
[;1m  open_port(PortName, PortSettings)[0m

  Returns a port identifier as the result of opening a new Erlang
  port. A port can be seen as an external Erlang process.

  The name of the executable as well as the arguments specified in [;;4m[0m
  [;;4mcd[0m, [;;4menv[0m, [;;4margs[0m, and [;;4marg0[0m are subject to Unicode filename
  translation if the system is running in Unicode filename mode. To
  avoid translation or to force, for example UTF-8, supply the
  executable and/or arguments as a binary in the correct encoding.
  For details, see the module [;;4mfile[0m, the function [;;4m[0m
  [;;4mfile:native_name_encoding/0[0m in Kernel, and the [;;4mUsing Unicode in[0m
  [;;4mErlang[0m User's Guide.

  [;;4mNote[0m

    The characters in the name (if specified as a list) can only
    be > 255 if the Erlang virtual machine is started in Unicode
    filename translation mode. Otherwise the name of the
    executable is limited to the ISO Latin-1 character set.

  [;;4mPortName[0ms:

   • [;;4m{spawn, Command}[0m - Starts an external program. [;;4mCommand[0m is
     the name of the external program to be run. [;;4mCommand[0m runs
     outside the Erlang work space unless an Erlang driver with
     the name [;;4mCommand[0m is found. If found, that driver is
     started. A driver runs in the Erlang work space, which means
     that it is linked with the Erlang runtime system.

     For external programs, [;;4mPATH[0m is searched (or an equivalent
     method is used to find programs, depending on the OS). This
     is done by invoking the shell on certain platforms. The
     first space-separated token of the command is considered as
     the name of the executable (or driver). This (among other
     things) makes this option unsuitable for running programs
     with spaces in filenames or directory names. If spaces in
     executable filenames are desired, use [;;4m{spawn_executable,[0m
     [;;4mCommand}[0m instead.

  [;;4mWarning[0m

       On Unix systems, arguments are passed to a new operating
       system process as an array of strings but on Windows it
       is up to the child process to parse them and some
       Windows programs may apply their own rules, which are
       inconsistent with the standard C runtime [;;4margv[0m parsing.
       This is particularly troublesome when invoking [;;4m.bat[0m, [;;4m[0m
       [;;4m.cmd[0m, or [;;4m.com[0m files as these run implicitly through [;;4m[0m
       [;;4mcmd.exe[0m, whose argument parsing is vulnerable to
       malicious input and can be used to run arbitrary shell
       commands. Therefore, if you are running on Windows and
       you execute batch files or [;;4m.com[0m applications, you must
       not pass untrusted input as arguments to the program.
       This affects both [;;4mspawn[0m and [;;4mspawn_executable[0m.

   • [;;4m{spawn_executable, FileName}[0m - Works like [;;4m{spawn,[0m
     [;;4mFileName}[0m, but only runs external executables. [;;4mFileName[0m
     in its whole is used as the name of the executable,
     including any spaces. If arguments are to be passed, the [;;4m[0m
     [;;4mPortSettings[0m [;;4margs[0m and [;;4marg0[0m can be used.

     The shell is usually not invoked to start the program, it is
     executed directly. [;;4mPATH[0m (or equivalent) is not searched.
     To find a program in [;;4mPATH[0m to execute, use [;;4m[0m
     [;;4mos:find_executable/1[0m.

     Only if a shell script or [;;4m.bat[0m file is executed, the
     appropriate command interpreter is invoked implicitly, but
     there is still no command-argument expansion or implicit [;;4m[0m
     [;;4mPATH[0m search.

     If [;;4mFileName[0m cannot be run, an error exception is raised,
     with the POSIX error code as the reason. The error reason
     can differ between OSs. Typically the error [;;4menoent[0m is
     raised when an attempt is made to run a program that is not
     found and [;;4meacces[0m is raised when the specified file is not
     executable.

   • [;;4m{spawn_driver, Command}[0m - Works like [;;4m{spawn, Command}[0m,
     but demands the first (space-separated) token of the command
     to be the name of a loaded driver. If no driver with that
     name is loaded, a [;;4mbadarg[0m error is raised.

