in phnt/include/ntrtl.h [1282:1293]
FORCEINLINE VOID RtlInitAnsiString(
_Out_ PANSI_STRING DestinationString,
_In_opt_ PCSTR SourceString
)
{
if (SourceString)
DestinationString->MaximumLength = (DestinationString->Length = (USHORT)strlen(SourceString)) + sizeof(ANSI_NULL);
else
DestinationString->MaximumLength = DestinationString->Length = 0;
DestinationString->Buffer = (PCHAR)SourceString;
}