clang000-2019-03-21

1.1

#include <stdio.h>
int main() {
    return 0;
}

(gdb) disas
        Dump of assembler code for function main:
0x00000000004004d6 <+0>:    push   %rbp
0x00000000004004d7 <+1>:    mov    %rsp,%rbp        建立堆栈
0x00000000004004da <+4>:    mov    $0x0,%eax        返回值0
0x00000000004004df <+9>:    pop    %rbp
0x00000000004004e0 <+10>:   retq
        End of assembler dump.

1.2

#include <stdio.h>
int main() {
    return 10;
}

Dump of assembler code for function main:
   0x00000000004004d6 <+0>: push   %rbp
   0x00000000004004d7 <+1>: mov    %rsp,%rbp
=> 0x00000000004004da <+4>: mov    $0xa,%eax
   0x00000000004004df <+9>: pop    %rbp
   0x00000000004004e0 <+10>:    retq   
End of assembler dump.

2.1

#include <stdio.h>
void fun1() {
}

int main() {
    fun1();
    return 10;
}

Dump of assembler code for function main:
   0x00000000004004dd <+0>: push   %rbp
   0x00000000004004de <+1>: mov    %rsp,%rbp
   0x00000000004004e1 <+4>: mov    $0x0,%eax
   0x00000000004004e6 <+9>: callq  0x4004d6 <fun1>
=> 0x00000000004004eb <+14>:    mov    $0xa,%eax
   0x00000000004004f0 <+19>:    pop    %rbp
   0x00000000004004f1 <+20>:    retq   
End of assembler dump.

Dump of assembler code for function fun1:
   0x00000000004004d6 <+0>: push   %rbp
   0x00000000004004d7 <+1>: mov    %rsp,%rbp
=> 0x00000000004004da <+4>: nop
   0x00000000004004db <+5>: pop    %rbp
   0x00000000004004dc <+6>: retq   
End of assembler dump.

2.2

#include <stdio.h>

int fun1() {
    return 16;
}

int main() {
    fun1();
    return 10;
}

Dump of assembler code for function fun1:
   0x00000000004004d6 <+0>: push   %rbp
   0x00000000004004d7 <+1>: mov    %rsp,%rbp
   0x00000000004004da <+4>: mov    $0x10,%eax
   0x00000000004004df <+9>: pop    %rbp
   0x00000000004004e0 <+10>:    retq   
End of assembler dump.
Dump of assembler code for function main:
   0x00000000004004e1 <+0>: push   %rbp
   0x00000000004004e2 <+1>: mov    %rsp,%rbp
   0x00000000004004e5 <+4>: mov    $0x0,%eax
   0x00000000004004ea <+9>: callq  0x4004d6 <fun1>
=> 0x00000000004004ef <+14>:    mov    $0xa,%eax
   0x00000000004004f4 <+19>:    pop    %rbp
   0x00000000004004f5 <+20>:    retq   
End of assembler dump.

2.3

#include <stdio.h>

int fun1(int m) {
    return m;
}

int main() {
    fun1(16);
    return 10;
}

Dump of assembler code for function fun1:
   0x00000000004004d6 <+0>: push   %rbp
   0x00000000004004d7 <+1>: mov    %rsp,%rbp
   0x00000000004004da <+4>: mov    %edi,-0x4(%rbp)
   0x00000000004004dd <+7>: mov    -0x4(%rbp),%eax
   0x00000000004004e0 <+10>:    pop    %rbp
   0x00000000004004e1 <+11>:    retq   
End of assembler dump.
Dump of assembler code for function main:
   0x00000000004004e2 <+0>: push   %rbp
   0x00000000004004e3 <+1>: mov    %rsp,%rbp
   0x00000000004004e6 <+4>: mov    $0x10,%edi
   0x00000000004004eb <+9>: callq  0x4004d6 <fun1>
=> 0x00000000004004f0 <+14>:    mov    $0xa,%eax
   0x00000000004004f5 <+19>:    pop    %rbp
   0x00000000004004f6 <+20>:    retq   
End of assembler dump.

