问题
I feel that a native extension is like libraries that you should install onto your system before trying to install those gems, which depend on the native extensions. Like the ImageMagic library. Is that correct? Is there something else that we should know about native extensions?
回答1:
A gem native extension might link to a separate library that needs to be pre-installed, and RMagick is an example of that.
However, the term really just means "includes some code that needs to be compiled for your machine". The code is compiled and linked so that the resulting executable file can be required by Ruby on the machine.
The usual reasons for writing a Ruby C or C++ extension are:
Speed. For some CPU-intense tasks, C code can be 100 times faster than Ruby. In this case a native extension can be completely stand-alone with all C source code included in the gem.
Third-party library already written in C. In this case the gem will have C source code that binds the library functions into Ruby modules, classes and methods.
You can view the C source code for gems with native extensions, it is installed alongside the Ruby source code. By convention, there is a folder inside the gem called ext/gem_name which contains a Ruby file extconf.rb that passes information to the compiler (technically it creates a make file). In addition the C source files are put there as well.
MRI Ruby is implemented as a very "flat" structure in C, basically consisting of a large number of C functions. This makes it relatively easy to learn how to implement a native extension, even if you don't know much C. You can read Extending Ruby 1.9 as an introduction to the topic.
Native extensions can fail to install or work properly. There are many questions on Stack Overflow asking for help with specific failed installs. The usual problems are:
Missing libraries. Hopefully the gem author will explain what you need to pre-install in the README, but is not always clear.
Compiler mismatches. It is difficult to test all target systems, so sometimes the instructions in
extconf.rbwon't work on a specific system, or the C code will raise warnings or not compile due to differences. In Windows, you probably won't have a working compiler unless you install the Ruby DevkitWill not work with all versions of Ruby. For instance, JRuby can use C native extensions, if it has been enabled but it is not always advisable - the topic is quite complex, but generally avoid mixing JRuby and native extensions.
回答2:
Native extension is just a gem which is written (entirely or partially) in C.
It may or may not depend on an external library, this is not a factor here. What matters is that such gem needs to be compiled and it is likely to be platform-dependant (there was a reason to use C, right? Maybe for using some low-level OS API or something. But most often it's to interface with a library).
回答3:
Quoting this article
“Native extensions” are the glue that connects a Ruby gem with some other non-Ruby software component or library present on your machine.
The native extensionis not the dependency. A native extension is generally a C code that interacts with a non-Ruby dependency.
For instance, a gem that uses ImageMagic have a native extension written in C that talks to ImageMagic and represents the bridge from the Ruby gem to ImageMagic.
When you install the gem and the native extension is compiled, you don't compile the C library (e.g ImageMagic), that library must be already present on your system. You compile the C bridge bundled with the gem.
回答4:
I'm not big ruby specialist, so take this with a grain of salt:
I'm fairly certain that it's just a gem that needs to install a native (e.g. C-Library) library in order to work. A lot of gems simply wrap existing C-libraries with a Ruby API.
The installation of the gem will trigger the download of C-libraries, which will then be built using gcc or another compiler. If your system configuration is not supported, you'll need to pass parameters to gem tool to indicate the right directories, etc.. if you're unlucky you might need to change the make files directly.
来源：https://stackoverflow.com/questions/31202707/what-exactly-is-a-gem-native-extension