Intel® C++ Compiler
Community support and assistance for creating C++ code that runs on platforms based on Intel® processors.
The Intel sign-in experience has changed to support enhanced security controls. If you sign in, click here for more information.
7782 Discussions

What is eax in 64bit machine and why the pointer address get 1 extend in the higher bits [compiled with ICC]?

Dear all~

I have a code section that tries to assign a pointer to a variable in 64bit machine. It's simple but something's wrong currently. The binary is run in Linux 2.6.9-89.ELlargesmp x86-64. The CPU is Intel Xeon CPU X5560 @ 2.80GHz.

[bash]int Func() 
int idx = 0;
struct myStruct * ptr = NULL;

... /* somewhere modify idx */

ptr = getPtr(idx);


struct myStruct* getPtr(int idx)
/* S_ptrTbl is of type struct myStruct** */
if (S_ptrTbl) {
return S_ptrTbl[idx];
return NULL;

In the function getPtr(), the idx is OK and in the return of getPtr(), the rax register is OK (value is 0x9d65f9a0). And in the assignment, it performs
[bash]mov %eax, -0x30(%rbp)
mov -0x30(%rbp), %eax
movslq %eax, %rax[/bash]
And then %rax is 0xffffffff9d65f9a0 which is an invalid memory address.

Before performing the getPtr(), xor %eax, %eax is performed and then I think %eax should be 0. But I can't
display the eax in gdb and I found in google that %eax is %rax in 64bit machine.

Can anyone show me how to print the %eax in gdb, and any hint about why the address is wrong?

Thanks a lot...



0 Kudos
2 Replies
Black Belt
Line 8 references getPtr() without any previous declaration of its type. By default the type is int and, depending on compiler options, int is 4-bytes long.

Try declaring getPtr() as follows before using it.
[cpp]extern struct myStruct* getPtr(int);
You could use the Intel debugger instead of GDB, or the DDD front-end for GDB. At the GDB prompt, try "help p" and it will tell you, among other things that to display the contents of %eax the command is "p $eax".

Even in 64-bit mode, registers al, ah, ax, eax, bl, bh, bx, ebx, etc. are very much available and used everywhere.
Hi mecej4~

Yes, you're right. I found that the declarasion of the function was removed from the include file by someone for some means and then this issue happened. I just declare it again and then it's ok now. Thanks a lot...

By the way, previously I used p $eax and I got void in return. Then if I cast it as char by p (char)$eax, it return a value. And I can't cast it as short or int or double* or whatever. It showed invalid cast. Do you have any idea about that? Thanks...