浅析C语言定义时赋值、定义后赋值、定义时不赋值
引言
其实在我看来定义一个变量后再赋值和定义的时候直接初始化应该是一个意思，现在就来验证一下，之前貌似验证过是一致的，但是我需要留下证据。
code
| 类型 | 内容 |
|---|---|
| 定义时赋值 | #include <stdio.h> int main(){ int a = 10; return 0;} |
| 定义后赋值 | #include <stdio.h> int main(){ int a; a = 10; return 0;} |
| 定义不赋值 | #include <stdio.h> int main(){ int a; return 0;} |
预编译
gcc -E -I./inc *.c -o *.i
这里都是一样的，由于比较长就不展示了。*表示对应的各个文件，这里用这个符号表示而已。
编译
gcc -S -I./inc *.c -o *.s
定义时赋值
.file "yubian.c"
.text
.globl main
.type main, @function
main:
.LFB0:
.cfi_startproc
pushq %rbp //将调用这个函数的函数的栈帧的底部栈指针存进本函数栈帧。用于恢复调用本函数的函数。
.cfi_def_cfa_offset 16
.cfi_offset 6, -16
movq %rsp, %rbp //将栈顶指针数据赋值给栈底指针。这时进入到了新的函数栈帧。
.cfi_def_cfa_register 6
movl $10, -4(%rbp) // 这里给int c 开辟栈空间 并赋值10
movl $0, %eax //返回值赋值0
popq %rbp //弹出数据，放入基址寄存器中。也就是本栈帧中存储的第一个数据：调用函数的栈底指针函数，用来返回,调用函数。
.cfi_def_cfa 7, 8
ret
.cfi_endproc
.LFE0:
.size main, .-main
.ident "GCC: (Ubuntu 5.4.0-6ubuntu1~16.04.12) 5.4.0 20160609"
.section .note.GNU-stack,"",@progbits
定义后赋值
.file "yubianyi.c"
.text
.globl main
.type main, @function
main:
.LFB0:
.cfi_startproc
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset 6, -16
movq %rsp, %rbp
.cfi_def_cfa_register 6
movl $10, -4(%rbp)
movl $0, %eax
popq %rbp
.cfi_def_cfa 7, 8
ret
.cfi_endproc
.LFE0:
.size main, .-main
.ident "GCC: (Ubuntu 5.4.0-6ubuntu1~16.04.12) 5.4.0 20160609"
.section .note.GNU-stack,"",@progbits
定义不赋值
.file "yu.c"
.text
.globl main
.type main, @function
main:
.LFB0:
.cfi_startproc
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset 6, -16
movq %rsp, %rbp
.cfi_def_cfa_register 6
movl $0, %eax
popq %rbp
.cfi_def_cfa 7, 8
ret
.cfi_endproc
.LFE0:
.size main, .-main
.ident "GCC: (Ubuntu 5.4.0-6ubuntu1~16.04.12) 5.4.0 20160609"
.section .note.GNU-stack,"",@progbits
定义直接赋值和定以后马上赋值在这里还是一样的。而不赋值的时候，少了一条movl $10, -4(%rbp),这句话的含义就是这里给int c 开辟栈空间 并赋值10。汇编以了解为主，目前不做深入。
汇编
汇编指令
gcc -c *.s -o *.o
查看指令
nm *.o
wzh@ubuntu:~/cipan/ctest$ nm *.o
yubian.o:
0000000000000000 T main
yubianyi.o:
0000000000000000 T main
yu.o:
0000000000000000 T main
查看指令
readelf -a yubianyi.o yubian.o yu.o
wzh@ubuntu:~/cipan/ctest$ readelf -a yubianyi.o yubian.o yu.o
File: yubianyi.o
ELF Header:
Magic: 7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00
Class: ELF64
Data: 2's complement, little endian
Version: 1 (current)
OS/ABI: UNIX - System V
ABI Version: 0
Type: REL (Relocatable file)
Machine: Advanced Micro Devices X86-64
Version: 0x1
Entry point address: 0x0
Start of program headers: 0 (bytes into file)
Start of section headers: 544 (bytes into file)
Flags: 0x0
Size of this header: 64 (bytes)
Size of program headers: 0 (bytes)
Number of program headers: 0
Size of section headers: 64 (bytes)
Number of section headers: 11
Section header string table index: 8
Section Headers:
[Nr] Name Type Address Offset
Size EntSize Flags Link Info Align
[ 0] NULL 0000000000000000 00000000
0000000000000000 0000000000000000 0 0 0
[ 1] .text PROGBITS 0000000000000000 00000040
0000000000000012 0000000000000000 AX 0 0 1
[ 2] .data PROGBITS 0000000000000000 00000052
0000000000000000 0000000000000000 WA 0 0 1
[ 3] .bss NOBITS 0000000000000000 00000052
0000000000000000 0000000000000000 WA 0 0 1
[ 4] .comment PROGBITS 0000000000000000 00000052
0000000000000036 0000000000000001 MS 0 0 1
[ 5] .note.GNU-stack PROGBITS 0000000000000000 00000088
0000000000000000 0000000000000000 0 0 1
[ 6] .eh_frame PROGBITS 0000000000000000 00000088
0000000000000038 0000000000000000 A 0 0 8
[ 7] .rela.eh_frame RELA 0000000000000000 000001b0
0000000000000018 0000000000000018 I 9 6 8
[ 8] .shstrtab STRTAB 0000000000000000 000001c8
0000000000000054 0000000000000000 0 0 1
[ 9] .symtab SYMTAB 0000000000000000 000000c0
00000000000000d8 0000000000000018 10 8 8
[10] .strtab STRTAB 0000000000000000 00000198
0000000000000011 0000000000000000 0 0 1
Key to Flags:
W (write), A (alloc), X (execute), M (merge), S (strings), l (large)
I (info), L (link order), G (group), T (TLS), E (exclude), x (unknown)
O (extra OS processing required) o (OS specific), p (processor specific)
There are no section groups in this file.
