Predefined function belongs to standard library functions in C programming language. Some predefined functions listed below, I'm sure these functions you definitely used in C programming like:
printf(); scanf(); puts(); gets();
Example of C Program
#include <stdio.h>
int main() // main function of this program//
{
printf("Hello, This is predefind function program example");
}
printf() is not used without including the stdio.h header file, the program gives error, so add #include <stdio.h> in header of program.