what does crypt(3) and whatnot mean

Question

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What do the parentheses and number after a Linux command or C function mean?

In Linux Manpages (http://linux.die.net/man/3/crypt) I often see sth like this: crypt(3), bla(4), bla(5)

What is the full meaning of these expressions?

Answer 1

The numbers signify what section the page belongs to, there are 8:

1 - General commands
2 - System calls
3 - C library functions
4 - Special files (usually devices, those found in /dev) and drivers
5 - File formats and conventions
6 - Games and screensavers
7 - Miscellanea
8  - System administration commands and daemons

for example, if you wanted to know how to use the crontab command, you could type man crontab. If you wanted to know the layout of the crontab file, you'd use man 5 crontab.

Not all items have all 8 pages. If the specific item is not a command, it will automatically direct you to the appropriate page as there was to entry in section 1 for the command. An example of this is man strcpy which will bring you to the C library function strcpy.

Answer 2

You can do a "man man" to have the manual of the manual. I'll show you the meaning of these sections.

Answer 3

The reason for the sections john describes is that the same word eg 'free' might be a user command or a C library function.

so "man free" or "man 1 free" would give you the page from section 1, if no number is given it gives you the lowest.

while "man 3 free" would give you the C library function to free memory.