osx raw syscall on arm

noheaders.c

#define STDOUT 1
#define SYS_WRITE 4

int strlen(const char* str) {
    int len = 0;
    for(const char* cur = str; *cur; ++cur) ++len;
    return len;
}

int write(int fd, const char* buf, int count) {
    register int _retval asm("x4");
    register int _syscall_no asm("x16") = SYS_WRITE;

    register int _fd asm("x0") = fd;
    register const char* _buf asm("x1") = buf;
    register int _count asm("x2") = count;

    __asm__ (
        "svc #0x80\n"
        "mov x4, x0"
        : "=r"(_retval)
        : "r"(_fd), "r"(_buf), "r"(_count), "r"(_syscall_no)
    );

    return _retval;
}

int putc(char c) {
    return write(STDOUT, &c, 1);
}

int puts(const char* str) {
    int len = strlen(str);
    return write(STDOUT, str, len);
}

int putd(int n) {
    char buf[32];
    char* cur = buf + 31;
    *cur = 0;

    if(n == 0) {
        --cur;
        *cur = '0';
    } else {
        while(n) {
            --cur;
            *cur = '0' + (n % 10);
            n /= 10;
        }
    }

    return puts(cur);
}

int main() {
    int rc = puts("Look ma, no headers!\n");
    puts("rc = ");
    putd(rc);
    putc('\n');
    
    return 0;
}