Is any direct function is available for the complement of the char variable
Is to complement your library with a 'pure' C text book (NOT "C for the '51"). Now for something tryly awful: you also have to read it.
Erik
"you also have to read it"
Or at least look at the index - and find where the Complement Operator is described!
just a wild guess
The OP is under the assumption that "C for the '51" (whatever the actual title) is a C book.