   • [;;4m{fd, In, Out}[0m - Allows an Erlang process to access any
     currently opened file descriptors used by Erlang. The file
     descriptor [;;4mIn[0m can be used for standard input, and the file
     descriptor [;;4mOut[0m for standard output. It is only used for
     various servers in the Erlang OS ([;;4mshell[0m and [;;4muser[0m).
     Hence, its use is limited.

  [;;4mPortSettings[0m is a list of settings for the port. The valid
  settings are as follows:

   • [;;4m{packet, N}[0m - Messages are preceded by their length, sent
     in [;;4mN[0m bytes, with the most significant byte first. The
     valid values for [;;4mN[0m are 1, 2, and 4.

   • [;;4mstream[0m - Output messages are sent without packet lengths. A
     user-defined protocol must be used between the Erlang
     process and the external object.

   • [;;4m{line, L}[0m - Messages are delivered on a per line basis.
     Each line (delimited by the OS-dependent newline sequence)
     is delivered in a single message. The message data format is [;;4m[0m
     [;;4m{Flag, Line}[0m, where [;;4mFlag[0m is [;;4meol[0m or [;;4mnoeol[0m, and [;;4mLine[0m
     is the data delivered (without the newline sequence).

     [;;4mL[0m specifies the maximum line length in bytes. Lines longer
     than this are delivered in more than one message, with [;;4mFlag[0m
     set to [;;4mnoeol[0m for all but the last message. If end of file
     is encountered anywhere else than immediately following a
     newline sequence, the last line is also delivered with [;;4mFlag[0m
     set to [;;4mnoeol[0m. Otherwise lines are delivered with [;;4mFlag[0m
     set to [;;4meol[0m.

     The [;;4m{packet, N}[0m and [;;4m{line, L}[0m settings are mutually
     exclusive.

   • [;;4m{cd, Dir}[0m - Only valid for [;;4m{spawn, Command}[0m and [;;4m[0m
     [;;4m{spawn_executable, FileName}[0m. The external program starts
     using [;;4mDir[0m as its working directory. [;;4mDir[0m must be a
     string.

   • [;;4m{env, Env}[0m - Only valid for [;;4m{spawn, Command}[0m, and [;;4m[0m
     [;;4m{spawn_executable, FileName}[0m. The environment of the
     started process is extended using the environment
     specifications in [;;4mEnv[0m.

     [;;4mEnv[0m is to be a list of tuples [;;4m{Name, Val}[0m, where [;;4mName[0m
     is a [;;4mos:env_var_name/0[0m representing the name of an
     environment variable, and [;;4mVal[0m is a [;;4mos:env_var_name/0[0m
     representing the value it is to have in the spawned port
     process. Both [;;4mName[0m and [;;4mVal[0m must be strings.

     If [;;4mVal[0m is set to the atom [;;4mfalse[0m or the empty string
     (that is [;;4m""[0m or [;;4m[][0m), open_port will consider those
     variables unset just as if [;;4mos:unsetenv/1[0m had been called.

     For information about encoding requirements, see
     documentation of the types for [;;4mName[0m and [;;4mVal[0m.

   • [;;4m{args, [ string() | binary() ]}[0m - Only valid for [;;4m[0m
     [;;4m{spawn_executable, FileName}[0m and specifies arguments to the
     executable. Each argument is specified as a separate string
     and (on Unix) eventually ends up as one element each in the
     argument vector. On other platforms, a similar behavior is
     mimicked.

     The arguments are not expanded by the shell before they are
     supplied to the executable. Most notably this means that
     file wildcard expansion does not occur. To expand wildcards
     for the arguments, use [;;4mfilelib:wildcard/1[0m. Notice that
     even if the program is a Unix shell script, meaning that the
     shell ultimately is invoked, wildcard expansion does not
     occur, and the script is provided with the untouched
     arguments. On Windows, wildcard expansion is always up to
     the program itself, therefore this is not an issue.

     The executable name (also known as [;;4margv[0][0m) is not to be
     specified in this list. The proper executable name is
     automatically used as [;;4margv[0][0m, where applicable.

