Refactor C sample

[Tiryoh]

Checklists

• [x] I have read the CONTRIBUTING document.
• [x] I have checked to ensure there aren't other open Pull Requests for the same change.

What does this implement/fix? Explain your changes.

現状のコードでもコンパイルして問題なく動いていますが、以下の理由によりmain関数をint型にしたほうが良いと思います。

• フリースタンディング環境ではなくホスト環境でのプログラムであること
• コンパイル時に明確にC99を指定していないこと

現状ではコンパイル時に以下のワーニングが出るので、まとめてリファクタリングしています。

$ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/aarch64-linux-gnu/7/lto-wrapper
Target: aarch64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu/Linaro 7.5.0-3ubuntu1~18.04' --with-bugurl=file:///usr/share/doc/gcc-7/README.Bugs --enable-languages=c,ada,c++,go,d,fortran,objc,obj-c++ --prefix=/usr --with-gcc-major-version-only --program-suffix=-7 --program-prefix=aarch64-linux-gnu- --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --libdir=/usr/lib --enable-nls --enable-bootstrap --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new --enable-gnu-unique-object --disable-libquadmath --disable-libquadmath-support --enable-plugin --enable-default-pie --with-system-zlib --enable-multiarch --enable-fix-cortex-a53-843419 --disable-werror --enable-checking=release --build=aarch64-linux-gnu --host=aarch64-linux-gnu --target=aarch64-linux-gnu
Thread model: posix
gcc version 7.5.0 (Ubuntu/Linaro 7.5.0-3ubuntu1~18.04) 

$ gcc step1.c -o step1 -Wall
step1.c:3:6: warning: return type of ‘main’ is not ‘int’ [-Wmain]
 void main(void)
      ^~~~
step1.c: In function ‘main’:
step1.c:17:13: warning: implicit declaration of function ‘write’ [-Wimplicit-function-declaration]
             write(led[i],"1",1);
             ^~~~~
step1.c:19:9: warning: implicit declaration of function ‘usleep’ [-Wimplicit-function-declaration]
         usleep(500*1000);
         ^~~~~~
step1.c:28:9: warning: implicit declaration of function ‘close’ [-Wimplicit-function-declaration]
         close(led[i]);
         ^~~~~

$ gcc step2.c -o step2 -Wall
step2.c: In function ‘_Getch’:
step2.c:6:2: warning: implicit declaration of function ‘system’ [-Wimplicit-function-declaration]
  system("stty -echo -icanon min 1 time 0");
  ^~~~~~
step2.c:7:7: warning: implicit declaration of function ‘getchar’ [-Wimplicit-function-declaration]
  ch = getchar();
       ^~~~~~~
step2.c: At top level:
step2.c:12:6: warning: return type of ‘main’ is not ‘int’ [-Wmain]
 void main(void){
      ^~~~
step2.c: In function ‘main’:
step2.c:19:5: warning: implicit declaration of function ‘write’ [-Wimplicit-function-declaration]
     write(buzzer,"0",2);
     ^~~~~
step2.c:66:2: warning: implicit declaration of function ‘close’ [-Wimplicit-function-declaration]
  close(buzzer);
  ^~~~~

$ gcc step3.c -o step3 -Wall
step3.c: In function ‘get_SW0’:
step3.c:7:2: warning: implicit declaration of function ‘read’; did you mean ‘creat’? [-Wimplicit-function-declaration]
  read(SW0,buf,2);
  ^~~~
  creat
step3.c:8:2: warning: implicit declaration of function ‘close’ [-Wimplicit-function-declaration]
  close(SW0);
  ^~~~~
step3.c: At top level:
step3.c:32:6: warning: return type of ‘main’ is not ‘int’ [-Wmain]
 void main(void){
      ^~~~
step3.c: In function ‘main’:
step3.c:45:4: warning: implicit declaration of function ‘usleep’ [-Wimplicit-function-declaration]
    usleep(10000);
    ^~~~~~
step3.c:50:5: warning: implicit declaration of function ‘write’ [-Wimplicit-function-declaration]
     write(LED3,"0",1);
     ^~~~~

$ gcc step4.c -o step4 -Wall
step4.c:5:6: warning: return type of ‘main’ is not ‘int’ [-Wmain]
 void main(void)
      ^~~~

$ gcc step5.c -o step5 -Wall
step5.c:5:6: warning: return type of ‘main’ is not ‘int’ [-Wmain]
 void main(void)
      ^~~~

Does this close any currently open issues?

しません。