There are no program headers in this file.
Relocation section '.rela.eh_frame' at offset 0x1b0 contains 1 entries:
Offset Info Type Sym. Value Sym. Name + Addend
000000000020 000200000002 R_X86_64_PC32 0000000000000000 .text + 0
The decoding of unwind sections for machine type Advanced Micro Devices X86-64 is not currently supported.
Symbol table '.symtab' contains 9 entries:
Num: Value Size Type Bind Vis Ndx Name
0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND
1: 0000000000000000 0 FILE LOCAL DEFAULT ABS yubianyi.c
2: 0000000000000000 0 SECTION LOCAL DEFAULT 1
3: 0000000000000000 0 SECTION LOCAL DEFAULT 2
4: 0000000000000000 0 SECTION LOCAL DEFAULT 3
5: 0000000000000000 0 SECTION LOCAL DEFAULT 5
6: 0000000000000000 0 SECTION LOCAL DEFAULT 6
7: 0000000000000000 0 SECTION LOCAL DEFAULT 4
8: 0000000000000000 18 FUNC GLOBAL DEFAULT 1 main
No version information found in this file.
File: yubian.o
ELF Header:
Magic: 7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00
Class: ELF64
Data: 2's complement, little endian
Version: 1 (current)
OS/ABI: UNIX - System V
ABI Version: 0
Type: REL (Relocatable file)
Machine: Advanced Micro Devices X86-64
Version: 0x1
Entry point address: 0x0
Start of program headers: 0 (bytes into file)
Start of section headers: 536 (bytes into file)
Flags: 0x0
Size of this header: 64 (bytes)
Size of program headers: 0 (bytes)
Number of program headers: 0
Size of section headers: 64 (bytes)
Number of section headers: 11
Section header string table index: 8
Section Headers:
[Nr] Name Type Address Offset
Size EntSize Flags Link Info Align
[ 0] NULL 0000000000000000 00000000
0000000000000000 0000000000000000 0 0 0
[ 1] .text PROGBITS 0000000000000000 00000040
0000000000000012 0000000000000000 AX 0 0 1
[ 2] .data PROGBITS 0000000000000000 00000052
0000000000000000 0000000000000000 WA 0 0 1
[ 3] .bss NOBITS 0000000000000000 00000052
0000000000000000 0000000000000000 WA 0 0 1
[ 4] .comment PROGBITS 0000000000000000 00000052
0000000000000036 0000000000000001 MS 0 0 1
[ 5] .note.GNU-stack PROGBITS 0000000000000000 00000088
0000000000000000 0000000000000000 0 0 1
[ 6] .eh_frame PROGBITS 0000000000000000 00000088
0000000000000038 0000000000000000 A 0 0 8
[ 7] .rela.eh_frame RELA 0000000000000000 000001a8
0000000000000018 0000000000000018 I 9 6 8
[ 8] .shstrtab STRTAB 0000000000000000 000001c0
0000000000000054 0000000000000000 0 0 1
[ 9] .symtab SYMTAB 0000000000000000 000000c0
00000000000000d8 0000000000000018 10 8 8
[10] .strtab STRTAB 0000000000000000 00000198
000000000000000f 0000000000000000 0 0 1
Key to Flags:
W (write), A (alloc), X (execute), M (merge), S (strings), l (large)
I (info), L (link order), G (group), T (TLS), E (exclude), x (unknown)
O (extra OS processing required) o (OS specific), p (processor specific)
There are no section groups in this file.
There are no program headers in this file.