     If you explicitly want to set the program name in the
     argument vector, option [;;4marg0[0m can be used.

   • [;;4m{arg0, string() | binary()}[0m - Only valid for [;;4m[0m
     [;;4m{spawn_executable, FileName}[0m and explicitly specifies the
     program name argument when running an executable. This can
     in some circumstances, on some OSs, be desirable. How the
     program responds to this is highly system-dependent and no
     specific effect is guaranteed.

   • [;;4mexit_status[0m - Only valid for [;;4m{spawn, Command}[0m, where [;;4m[0m
     [;;4mCommand[0m refers to an external program, and for [;;4m[0m
     [;;4m{spawn_executable, FileName}[0m.

     When the external process connected to the port exits, a
     message of the form [;;4m{Port,{exit_status,Status}}[0m is sent to
     the connected process, where [;;4mStatus[0m is the exit status of
     the external process. If the program aborts on Unix, the
     same convention is used as the shells do (that is,
     128+signal).

     If option [;;4meof[0m is specified also, the messages [;;4meof[0m and [;;4m[0m
     [;;4mexit_status[0m appear in an unspecified order.

   • [;;4muse_stdio[0m - Only valid for [;;4m{spawn, Command}[0m and [;;4m[0m
     [;;4m{spawn_executable, FileName}[0m. It allows the standard input
     and output (file descriptors 0 and 1) of the spawned (Unix)
     process for communication with Erlang.

   • [;;4mnouse_stdio[0m - The opposite of [;;4muse_stdio[0m. It uses file
     descriptors 3 and 4 for communication with Erlang.

   • [;;4mstderr_to_stdout[0m - Affects ports to external programs. The
     executed program gets its standard error file redirected to
     its standard output file. [;;4mstderr_to_stdout[0m and [;;4m[0m
     [;;4mnouse_stdio[0m are mutually exclusive.

   • [;;4moverlapped_io[0m - Affects ports to external programs on
     Windows only. The standard input and standard output handles
     of the port program are, if this option is supplied, opened
     with flag [;;4mFILE_FLAG_OVERLAPPED[0m, so that the port program
     can (and must) do overlapped I/O on its standard handles.
     This is not normally the case for simple port programs, but
     an option of value for the experienced Windows programmer. 
     On all other platforms, this option is silently discarded.

   • [;;4min[0m - The port can only be used for input.

   • [;;4mout[0m - The port can only be used for output.

   • [;;4mbinary[0m - All I/O from the port is binary data objects as
     opposed to lists of bytes.

   • [;;4meof[0m - The port is not closed at the end of the file and
     does not produce an exit signal. Instead, it remains open
     and a [;;4m{Port, eof}[0m message is sent to the process holding
     the port.

   • [;;4mhide[0m - When running on Windows, suppresses creation of a
     new console window when spawning the port program. (This
     option has no effect on other platforms.)

   • [;;4m{parallelism, Boolean}[0m - Sets scheduler hint for port
     parallelism. If set to [;;4mtrue[0m, the virtual machine schedules
     port tasks; when doing so, it improves parallelism in the
     system. If set to [;;4mfalse[0m, the virtual machine tries to
     perform port tasks immediately, improving latency at the
     expense of parallelism. The default can be set at system
     startup by passing command-line argument [;;4m+spp[0m to erl.

   • [;;4m{busy_limits_port, {Low, High} | disabled}[0m - Sets limits
     that will be used for controlling the busy state of the
     port.

     When the ports internal output queue size becomes larger
     than or equal to [;;4mHigh[0m bytes, it enters the busy state.
     When it becomes less than [;;4mLow[0m bytes it leaves the busy
     state. When the port is in the busy state, processes sending
     commands to it will be suspended until the port leaves the
     busy state. Commands are in this context either [;;4mPort ![0m
     [;;4m{Owner, {command, Data}}[0m or [;;4mport_command/[2,3][0m.

