feat(unistd): cwd/link/readlink/path ops

src/unistd/getlogin.c

@@ -0,0 +1,149 @@
+#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;
+}