Analysis of differences between C language (1) char * p and char p [] from the perspective of Assembly

 

C language code:

 

 

 

# Include <stdlib. h>

# Include <stdio. h>

 

Void main ()

{

Char array1 [] = {"123456 "};

Char * pointer1 = "123456 ";

}

Assembly code:

 

 

 

(Gdb) disassemble main

Dump of worker er code for function main:

0x08048394 <+ 0>: push % ebp

0x08048395 <+ 1>: mov % esp, % ebp

0x08048397 <+ 3>: sub $0x10, % esp

0x0804839a <+ 6>: movl $0x34333231,-0xb (% ebp)

0x080483a1 <+ 13>: movw $0x3635,-0x7 (% ebp)

0x080483a7 <+ 19>: movb $0x0,-0x5 (% ebp)

0x080483ab <+ 23>: movl $0x8048484,-0x4 (% ebp)

0x080483b2 <+ 30>: leave

0x080483b3 <+ 31>: ret

End of worker er dump.

(Gdb) x/7xb 0x8048484

0x8048484 <__dso_handle + 4>: 0x31 0x32 0x33 0x34 0x35 0x36 0x00

(Gdb)

From the <+ 6> <+ 13> <+ 19> lines, we can see that the program allocates 7 bytes of memory space for array1 to store "123456" + '\ 0 '.

The <+ 23> line indicates that the address 0x8048484 is assigned to pointer1. We can view the content after memory 0x8048484. The value of 7bytes is "123456" + '\ 0 ', here pointer1 is just a pointer and no memory units are allocated for it.

The following code is easy to understand.

 

 

 

# Include <stdlib. h>

# Include <stdio. h>

 

Void main ()

{

Char array1 [] = {"123456 "};

Char array2 [] = {"123456 "};

Char * pointer1 = "123456 ";

Char * pointer2 = "123456 ";

}

Assembly code:

 

 

(Gdb) disassemble main

Dump of worker er code for function main:

0x08048394 <+ 0>: push % ebp

0x08048395 <+ 1>: mov % esp, % ebp

0x08048397 <+ 3>: sub $0x20, % esp

0x0804839a <+ 6>: movl $0x34333231,-0xf (% ebp)

0x080483a1 <+ 13>: movw $0x3635,-0xb (% ebp)

0x080483a7 <+ 19>: movb $0x0,-0x9 (% ebp)

0x080483ab <+ 23>: movl $0x34333231,-0x16 (% ebp)

0x080483b2 <+ 30>: movw $0x3635,-0x12 (% ebp)

0x080483b8 <+ 36>: movb $0x0,-0x10 (% ebp)

0x080483bc <+ 40>: movl $0x8048494,-0x4 (% ebp)

0x080483c3 <+ 47>: movl $0x8048494,-0x8 (% ebp)

0x080483ca <+ 54>: leave

0x080483cb <+ 55>: ret

End of worker er dump.

(Gdb) x/7xb 0x8048494

0x8048494 <__dso_handle + 4>: 0x31 0x32 0x33 0x34 0x35 0x36 0x00

(Gdb)

 

Then the code will let it go!

 

(Gdb) disassemble main

Dump of worker er code for function main:

0x08048394 <+ 0>: push % ebp

0x08048395 <+ 1>: mov % esp, % ebp

0x08048397 <+ 3>: sub $0x20, % esp

0x0804839a <+ 6>: movl $0x34333231,-0xf (% ebp)

0x080483a1 <+ 13>: movw $0x3635,-0xb (% ebp)

0x080483a7 <+ 19>: movb $0x0,-0x9 (% ebp)

0x080483ab <+ 23>: movl $0x34333231,-0x16 (% ebp)

0x080483b2 <+ 30>: movw $0x3635,-0x12 (% ebp)

0x080483b8 <+ 36>: movb $0x0,-0x10 (% ebp)

0x080483bc <+ 40>: movl $0x8048494,-0x4 (% ebp)

0x080483c3 <+ 47>: movl $0x804849b,-0x8 (% ebp)

0x080483ca <+ 54>: leave

0x080483cb <+ 55>: ret

End of worker er dump.

(Gdb) x/7xb 0x8048494

0x8048494 <__dso_handle + 4>: 0x31 0x32 0x33 0x34 0x35 0x36 0x00

(Gdb) x/9xb 0x804849b

0x804849b <__dso_handle + 11>: 0x31 0x32 0x33 0x34 0x35 0x36 0x37 0x38

0x80484a3 <__ dso_handle + 19>: 0x00

(Gdb)

 

From Study Notes of sg131971 (seven nights)