Inline assembly in C program on x86_64 linux - c

Question

I've built a short program written on C and inline assembly on my linux x86_64. It is supposed to write a string to stdout. I found it in an article on the internet:
int write_call( int fd, const char * str, int len ){
long __res;
__asm__ volatile ( "int $0x80":
"=a" (__res):"0"(__NR_write),"b"((long)(fd)),"c"((long)(str)),"d"((long)(len)) );
return (int) __res;
}
void do_write( void ){
char * str = "Paragon output string.\n";
int len = strlen( str ), n;
printf( "string for write length = %d\n", len );
n = write_call( 1, str, len );
printf( "write return : %d\n", n );
}
int main( int argc, char * argv[] ){
do_write();
return EXIT_SUCCESS;
}
But as I run it, it works incorrectly, making output
"write return : -14"
If I build and run it on 32-bit linux it does what is expected.
After some research I fount out that instruction "int $0x80" is a x86 instruction and truncates arguments in registers if called on x86_64.
But I couldn't find a proper substitution of "int $0x80" for x86_64 architecture. I have zero experience in assembly.
What should I put instead of "int $0x80" to receive expected result?

Answer 1

For amd64, you need to use "syscall" - and use different registers - instead of "int 0x80":
http://cs.lmu.edu/~ray/notes/linuxsyscalls/
http://blog.rchapman.org/post/36801038863/linux-system-call-table-for-x86-64
http://crypto.stanford.edu/~blynn/rop/
Here's a good example:
How to invoke a system call via sysenter in inline assembly (x86/amd64 linux)?
#include <unistd.h>
int main(void)
{
const char hello[] = "Hello World!\n";
const size_t hello_size = sizeof(hello);
ssize_t ret;
asm volatile
(
"movl $1, %%eax\n\t"
"movl $1, %%edi\n\t"
"movq %1, %%rsi\n\t"
"movl %2, %%edx\n\t"
"syscall"
: "=a"(ret)
: "g"(hello), "g"(hello_size)
: "%rdi", "%rsi", "%rdx", "%rcx", "%r11"
);
return 0;