系统技术非业余研究

Erlang的port相当于系统的IO，打开了Erlang世界通往外界的通道，可以很方便的执行外部程序。 但是open_port的性能对整个系统来讲非常的重要，我就带领大家看看open_port影响性能的因素。

首先看下open_port的文档：

{spawn, Command}

Starts an external program. Command is the name of the external program which will be run. Command runs outside the Erlang work space unless an Erlang driver with the name Command is found. If found, that driver will be started. A driver runs in the Erlang workspace, which means that it is linked with the Erlang runtime system.

When starting external programs on Solaris, the system call vfork is used in preference to fork for performance reasons, although it has a history of being less robust. If there are problems with using vfork, setting the environment variable ERL_NO_VFORK to any value will cause fork to be used instead.

For external programs, the PATH is searched (or an equivalent method is used to find programs, depending on operating system). This is done by invoking the shell och certain platforms. The first space separated token of the command will be considered as the name of the executable (or driver). This (among other things) makes this option unsuitable for running programs having spaces in file or directory names. Use {spawn_executable, Command} instead if spaces in executable file names is desired.

open_port一个外部程序的时候流程大概是这样的：beam.smp先vfork, 子进程调用child_setup程序，做进一步的清理操作。 清理完成后才真正exec我们的外部程序。

再来看下open_port实现的代码：
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Erlang与外面世界的交互主要通过port来进行的，特别是和外部程序的协作，通常是通过管道进行的。
基本上有2种方法可以调用外部程序: 1. os:cmd 2. erlang:open_port, 这二种方式各有利弊，先看文档：
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