Question

what is the meaning of stdin as applied in c language

Answer 1

Stdin is the Standard Input buffer where all the user input is read in from (when you use scanf or other various functions)

Answer 2

STDIN is the standard input or default stream to your program, usually this refers to the keyboard (unless you redirect STDIN). The STDIN function is defined in STDIO.H header module, and the functions contained within this module are typically used with text. With STDIO functions you can use commands like GETS, SCANF, and GETC or GETCHAR. SCANF reads formatted input from STDIN, GETS reads a string from STDIN and GETC or GETCHAR read a character from STDIN.