Relocation section '.rela.eh_frame' at offset 0x1a8 contains 1 entries:
Offset Info Type Sym. Value Sym. Name + Addend
000000000020 000200000002 R_X86_64_PC32 0000000000000000 .text + 0
The decoding of unwind sections for machine type Advanced Micro Devices X86-64 is not currently supported.
Symbol table '.symtab' contains 9 entries:
Num: Value Size Type Bind Vis Ndx Name
0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND
1: 0000000000000000 0 FILE LOCAL DEFAULT ABS yubian.c
2: 0000000000000000 0 SECTION LOCAL DEFAULT 1
3: 0000000000000000 0 SECTION LOCAL DEFAULT 2
4: 0000000000000000 0 SECTION LOCAL DEFAULT 3
5: 0000000000000000 0 SECTION LOCAL DEFAULT 5
6: 0000000000000000 0 SECTION LOCAL DEFAULT 6
7: 0000000000000000 0 SECTION LOCAL DEFAULT 4
8: 0000000000000000 18 FUNC GLOBAL DEFAULT 1 main
No version information found in this file.
File: yu.o
ELF Header:
Magic: 7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00
Class: ELF64
Data: 2's complement, little endian
Version: 1 (current)
OS/ABI: UNIX - System V
ABI Version: 0
Type: REL (Relocatable file)
Machine: Advanced Micro Devices X86-64
Version: 0x1
Entry point address: 0x0
Start of program headers: 0 (bytes into file)
Start of section headers: 536 (bytes into file)
Flags: 0x0
Size of this header: 64 (bytes)
Size of program headers: 0 (bytes)
Number of program headers: 0
Size of section headers: 64 (bytes)
Number of section headers: 11
Section header string table index: 8
Section Headers:
[Nr] Name Type Address Offset
Size EntSize Flags Link Info Align
[ 0] NULL 0000000000000000 00000000
0000000000000000 0000000000000000 0 0 0
[ 1] .text PROGBITS 0000000000000000 00000040
000000000000000b 0000000000000000 AX 0 0 1
[ 2] .data PROGBITS 0000000000000000 0000004b
0000000000000000 0000000000000000 WA 0 0 1
[ 3] .bss NOBITS 0000000000000000 0000004b
0000000000000000 0000000000000000 WA 0 0 1
[ 4] .comment PROGBITS 0000000000000000 0000004b
0000000000000036 0000000000000001 MS 0 0 1
[ 5] .note.GNU-stack PROGBITS 0000000000000000 00000081
0000000000000000 0000000000000000 0 0 1
[ 6] .eh_frame PROGBITS 0000000000000000 00000088
0000000000000038 0000000000000000 A 0 0 8
[ 7] .rela.eh_frame RELA 0000000000000000 000001a8
0000000000000018 0000000000000018 I 9 6 8
[ 8] .shstrtab STRTAB 0000000000000000 000001c0
0000000000000054 0000000000000000 0 0 1
[ 9] .symtab SYMTAB 0000000000000000 000000c0
00000000000000d8 0000000000000018 10 8 8
[10] .strtab STRTAB 0000000000000000 00000198
000000000000000b 0000000000000000 0 0 1
Key to Flags:
W (write), A (alloc), X (execute), M (merge), S (strings), l (large)
I (info), L (link order), G (group), T (TLS), E (exclude), x (unknown)
O (extra OS processing required) o (OS specific), p (processor specific)
There are no section groups in this file.
There are no program headers in this file.
Relocation section '.rela.eh_frame' at offset 0x1a8 contains 1 entries:
Offset Info Type Sym. Value Sym. Name + Addend
000000000020 000200000002 R_X86_64_PC32 0000000000000000 .text + 0
The decoding of unwind sections for machine type Advanced Micro Devices X86-64 is not currently supported.
Symbol table '.symtab' contains 9 entries:
Num: Value Size Type Bind Vis Ndx Name
0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND
1: 0000000000000000 0 FILE LOCAL DEFAULT ABS yu.c
2: 0000000000000000 0 SECTION LOCAL DEFAULT 1
3: 0000000000000000 0 SECTION LOCAL DEFAULT 2
4: 0000000000000000 0 SECTION LOCAL DEFAULT 3
5: 0000000000000000 0 SECTION LOCAL DEFAULT 5
6: 0000000000000000 0 SECTION LOCAL DEFAULT 6
7: 0000000000000000 0 SECTION LOCAL DEFAULT 4
8: 0000000000000000 11 FUNC GLOBAL DEFAULT 1 main
No version information found in this file.
定义时赋值和定义后赋值，区别就在文件的大小不同，定以后赋值会稍微大一点。其他一致。
定义时赋值和定义时不赋值，文件大小基本保持一致。
定义时赋值和定以后赋值的.text相同，而定义后不赋值，执行的语句会少一条，这个会在.text中体现。
这里算是对编译时的一个详细说明了，其实和编译时产生的不同基本保持一致
链接
这里保持一致。
