在C中将整个文件转换为小写的最佳方式

I was wondering if theres a realy good (performant) solution how to Convert a whole file to lower Case in C. I use fgetc convert the char to lower case and write it in another temp-file with fputc. At the end i remove the original and rename the tempfile to the old originals name. But i think there must be a better Solution for it. I was wondering if theres a realy good (perform