man man 

MANUAL SECTIONS       The standard sections of the manual include:       1      User Commands       2      System Calls       3      C Library Functions       4      Devices and Special Files       5      File Formats and Conventions       6      Games et. Al.       7      Miscellanea       8      System Administration tools and Deamons       Distributions customize the manual section to their  specifics,       which often include additional sections.

man 3 printf

PRINTF(3)                  Linux Programmer’s Manual                 PRINTF(3)NAME       printf,  fprintf, sprintf, snprintf, vprintf, vfprintf, vsprintf, vsnprintf -       formatted output conversionSYNOPSIS       #include <stdio.h>       int printf(const char *format, ...);       int fprintf(FILE *stream, const char *format, ...);       int sprintf(char *str, const char *format, ...);       int snprintf(char *str, size_t size, const char *format, ...);       #include <stdarg.h>       int vprintf(const char *format, va_list ap);       int vfprintf(FILE *stream, const char *format, va_list ap);       int vsprintf(char *str, const char *format, va_list ap);       int vsnprintf(char *str, size_t size, const char *format, va_list ap);   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):       snprintf(), vsnprintf(): _BSD_SOURCE || _XOPEN_SOURCE >= 500 ||       _ISOC99_SOURCE; or cc -std=c99