/*
 * 2015 by Marek Behun <kabel@blackhole.sk>
 * Only for Linux x86_64
 * gcc -nostdlib -nostdinc -o sys sys.c
 */

/* nadefinujeme si typy size_t a ssize_t */
typedef unsigned long int size_t;
typedef long int ssize_t;

/* toto bude funkcia na pristup do kernelu */
long (*syscall) (long int, ...);

void init_syscall (void)
{
    /* a tu je jej kod, skompilovany assembler pre x86_64 pre Linux */
    static const char syscall_code[] = {
        0x48, 0x89, 0xf8,               // mov %rdi, %rax
        0x48, 0x89, 0xf7,               // mov %rsi, %rdi
        0x48, 0x89, 0xd6,               // mov %rdx, %rsi
        0x48, 0x89, 0xca,               // mov %rcx, %rdx
        0x4d, 0x89, 0xc2,               // mov %r8, %r10
        0x4d, 0x89, 0xc8,               // mov %r9, %r8
        0x4c, 0x8b, 0x4c, 0x24, 0x08,   // mov 0x8(%rsp), %r9
        0x0f, 0x05,                     // syscall
        0xc3,                           // retq
    };

    /* nastavime, aby pointer syscall ukazoval na zaciatok kodu */
    syscall = (void *) &syscall_code[0];
}

ssize_t sys_read (int fd, void * buf, size_t count)
{
    return syscall (0, fd, buf, count);
}

ssize_t sys_write (int fd, void * buf, size_t count)
{
    return syscall (1, fd, buf, count);
}

void sys_exit (int status)
{
    syscall (60, status);
}

void _start (int argc, char ** argv, char ** envp)
{
    init_syscall ();

    sys_write (1, "abc\n", 4);

    sys_exit (0);
}