#include <stdio.h>
#include <pwd.h>
#include <limits.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
#include <errno.h>
#include <ctype.h>
#include <sys/stat.h>
#include <sys/sysmacros.h>

int getlogin_r(char *buf, size_t size)
{
	struct passwd pw, *ppw;
	char pw_buf[1024];
	struct stat st;
	char tty_buf[TTY_NAME_MAX];
	int i;
	dev_t dev;

	char procbuf[1024];
	int fd = open("/proc/self/stat", O_RDONLY|O_CLOEXEC);
	if (fd < 0) switch (errno) {
	case EMFILE:
	case ENFILE:
		return errno;
	default:
		return ENOSYS;
	}
	int n = read(fd, procbuf, sizeof procbuf - 1);
	close(fd);
	if (n < 0) return ENOSYS;
	procbuf[n] = 0;

	char *s = strrchr(procbuf, ')');
	if (!s) return ENOSYS;

	for (i=0; i<5 && *s; i+=(*s++==' '));
	if (!*s) return ENOSYS;

	for (dev=0; isdigit(*s); s++)
		dev = 10*dev + (*s-'0');

	if (!dev) return ENXIO;

	for (i=0; i<3; i++)
		if (!fstat(i, &st) && S_ISCHR(st.st_mode) && st.st_rdev == dev)
			break;

	snprintf(tty_buf, sizeof tty_buf, "/dev/pts/%u", (unsigned)minor(dev));
	if (i<3 || (!stat(tty_buf, &st) && S_ISCHR(st.st_mode) && st.st_rdev == dev)) {
		getpwuid_r(st.st_uid, &pw, pw_buf, sizeof pw_buf, &ppw);
		size_t l = strlen(pw.pw_name);
		if (l >= LOGIN_NAME_MAX) return EOVERFLOW;
		if (l >= size) return ERANGE;
		memcpy(buf, pw.pw_name, l+1);
		return 0;
	}

	return ENOTTY;
}

char *getlogin()
{
	static char buf[LOGIN_NAME_MAX];
	int r = getlogin_r(buf, sizeof buf);
	if (r) {
		errno = r;
		return 0;
	}
	return buf;
}

int main()
{
	printf("%s %m\n", getlogin());
	return 0;
}