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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.

例子：

man 3 printf

PRINTF(3)                  Linux Programmer’s Manual                 PRINTF(3)

NAME
       printf,  fprintf, sprintf, snprintf, vprintf, vfprintf, vsprintf, vsnprintf -
       formatted output conversion

SYNOPSIS
       #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