Sb-posix is the supported interface for calling out to the operating system.1
The scope of this interface is “operating system calls on a typical
Unixlike platform”. This is section 2 of the Unix manual, plus section
3 calls that are (a) typically found in libc, but (b) not part of the C
standard. For example, we intend to provide support for
readdir(), but not for
That said, if your favourite system call is not included yet, you are
encouraged to submit a patch to the SBCL mailing list.
Some facilities are omitted where they offer absolutely no additional use over some portable function, or would be actively dangerous to the consistency of Lisp. Not all functions are available on all platforms.
 The functionality contained in the package
SB-UNIX is for SBCL internal use only; its contents are likely to
change from version to version.