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.
