Re: Struggling with conversion to AMD64/Linux syscalls in inline asm

[me <mgbg25171 at blueyonder dot co dot uk>]

I now get a file descriptor of 3 when opening /dev/vcsa1 thanks to Bob
Platz, Jie Zhang, Thomas Martiz & Cedrix Roux. Bob and Cedric showed me
that the registers used in 64bit linux are different. Both codes now
give 3 which seems reasonable.

Unfortunately I'm now stuck on the read bit. Here's where I've got to
so far...

//My amd64 inline asm version
//this now returns a file descriptor of 3
#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;                    //fails unless replaced by value 
   unsigned char buf[] = {0,0,0,0};

   //1 open /dev/vcsa1 & return file descriptor in rax  
   __asm__ __volatile__(
   ".equ sys_open,2\n"
   ".equ O_RDWR,02\n"
   "movq $sys_open,%%rax\n" 
   "movq $O_RDWR,%%rsi\n" 
   "movq $0600,%%rdx\n" //read/write for user in x86. Not sure for AMD64?
   "syscall\n" 
   :"=D"(test_str),"=a"(test_int)
   :"D"(fl)              //rdi
   :"rsi","rdx","rcx","r10","r11","cc" //Is this enough
   ); 

   //cout << test_str << endl; // /dev/vcsa1    ie pass
   //cout << test_int << endl; // 3             ie pass

   //read 1st 4 bytes of file into buf[]
   __asm__ __volatile__(
   "movq %%rax, %%rsi\n"   //returned file descriptor into rsi
   "movq $0,%%rax\n"      //syscall read into rax
   "movq $4,%%rdx\n"      //qty of bytes to read into rdx
   "syscall\n" 
   :"=a"(test_int)
   :"D"(&buf[0])          //ptr of where to place 1st byte into rdi
   :"rsi","rdx","rcx","r10","r11","cc"
   );
   cout << test_int << endl; //returning -9 expected 4   ie fail 
}

int main(){
   my_fn();
   return 0;
}

Yes the syscall numbers in 64 bit linux is different to x86 and I do
take the point that I'm lucky with my output variables.

I expect read to return 4 instead of -9.

BTW what a great mailing list. Thx for your responsiveness.