9
man 2 syscalls
Aside from that, you can look in /usr/include/sys/syscall.h (which on my system merely #includes /usr/include/bits/syscall.h). That's generated at libc build time from kernel syscall list.
You can also grep the Linux kernel source for SYSCALL_DEFINE. (I'm not a BSD expert, but I think the equivalent in FreeBSD is SYSCALL_MODULE)
For the official, authoritative IEEE Std. 1003.1 / Single UNIX Specification (UNIX 2004) manpages, see:
A full list of functions (system interfaces) may be found under "System Interfaces" or at the link. I would also like to use this as an opportunity to plug my Development / Coding Search custom search engine, which includes and is heavily biased towards the Single UNIX Specification / IEEE Std. 1003.1. For example, a standard search for fopen, close, unix, etc. have promotions taken out to ensure that results from the authoritative documentation are at the very top. Adding "man" in front of a query heavily weights the result in favor of the IEEE Std. 1003.1 man page as in man find, man free, answered 2010-04-21T05:35:56.880