x86-64 Linux buffered "hello world"

example.c

// x86-64 Linux buffered "hello world"
// $ cc example.c && ./a.out
// Ref: https://old.reddit.com/r/C_Programming/comments/1bxvs3u
// Ref: https://thecoder08.github.io/hello-world.html
// Ref: https://nullprogram.com/blog/2023/02/13/
#include <stddef.h>
#include <string.h>

ptrdiff_t xwrite(int fd, void *buf, ptrdiff_t len)
{
    enum { SYS_write = 1 };
    asm volatile (
        "syscall"
        : "=a"(len)
        : "a"(SYS_write), "D"(fd), "S"(buf), "d"(len)
        : "rcx", "r11", "memory"
    );
    return len;
}

typedef struct {
    char buf[1<<12];
    int  len;
    int  fd;
    int  err;
} xbuf;

xbuf xstdout[1] = {{.fd = 1}};

void xflush(xbuf *b)
{
    if (b->err) return;
    for (int off = 0; off < b->len;) {
        int r = xwrite(b->fd, b->buf+off, b->len-off);
        if (r < 1) {
            b->err = 1;
            return;
        }
        off += r;
    }
    b->len = 0;
}

void xprint(xbuf *b, char *buf, ptrdiff_t len)
{
    for (ptrdiff_t off = 0; !b->err && off < len;) {
        int avail = (int)sizeof(b->buf) - b->len;
        int count = avail<len-off ? avail : len-off;
        memcpy(b->buf+b->len, buf+off, count);
        off += count;
        b->len += count;
        if (b->len == (int)sizeof(b->buf)) {
            xflush(b);
        }
    }
}

void xputs(char *s)
{
    xprint(xstdout, s, strlen(s));
    xprint(xstdout, "\n", 1);
}

int main(void)
{
    xputs("hello world");
    xflush(xstdout);
    return xstdout->err;
}