#ifdef HAVE_CONFIG_H
#include <config.h>
#endif

#include <errno.h>

#include <fcntl.h>
#include <unistd.h>

/*
 * getlogin/getlogin_r: the login name associated with the calling
 * process — the user who owns its controlling terminal.
 *
 * POSIX.1-2008: getlogin() returns a pointer to the login name, or NULL
 * when the process is not associated with a terminal; getlogin_r may
 * fail with ENOTTY when the calling process has no controlling terminal.
 * The "controlling terminal" is the terminal of the session the process
 * belongs to; Linux reports it as field 7 (tty_nr) of /proc/self/stat
 * (0 when the process has none).
 *
 * Resolving the NAME of that terminal's user requires the session/utmp
 * database, which this libc does not yet carry (the passwd/group todo
 * owns /etc/passwd and the session records). This stage therefore never
 * fabricates a name: it distinguishes the no-terminal case (ENOTTY,
 * fully specified by POSIX) from the has-a-terminal-but-no-database
 * case (ENOENT), and getlogin() reports both as NULL with errno set.
 * The name resolution is completed by the session-database todo.
 */

/* Static buffer for getlogin(); 256 bytes comfortably holds a login name. */
#define VLIBC_UNISTD_LOGIN_BUF 256

static char getlogin_buf[VLIBC_UNISTD_LOGIN_BUF];

/*
 * Read field 7 (tty_nr) of /proc/self/stat. After the parenthesized
 * comm field — located by scanning back from the end for the last ')' —
 * the fields run state(3) ppid(4) pgrp(5) session(6) tty_nr(7), so
 * tty_nr is the token after four more whitespace-separated tokens.
 * Returns 1 and the value when the file could be read, 0 otherwise.
 */
static int
login_tty_nr(long *tty_out)
{
    char buf[512];
    char *p;
    ssize_t n;
    int fd;
    int tok;
    long value = 0;
    int sign = 1;
    int have_digit = 0;

    fd = open("/proc/self/stat", O_RDONLY);
    if (fd < 0)
    {
        return 0;
    }
    n = read(fd, buf, sizeof(buf) - 1);
    close(fd);
    if (n <= 0)
    {
        return 0;
    }
    buf[n] = '\0';

    p = buf + n;
    while (p > buf && *p != ')')
    {
        p--;
    }
    if (*p != ')')
    {
        return 0;
    }
    p++;

    for (tok = 0; tok < 4; tok++)
    {
        while (*p == ' ' || *p == '\t')
        {
            p++;
        }
        while (*p != '\0' && *p != ' ' && *p != '\t' && *p != '\n')
        {
            p++;
        }
        if (*p == '\0')
        {
            return 0;
        }
    }
    while (*p == ' ' || *p == '\t')
    {
        p++;
    }
    if (*p == '-')
    {
        sign = -1;
        p++;
    }
    while (*p >= '0' && *p <= '9')
    {
        value = value * 10 + (*p - '0');
        p++;
        have_digit = 1;
    }
    if (!have_digit)
    {
        return 0;
    }
    *tty_out = sign * value;
    return 1;
}

int
getlogin_r(char *name, size_t namesize)
{
    long tty_nr;

    if (name == NULL || namesize == 0)
    {
        errno = EINVAL;
        return EINVAL;
    }
    if (!login_tty_nr(&tty_nr) || tty_nr == 0)
    {
        /* No controlling terminal (or procfs cannot be read). */
        errno = ENOTTY;
        return ENOTTY;
    }
    /* A controlling terminal exists, but no session database to resolve
     * its owner's login name yet — never fabricate one. */
    errno = ENOENT;
    return ENOENT;
}

char *
getlogin(void)
{
    int err = getlogin_r(getlogin_buf, sizeof(getlogin_buf));

    if (err != 0)
    {
        errno = err;
        return NULL;
    }
    return getlogin_buf;
}