How to learn about functions

The best online source for c-functions are the man pages on a Unix system. man - S 3 printf will print a lot of information on your screen:

PRINTF(3) Linux Programmers Manual PRINTF(3) Name printf, fprintf, sprintf, snprintf, vprintf, vfprintf, vsprintf, vsnprintf, - formatted output conversion SYNOPSIS #include <stdio.h> int printf(const char *FORMAT [, ARG, ...]); int fprintf(FILE *FD, const char *FORMAT [, ARG, ...]); int sprintf(char *STR, const char *FORMAT [, ARG, ...]); int saprintf(char **STRP, const char *FORMAT [, ARG, ...]); int snprintf(char *STR, size_t SIZE, const char *FORMAT [, ARG, ...]); DESCRIPTION `printf' accepts a series of arguments, applies to each a format speci- fier from `*FORMAT', and writes the formatted data to `stdout', termi- nated with a null character. The behavior of `printf' is undefined if there are not enough arguments for the format. `printf' returns when .............. :