clang簡介

昨天在練習c語言的時候，無聊的想查一下電腦的gcc版本是什麼，
使用的是 gcc --version 這個指令。
Configured with: --prefix=/Library/Developer/CommandLineTools/usr --with-gxx-include-dir=/usr/include/c++/4.2.1
Apple LLVM version 8.1.0 (clang-802.0.38)
Target: x86_64-apple-darwin16.5.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin
一查才知道原來電腦裡裝的是另一個代替gcc的編譯器---clang

[ 關於clang ]

是一個C、C++、Objective-C和Objective-C++程式語言的編譯器前端。它採用了底層虛擬機（LLVM）作為其後端。它的目標是提供一個GNU編譯器套裝（GCC）的替代品。作者是克里斯·拉特納，在蘋果公司的贊助支援下進行開發，而原始碼授權是使用類BSD的伊利諾伊大學厄巴納-香檳分校開源碼許可。

Clang專案包括Clang前端和Clang靜態分析器等。

這個軟體專案在2005年由蘋果電腦發起，是LLVM編譯器工具集的前端（front-end），目的是輸出代碼對應的抽象語法樹（Abstract Syntax Tree, AST），並將程式碼編譯成LLVM Bitcode。接著在後端（back-end）使用LLVM編譯成平台相關的機器語言 。Clang支援C、C++、Objective C。

在Clang語言中，使用Stmt來代表statement。Clang程式碼的單元（unit）皆為語句（statement），語法樹的節點（node）類型就是Stmt。另外Clang的運算式（Expression）也是語句的一種，Clang使用Expr來代表Expression，Expr本身繼承自Stmt。節點之下有子節點列表（sub-node-list）。

Clang本身效能優異，其生成的AST所耗用掉的記憶體僅僅是GCC的20%左右。FreeBSD 10將Clang/LLVM作為預設編譯器。

[ 效能 ]

測試證明Clang編譯Objective-C代碼時速度為GCC的3倍，還能針對使用者發生的編譯錯誤準確地給出建議

來源 : wikipedia

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註：

在stackexchange上面查到可以找出電腦gcc的指令(但不知是取自哪)

gcc -dumpversion | cut -f1,2,3 -d. 


我的是4.2.1 (改-fl後面的1,2,3可選擇顯示版本小數點數 如 -fl1 輸出為4)

安裝binary檔案到osx/linux並使用其terminal指令

有時候我們在網路上找到的程式是binary檔而不是osx上常見的dmg，這時候就要將其移動到系統當中的一個特定地點 /usr/local/bin，osx/linux只會從這些資料夾查看可以使用的程式。

安裝方法：
將下載來的binary檔案移動到 /usr/local/bin 即可在terminal中使用其指令。

可以使用 mv 或者 cp 這兩個指令
用法：
mv [source] [destination]
cp [source] [destination]


說明：
When you type commands like ngrok in the terminal, Macs (and other Unix OSs) look for these programs in the folders specified in your PATH. The PATH is a list of folders that's specified by each user. To check your path, open the terminal and type: echo $PATH.
You'll see output that looks something like: /usr/local/bin:/usr/bin:/bin. This is a : separated list of folders.
So when you type ngrok in the terminal, your Mac will look for this executable in the following folders: /usr/local/bin, /usr/bin/ and /bin.


像我目前如果在terminal中輸入 echo $PATH 則會出現：
/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin:/opt/X11/bin:/Library/Frameworks/Mono.framework/Versions/Current/Commands:/Library/TeX/texbin
代表這些地方都有存放可在terminal中執行指令的程式


各個path的比較：

1. /bin (and /sbin) were intended for programs that needed to be on a small / partition before the larger /usr, etc. partitions were mounted. These days, it mostly serves as a standard location for key programs like /bin/sh, although the original intent may still be relevant for e.g. installations on small embedded devices.
2. /sbin, as distinct from /bin, is for system management programs (not normally used by ordinary users) needed before /usr is mounted.
3. /usr/bin is for distribution-managed normal user programs.
4. There is a /usr/sbin with the same relationship to /usr/bin as /sbin has to /bin.
5. /usr/local/bin is for normal user programs not managed by the distribution package manager, e.g. locally compiled packages. You should not install them into /usr/bin because future distribution upgrades may modify or delete them without warning.
6. /usr/local/sbin, as you can probably guess at this point, is to /usr/local/bin as /usr/sbin to /usr/bin.
   
     In addition, there is also /opt which is for monolithic non-distribution packages, although before they were properly integrated various distributions put Gnome and KDE there. Generally you should reserve it for large, poorly behaved third party packages such as Oracle.

參考資料：
https://stackoverflow.com/questions/30188582/ngrok-command-not-found
https://unix.stackexchange.com/questions/8656/usr-bin-vs-usr-local-bin-on-linux 
https://linux.die.net/man/7/hier