Most important things POSIX 7 defines
C API
Greatly extends ANSI C with things like:
- more file operations:
mkdir, dirname, symlink, readlink, link (hardlinks), poll(), stat, sync, nftw()
- process and threads:
fork, execl, wait, pipe, semaphors sem_*, shared memory (shm_*), kill, scheduling parameters (nice, sched_*), sleep, mkfifo, setpgid()
- networking:
socket()
- memory management:
mmap, mlock, mprotect, madvise, brk()
- utilities: regular expressions (
reg*)
Those APIs also determine underlying system concepts on which they depend, e.g. fork requires a concept of a process.
Many Linux system calls exist to implement a specific POSIX C API function and make Linux compliant, e.g. sys_write, sys_read, ... Many of those syscalls also have Linux-specific extensions however.
Major Linux desktop implementation: glibc, which in many cases just provides a shallow wrapper to system calls.
CLI utilities
E.g.: cd, ls, echo, ...
Many utilities are direct shell front ends for a corresponding C API function, e.g. mkdir.
Major Linux desktop implementation: GNU Coreutils for the small ones, separate GNU projects for the big ones: sed, grep, awk, ... Some CLI utilities are implemented by Bash as built-ins.
Shell language
E.g., a=b; echo "$a"
Major Linux desktop implementation: GNU Bash.
Environment variables
E.g.: HOME, PATH.
PATH search semantics are specified, including how slashes prevent PATH search.
Program exit status
ANSI C says 0 or EXIT_SUCCESS for success, EXIT_FAILURE for failure, and leaves the rest implementation defined.
POSIX adds:
See also: What is the meaning of $? (dollar question mark) in shell scripts?
Regular expression
There are two types: BRE (Basic) and ERE (Extended). Basic is deprecated and only kept to not break APIs.
Those are implemented by C API functions, and used throughout CLI utilities, e.g. grep accepts BREs by default, and EREs with -E.
E.g.: echo 'a.1' | grep -E 'a.[[:digit:]]'
Major Linux implementation: glibc implements the functions under regex.h which programs like grep can use as backend.
Directory struture
E.g.: /dev/null, /tmp
The Linux FHS greatly extends POSIX.
Filenames
/ is the path separatorNUL cannot be used. is cwd, .. parent- portable filenames
- use at most max 14 chars and 256 for the full path
- can only contain:
a-zA-Z0-9._-
See also: https://stackoverflow.com/questions/18550253/what-is-posix-compliance-for-filesystem
Command line utility API conventions
Not mandatory, used by POSIX, but almost nowhere else, notably not in GNU. But true, it is too restrictive, e.g. single letter flags only (e.g. -a), no double hyphen long versions (e.g. --all).
A few widely used conventions:
- means stdin where a file is expected-- terminates flags, e.g. ls -- -l to list a directory named -l
See also: https://stackoverflow.com/questions/8957222/are-there-standards-for-linux-command-line-switches-and-arguments
"POSIX ACLs" (Access Control Lists), e.g. as used as backend for setfacl.
This was withdrawn but it was implemented in several OSes, including in Linux with setxattr.
Who conforms to POSIX?
Many systems follow POSIX closely, but few are actually certified by the Open Group which maintains the standard. Notable certified ones include:
Most Linux distros are very compliant, but not certified because they don't want to pay the compliance check. Inspur's K-UX and Huawei's EulerOS are two certified examples.
The official list of certified systems be found at: https://www.opengroup.org/openbrand/register/ and also at the wiki page.
Windows
Windows implemented POSIX on some of its professional distributions.
Since it was an optional feature, programmers could not rely on it for most end user applications.
Support was deprecated in Windows 8:
In 2016 a new official Linux-like API called "Windows Subsystem for Linux" was announced. It includes Linux system calls, ELF running, parts of the /proc filesystem, Bash, GCC, (TODO likely glibc?), apt-get and more: https://channel9.msdn.com/Events/Build/2016/P488 so I believe that it will allow Windows to run much, if not all, of POSIX. However, it is focused on developers / deployment instead of end users. In particular, there were no plans to allow access to the Windows GUI.
Historical overview of the official Microsoft POSIX compatibility: http://brianreiter.org/2010/08/24/the-sad-history-of-the-microsoft-posix-subsystem/
Cygwin is a well known GPL third-party project for that "provides substantial POSIX API functionality" for Windows, but requires that you "rebuild your application from source if you want it to run on Windows". MSYS2 is a related project that seems to add more functionality on top of Cygwin.
Android
Android has its own C library (Bionic) which does not fully support POSIX as of Android O: https://stackoverflow.com/questions/27604455/is-android-posix-compatible
Bonus level
The Linux Standard Base further extends POSIX.
Use the non-frames indexes, they are much more readable and searchable: http://pubs.opengroup.org/onlinepubs/9699919799/nfindex.html
Get a full zipped version of the HTML pages for grepping: https://stackoverflow.com/questions/453993/is-there-a-listing-of-the-posix-api-functions/45832939#45832939
