posix.md

POSIX

An operating system standardization by both IEEE and the open group (merger of the Open software foundation with X/Open)

For a cheat on the POSIX C API and all the concepts necessary to understand it see: https://github.com/cirosantilli/cpp/tree/99ad8ab9aee7bf2d8c47c9e25f4631eddb556ccf/posix

POSIX command line utilities shall not be discussed here because it is better to group utilities together with other non-POSIX utilities which have similar functions, for example discussing both kill (POSIX) and killall (non-POSIX) side by side.

POSIX means: Portable Operating System Interface for uniX.

POSIX is also known as: Single Unix specification (SUS)

http://www.opengroup.org/openbrand/register/

Most GNU/Linux systems are largely POSIX compliant but not certified, Windows is largely not compliant (feature request :-)), Mac OS X Yosemite is certified. The list of all certified products is found at: http://www.opengroup.org/openbrand/register/

The specification if free to view.

Has several versions.

POSIX issue 7: IEEE formal name: IEEE Std 1003.1-2008 highly recommended link: http://pubs.opengroup.org/onlinepubs/9699919799/

Single UNIX Specification, Version 4: http://www.unix.org/version4/

Unix

https://en.wikipedia.org/wiki/Unix#History

Based on https://en.wikipedia.org/wiki/Multics from Bell Labs.

The Open Group

Major open group supporters whose major supporters include:

• Fujitsu
• Oracle
• Hitachi
• HP
• IBM,
• US Department of Defense
• NASA

Therefore some of the top users/creators of software.

Examples of what POSIX specifies

Shell command language

A shell language including almost all the basic bash syntax.

bash is compliant with extensions

Utilities

Utilities that should be available to the shell (either as programs in path or shell builtins, this is not specified).

Examples:

• cd
• ls
• cat
• mkdir
• c99 and fortr77: compiler interfaces for C99 and Fortran 77!

and tons of others which most people never heard of

System interfaces

Standard C interfaces to the system.

They allow for operations such as:

• threads
• IPC
• filesystem operations
• user/group info
• maths: cos, sin()
• path operations: basename(), dirname()

It does not however specify the exact system calls, and those are then implemented using a given OS system calls.

However many of the Linux system calls resemble those closely because of the compliance, so when you learn the POSIX interface you are already learning the Linux interface too.

In most Linux, the POSIX C interface is implemented as part of glibc, which also implements ANSI libc and Linux extensions.

Directory structure

http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap10.html#tag_10

Specifies very few files and directories:

• /
• /tmp
• /dev/null
• /dev/tty
• /dev/console

Extended notably by the FHS.

Environment variables

http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html#tag_08

POSIX specifies:

• a few environment variables and their functions

• many environment variables which it is unwise (quote) to override because they are frequently used in implementations, but does not specify their exact function.

The following variables have fixed purposes in POSIX 7:

• PATH
• HOME
• PWD
• TMPDIR
• COLUMNS and LINES: current width of terminal
• SHELL: this is not the current shell! It contains the path of the default shell.
• TERM: analogous to shell

The following variables determine the locale applications should use:

• LC_CTYPE: character classification and case conversion
• LC_COLLATE: collation order
• LC_MONETARY: monetary formatting
• LC_NUMERIC: numeric, non-monetary formatting
• LC_TIME: date and time formats
• LC_MESSAGES: formats of informative and diagnostic messages and interactive responses

They are defined at: http://pubs.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap07.html POSIX also offers interfaces to modify those variables. The exact location where they are implemented is not specified. E.g., on Ubuntu 14.04, they are stored under /etc/default/locale.

The following variables don't have fixed purposes, but POSIX says that they must be used with caution:

• EDITOR: default text editor
• PAGER: default pager program ( more, less, etc. )
• PPID: PID of parent process

Exit status

http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_08

Extends ANSI C (which specifies 0 for success):

• 126: command found but not executable.

• 127: command not found. Try:

  asdfqwer
  echo $?
  

• Values greater than 128 indicate that a process was terminated by a signal.

  Linux goes further and specifies that when terminated by a signal, the exit status shall be 128 + the normal error status.

Command line interface

http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap12.html

Suggests the following format for documenting CLIs:

utility_name[-a][-b][-c option_argument][-d|-e][-f[option_argument]][operand...]

A few of the most interesting suggests for argument and utility name syntax:

• utility names are lowercase alphanumeric.

• options start with one hyphen -

• options contain a single char only. Do not allow multi character options.

  One notable extension is the GNU convention of double hyphen for multi character options --optionname.

  Or even worse by some programs in the wild which use -option...

  Good discussion about command line interface conventions: http://stackoverflow.com/questions/10818443/short-long-options-with-option-argument-is-this-some-sort-of-convention

• -- marks the last of the options

• - means stdin/stdout when the utility expects a filename as argument

• -W is reserved for vendor options

Filenames

http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap04.html#tag_04_12

POSIX filenames are extremely restrictive: more than any modern operating system:

• only a-zA-Z0-9._- characters allowed
• maximum filename is 14 bits. 255 on Linux.

It also has rules which are enforced on all modern operating systems:

• reserved characters: slash / and null

TODO

• understand difference between the two: http://unix.stackexchange.com/questions/14368/difference-between-posix-single-unix-specification-and-open-group-base-specifi