Earlier major BSD releases included 3BSD (1980), 4BSD (1980), and 4.1BSD (1981).
The standard defines two levels of conformance: POSIX conformance, which is a baseline set of interfaces required of a conforming system; and XSI Conformance, which additionally mandates a set of interfaces (the "XSI extension") which are only optional for POSIX conformance. XSI-conformant systems can be branded UNIX 03. (XSI conformance constitutes the Single UNIX Specification version 3 (SUSv3).)
The POSIX.1-2001 document is broken into four parts:
XBD: Definitions, terms and concepts, header file specifications.
XSH: Specifications of functions (i.e., system calls and library functions in actual implementations).
XCU: Specifications of commands and utilities (i.e., the area formerly described by POSIX.2).
XRAT: Informative text on the other parts of the standard.
POSIX.1-2001 is aligned with C99, so that all of the library functions standardized in C99 are also standardized in POSIX.1-2001.
Two Technical Corrigenda (minor fixes and improvements) of the original 2001 standard have occurred: TC1 in 2003 (referred to as POSIX.1-2003), and TC2 in 2004 (referred to as POSIX.1-2004).
The changes in this revision are not as large as those that occurred for POSIX.1-2001/SUSv3, but a number of new interfaces are added and various details of existing specifications are modified. Many of the interfaces that were optional in POSIX.1-2001 become mandatory in the 2008 revision of the standard. A few interfaces that are present in POSIX.1-2001 are marked as obsolete in POSIX.1-2008, or removed from the standard altogether.
The revised standard is broken into the same four parts as POSIX.1-2001, and again there are two levels of conformance: the baseline POSIX Conformance, and XSI Conformance, which mandates an additional set of interfaces beyond those in the base specification.
In general, where the CONFORMING TO section of a manual page lists POSIX.1-2001, it can be assumed that the interface also conforms to POSIX.1-2008, unless otherwise noted.
Technical Corrigenda 1 (minor fixes and improvements) of this standard was released in 2013 (referred to as POSIX.1-2013).
Further information can be found on the Austin group web site,