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What do the numbers in a man page mean?

All system calls described in manpages have an associated number such as exec(3). What is the meaning of this number?

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Just an FYI, true system calls would have a "2": "read(2)" for example. Library calls have a "3", so "exec(3)" means a library call, one that ends up calling "execve(2)", the actual system call. – Bruce Ediger Nov 7 '12 at 18:26
@BruceEdiger after reading the answer I came to the same conclusion. :) – borges Nov 7 '12 at 18:39

marked as duplicate by Gilles, Renan, jasonwryan, Mat, Caleb Nov 8 '12 at 9:48

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The number is the man section. The 3 is for library calls. Here is the full list from man(1):

   1   Executable programs or shell commands
   2   System calls (functions provided by the kernel)
   3   Library calls (functions within program libraries)
   4   Special files (usually found in /dev)
   5   File formats and conventions eg /etc/passwd
   6   Games
   7   Miscellaneous  (including  macro  packages and conven‐
       tions), e.g. man(7), groff(7)
   8   System administration commands (usually only for root)
   9   Kernel routines [Non standard]

Some things have the same name in different sections. For example, man(1) is for the man binary, while man(7) explains how the macros should be written for manpage formatting. To access a specific section, you can put the numerical argument before the item:

man 7 man
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