     The [;;4mLow[0m limit is automatically adjusted to the same as [;;4m[0m
     [;;4mHigh[0m if it is set larger then [;;4mHigh[0m. Valid range of values
     for [;;4mLow[0m and [;;4mHigh[0m is [;;4m[1, (1 bsl[0m
     [;;4m(8*erlang:system_info(wordsize)))-2][0m. If the atom [;;4mdisabled[0m
     is passed, the port will never enter the busy state.

     The defaults are [;;4mLow = 4096[0m and [;;4mHigh = 8192[0m.

     Note that this option is only valid when spawning an
     executable (port program) by opening the spawn driver and
     when opening the [;;4mfd[0m driver. This option will cause a
     failure with a [;;4mbadarg[0m exception when opening other
     drivers.

   • [;;4m{busy_limits_msgq, {Low, High} | disabled}[0m - Sets limits
     that will be used for controlling the busy state of the port
     message queue.

     When the ports message queue size becomes larger than or
     equal to [;;4mHigh[0m bytes it enters the busy state. When it
     becomes less than [;;4mLow[0m bytes it leaves the busy state. When
     the port message queue is in the busy state, processes
     sending commands to it will be suspended until the port
     message queue leaves the busy state. Commands are in this
     context either [;;4mPort ! {Owner, {command, Data}}[0m or [;;4m[0m
     [;;4mport_command/[2,3][0m.

     The [;;4mLow[0m limit is automatically adjusted to the same as [;;4m[0m
     [;;4mHigh[0m if it is set larger then [;;4mHigh[0m. Valid range of values
     for [;;4mLow[0m and [;;4mHigh[0m is [;;4m[1, (1 bsl[0m
     [;;4m(8*erlang:system_info(wordsize)))-2][0m. If the atom [;;4mdisabled[0m
     is passed, the port message queue will never enter the busy
     state.

     Note that if the driver statically has disabled the use of
     this feature, a failure with a [;;4mbadarg[0m exception will be
     raised unless this option also is set to [;;4mdisable[0m or not
     passed at all.

     The defaults are [;;4mLow = 4096[0m and [;;4mHigh = 8192[0m unless the
     driver itself does modifications of these values.

     Note that the driver might fail if it also adjust these
     limits by itself and you have disabled this feature.

     The spawn driver (used when spawning an executable) and the [;;4m[0m
     [;;4mfd[0m driver do not disable this feature and do not adjust
     these limits by themselves.

     For more information see the documentation [;;4m[0m
     [;;4merl_drv_busy_msgq_limits()[0m.

  Default is [;;4mstream[0m for all port types and [;;4muse_stdio[0m for spawned
  ports.

  Failure: if the port cannot be opened, the exit reason is [;;4mbadarg[0m, [;;4m[0m
  [;;4msystem_limit[0m, or the POSIX error code that most closely describes
  the error, or [;;4meinval[0m if no POSIX code is appropriate:

   • [;;4mbadarg[0m - Bad input arguments to [;;4mopen_port[0m.

   • [;;4msystem_limit[0m - All available ports in the Erlang emulator
     are in use.

   • [;;4menomem[0m - Not enough memory to create the port.

   • [;;4meagain[0m - No more available OS processes.

   • [;;4menametoolong[0m - Too long external command.

   • [;;4memfile[0m - No more available file descriptors (for the OS
     process that the Erlang emulator runs in).

   • [;;4menfile[0m - Full file table (for the entire OS).

   • [;;4meacces[0m - [;;4mCommand[0m specified in [;;4m{spawn_executable,[0m
     [;;4mCommand}[0m does not point out an executable file.

   • [;;4menoent[0m - [;;4mFileName[0m specified in [;;4m{spawn_executable,[0m
     [;;4mFileName}[0m does not point out an existing file.

  During use of a port opened using [;;4m{spawn, Name}[0m, [;;4m{spawn_driver,[0m
  [;;4mName}[0m, or [;;4m{spawn_executable, Name}[0m, errors arising when sending
  messages to it are reported to the owning process using signals of
  the form [;;4m{'EXIT', Port, PosixCode}[0m. For the possible values of [;;4m[0m
  [;;4mPosixCode[0m, see [;;4mfile[0m.

  The maximum number of ports that can be open at the same time can
  be configured by passing command-line flag [;;4m+Q[0m to erl.
