Struggling with conversion to AMD64/Linux syscalls in inline asm

[Jie Zhang]

On 01/24/2010 10:51 PM, me wrote:
> I'm trying to convert some 32 bit linked asm (which runs fine)
> into 64 bit inline asm (g++) but am stumbling at the first hurdle
> ie I'm getting -14 returned as the file descriptor for /dev/vcsa1.
>
> Here's my attempt.
>
> [code]
> //My amd64 inline asm version
> #include<iostream>
> using namespace std;
>
> void my_fn(){
>     const char * fl =       "/dev/vcsa1";
>     const char * test_str = "          "; //make same size as fl
>     int test_int = -1; //ie initialised to fail unless replaced by value
>
>     //1 open /dev/vcsa1&  return file descriptor in rax
>     __asm__ __volatile__(
>     ".equ sys_open, 2\n"
>     ".equ O_RDWR, 02\n"
>     "mov $sys_open, %%rax\n"
>     "mov $O_RDWR, %%rcx\n"
>     "mov $0600, %%rdx\n" //read/write for user in x86. Not sure for AMD64?
>     "syscall\n"
>     :"=b"(test_str), "=a"(test_int)
>     :"b"(fl)

This statement does not tell gcc the connections between registers and C 
variables. So there is no guarantee that those variables will be put 
into the registers you expect by the above statement.


Jie