// gcc -std=gnu99 -Wall -Wextra writer.c -o writer

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <errno.h>

static void
fail(const char *str)
{
	fprintf(stderr, "W: %s\n", str);
	exit(EXIT_FAILURE);
}

int
main(void)
{
	int flags = fcntl(STDOUT_FILENO, F_GETFL);
	if (flags == -1)
		fail("Cannot get stdout file status flags");

	if (fcntl(STDOUT_FILENO, F_SETFL, flags | O_NONBLOCK) == -1)
		fail("Setting stdout to non-blocking mode failed");

	static unsigned char buf[64 << 10];
	int blocking = 0;
	int i = 0;

	while (i < 256)
	{
		int ret = write(STDOUT_FILENO, buf, sizeof(buf));
		if (ret == -1 && errno == EAGAIN)
		{
			blocking++;
			continue;
		}

		if (ret != sizeof(buf))
			fail("Unexpected number of bytes");

		memset(buf, ++i, sizeof(buf));
	}

	if (!blocking)
		fail("Was never blocking, read was too fast");

	if (fcntl(STDOUT_FILENO, F_SETFL, flags) == -1)
		fail("Restoring stdout file status flags failed");

	if (close(STDOUT_FILENO))
		fail("Error closing stdout");

	return EXIT_SUCCESS;
}