2.4

#include <stdio.h>

int fun1(int m) {
    return m;
}

int main() {
    int a;
    a = fun1(16);
    return 10;
}

Dump of assembler code for function fun1:
   0x00000000004004d6 <+0>: push   %rbp
   0x00000000004004d7 <+1>: mov    %rsp,%rbp
   0x00000000004004da <+4>: mov    %edi,-0x4(%rbp)
=> 0x00000000004004dd <+7>: mov    -0x4(%rbp),%eax
   0x00000000004004e0 <+10>:    pop    %rbp
   0x00000000004004e1 <+11>:    retq   
End of assembler dump.
Dump of assembler code for function main:
   0x00000000004004e2 <+0>: push   %rbp
   0x00000000004004e3 <+1>: mov    %rsp,%rbp
   0x00000000004004e6 <+4>: sub    $0x10,%rsp
   0x00000000004004ea <+8>: mov    $0x10,%edi
   0x00000000004004ef <+13>:    callq  0x4004d6 <fun1>
   0x00000000004004f4 <+18>:    mov    %eax,-0x4(%rbp)
=> 0x00000000004004f7 <+21>:    mov    $0xa,%eax
   0x00000000004004fc <+26>:    leaveq 
   0x00000000004004fd <+27>:    retq   
End of assembler dump.

---

3.1

printf("*******%s,%s,%d*******\n",__FUNCTION__,__FILE__,__LINE__);

#include <stdio.h>
#define NUM 100
int main(){
#ifdef DEBUG
        printf("*******%s,%s,%d*******\n",__FUNCTION__,__FILE__,__LINE__);
#endif
        printf("Hello!\n");
        return 0;
}

root@vultr:~/clang# ls
002.c
root@vultr:~/clang# gcc -DDEBUG -o 002 002.c 
root@vultr:~/clang# ls
002  002.c
root@vultr:~/clang# ./002
*******main,002.c,5*******
Hello!
root@vultr:~/clang# rm 002
root@vultr:~/clang# ls
002.c
root@vultr:~/clang# gcc -o 002 002.c
root@vultr:~/clang# ls
002  002.c
root@vultr:~/clang# ./002
Hello!
root@vultr:~/clang# ^C
root@vultr:~/clang# 

3.2

ANSI C一共只有32个关键字，9种控制语句，程序书写形式自由，区分大小写。把高级语言的基本结构和语句与低级语言的实用性结合起来。 C 语言可以像汇编语言一样对位、字节和地址进行操作，而这三者是计算机最基本的工作单元。
C语言的运算符包含的范围很广泛，共有34种运算符。C语言把括号、赋值、强制类型转换等都作为运算符处理。从而使C语言的运算类型极其丰富，表达式类型多样化。灵活使用各种运算符可以实现在其它高级语言中难以实现的运算。

关键字 sizeof,非函数！所以在任何环境下都可以使用。
关键字 return,函数返回。

char int short long unsigned signed float double void
struct union enum typedef,定义数据类型，圈地用的。

if 、else
switch、case、default
do 、while、for
continue、break、goto，逻辑结构，控制干活儿的顺序：顺序执行、分支执行、循环执行。

auto、register、static、const、extern、volatile，类型修饰符，（修饰数据类型，用来修饰上面那些个限制内存大小的数据类型），对内存资源存放位置的限定。

部分关键字解释：break：跳出当前循环
continue：结束当前循环，开始下一轮循环
case：开关语句分支
default：开关语句中的“其它”分支
goto：无条件跳转语句
return ：子程序返回语句（可以带参数，也可不带参数）
register：声明寄存器变量
sizeof：计算数据类型或变量长度（即所占字节数）
volatile：说明变量在程序执行中可被隐含地改变
auto ：声明自动变量
extern：声明变量或函数是在其它文件或本文件的其他位置定义

Relevant Link:
c语言关键字总结 - 二郎三郎 - 博客园

算术操作运算：
+ - * / %
逻辑运算（返回结果0或1，假就是零，非零就是真）：
|| &&
> >= < <=
!
? :
位运算：
<< >>
| &
^ ~
赋值运算：
=
+= -= &= ...
内存访问符号：
()
[]
{}
-> .
& *

Relevant Link:
异或运算的性质及用途 - 氏名無 - CSDN博客

3.3

register关键字

//"register.c"
#include <stdio.h>
int main(){
        register int a;
        a=0x10;
        printf("the a is %d\n",a);
        return 0;
}

//"register2.c"
#include <stdio.h>
int main(){
        int a;
        a=0x10;
        printf("the a is %d\n",a);
        return 0;
}

root@vultr:~/clang# ls
register2.c  register.c
root@vultr:~/clang# gcc -o register register.c
root@vultr:~/clang# gcc -o register2 register2.c 
root@vultr:~/clang# ls
register  register2  register2.c  register.c
root@vultr:~/clang# ./register
the a is 16
root@vultr:~/clang# ./register2
the a is 16
root@vultr:~/clang# rm register register2

root@vultr:~/clang# ls
register2.c  register.c
root@vultr:~/clang# gcc -c -o register register.c
root@vultr:~/clang# gcc -c -o register2 register2.c
root@vultr:~/clang# ls
register  register2  register2.c  register.c
root@vultr:~/clang# objdump -d register

register:     file format elf64-x86-64


Disassembly of section .text:

