Howto read file to char* buff?

Hi, I want to read file conten to a char buff, char buff[MAX_LENGTH]. But I can not use char type in UE4, then which type should I use? TChar? Char?

FString Data;

if (FFileHelper::LoadFileToString(Data, *Filename))
{
//....
}

peek around FFileHelper there are additional ways to read a file.

Thank you。