICIENT_BUFFER)
    {
        lpTargetPath[10000]; // in case the buffer got filled, increase size    of the buffer.
        continue;
    }
}
return;

}