0000000000000000 <main>:
   0:   55                      push   %rbp
   1:   48 89 e5                mov    %rsp,%rbp
   4:   53                      push   %rbx
   5:   48 83 ec 08             sub    $0x8,%rsp
   9:   bb 10 00 00 00          mov    $0x10,%ebx
   e:   89 de                   mov    %ebx,%esi
  10:   bf 00 00 00 00          mov    $0x0,%edi
  15:   b8 00 00 00 00          mov    $0x0,%eax
  1a:   e8 00 00 00 00          callq  1f <main+0x1f>
  1f:   b8 00 00 00 00          mov    $0x0,%eax
  24:   48 83 c4 08             add    $0x8,%rsp
  28:   5b                      pop    %rbx
  29:   5d                      pop    %rbp
  2a:   c3                      retq   
root@vultr:~/clang# objdump -d register2

register2:     file format elf64-x86-64


Disassembly of section .text:

0000000000000000 <main>:
   0:   55                      push   %rbp
   1:   48 89 e5                mov    %rsp,%rbp
   4:   48 83 ec 10             sub    $0x10,%rsp
   8:   c7 45 fc 10 00 00 00    movl   $0x10,-0x4(%rbp)
   f:   8b 45 fc                mov    -0x4(%rbp),%eax
  12:   89 c6                   mov    %eax,%esi
  14:   bf 00 00 00 00          mov    $0x0,%edi
  19:   b8 00 00 00 00          mov    $0x0,%eax
  1e:   e8 00 00 00 00          callq  23 <main+0x23>
  23:   b8 00 00 00 00          mov    $0x0,%eax
  28:   c9                      leaveq 
  29:   c3                      retq   
root@vultr:~/clang# 

3.4

//"shiftr.c"
#include <stdio.h>
int main(){
        int a=32;
        while(a){
           a=a>>1;
        }
        printf("**************\n");
}

root@vultr:~/clang# gcc -o shiftr shiftr.c 
root@vultr:~/clang# ls
shiftr  shiftr.c
root@vultr:~/clang# ./shiftr 
**************
root@vultr:~/clang# gcc -c -o shiftr.o shiftr.c 
root@vultr:~/clang# ls
shiftr  shiftr.c  shiftr.o
root@vultr:~/clang# objdump -d shiftr.o

shiftr.o:     file format elf64-x86-64


Disassembly of section .text:

0000000000000000 <main>:
   0:   55                      push   %rbp
   1:   48 89 e5                mov    %rsp,%rbp
   4:   48 83 ec 10             sub    $0x10,%rsp
   8:   c7 45 fc 20 00 00 00    movl   $0x20,-0x4(%rbp)
   f:   eb 03                   jmp    14 <main+0x14>
  11:   d1 7d fc                sarl   -0x4(%rbp)
  14:   83 7d fc 00             cmpl   $0x0,-0x4(%rbp)
  18:   75 f7                   jne    11 <main+0x11>
  1a:   bf 00 00 00 00          mov    $0x0,%edi
  1f:   e8 00 00 00 00          callq  24 <main+0x24>
  24:   b8 00 00 00 00          mov    $0x0,%eax
  29:   c9                      leaveq 
  2a:   c3                      retq   
root@vultr:~/clang# 

修改为int a=-32;，会让程序陷入死循环。

root@vultr:~/clang# gcc -o shiftr shiftr.c 
root@vultr:~/clang# ./shiftr 
^C
root@vultr:~/clang# gcc -c -o shiftr.o shiftr.c 
root@vultr:~/clang# ls
shiftr  shiftr.c  shiftr.o
root@vultr:~/clang# objdump -d shiftr.o

shiftr.o:     file format elf64-x86-64


Disassembly of section .text:

0000000000000000 <main>:
   0:   55                      push   %rbp
   1:   48 89 e5                mov    %rsp,%rbp
   4:   48 83 ec 10             sub    $0x10,%rsp
   8:   c7 45 fc e0 ff ff ff    movl   $0xffffffe0,-0x4(%rbp)
   f:   eb 03                   jmp    14 <main+0x14>
  11:   d1 7d fc                sarl   -0x4(%rbp)
  14:   83 7d fc 00             cmpl   $0x0,-0x4(%rbp)
  18:   75 f7                   jne    11 <main+0x11>
  1a:   bf 00 00 00 00          mov    $0x0,%edi
  1f:   e8 00 00 00 00          callq  24 <main+0x24>
  24:   b8 00 00 00 00          mov    $0x0,%eax
  29:   c9                      leaveq 
  2a:   c3                      retq   
root@vultr:~/clang#