libs/libc/pwd: Add getlogin() and getlogin_r().

POSIX specifies getlogin() and getlogin_r(), and NuttX did not have
them.  NuttX has no login sessions, so the login name is the name of
the real user ID in the user database, from getpwuid_r(): "root" when
there is no passwd file.

Assisted-by: Claude Code:claude-opus-5-5
Signed-off-by: Marco Casaroli <marco.casaroli@gmail.com>

include/unistd.h

@@ -399,6 +399,11 @@ int     isatty(int fd);
 FAR char *ttyname(int fd);
 int       ttyname_r(int fd, FAR char *buf, size_t buflen);
 
+/* The name of the user */
+
+FAR char *getlogin(void);
+int       getlogin_r(FAR char *buf, size_t bufsize);
+
 /* Memory management */
 
 #if defined(CONFIG_ARCH_ADDRENV) && defined(CONFIG_MM_PGALLOC) && \

libs/libc/pwd/CMakeLists.txt

@@ -26,7 +26,8 @@ set(SRCS
     lib_getpwuidr.c
     lib_getpwent.c
     lib_pwd_globals.c
-    lib_getspnam.c)
+    lib_getspnam.c
+    lib_getlogin.c)
 
 if(CONFIG_LIBC_PASSWD_FILE)
   list(APPEND SRCS lib_find_pwdfile.c lib_getspnamr.c)

libs/libc/pwd/Make.defs

@@ -23,7 +23,7 @@
 # Add the pwd C files to the build
 
 CSRCS += lib_getpwnam.c lib_getpwnamr.c lib_getpwuid.c lib_getpwuidr.c
-CSRCS += lib_getpwent.c lib_pwd_globals.c lib_getspnam.c
+CSRCS += lib_getpwent.c lib_pwd_globals.c lib_getspnam.c lib_getlogin.c
 
 ifeq ($(CONFIG_LIBC_PASSWD_FILE),y)
 CSRCS += lib_find_pwdfile.c lib_getspnamr.c

libs/libc/pwd/lib_getlogin.c

@@ -0,0 +1,113 @@
+/****************************************************************************
+ * libs/libc/pwd/lib_getlogin.c
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.  The
+ * ASF licenses this file to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance with the
+ * License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.  See the
+ * License for the specific language governing permissions and limitations
+ * under the License.
+ *
+ ****************************************************************************/
+
+/****************************************************************************
+ * Included Files
+ ****************************************************************************/
+
+#include <errno.h>
+#include <limits.h>
+#include <pwd.h>
+#include <string.h>
+#include <unistd.h>
+
+/****************************************************************************
+ * Pre-processor Definitions
+ ****************************************************************************/
+
+/* Size of the buffer for the strings of one user database entry */
+
+#define GETLOGIN_PWBUF_SIZE 256
+
+/****************************************************************************
+ * Public Functions
+ ****************************************************************************/
+
+/****************************************************************************
+ * Name: getlogin_r
+ *
+ * Description:
+ *   Put the login name of the user into buf.  NuttX has no login sessions,
+ *   so the login name is the name of the real user ID in the user database.
+ *
+ * Input Parameters:
+ *   buf     - The buffer for the name.
+ *   bufsize - The size of buf in bytes.
+ *
+ * Returned Value:
+ *   Zero on success.  ERANGE if the name does not fit in buf, ENXIO if the
+ *   user database has no entry for the real user ID.
+ *
+ ****************************************************************************/
+
+int getlogin_r(FAR char *buf, size_t bufsize)
+{
+  char pwbuf[GETLOGIN_PWBUF_SIZE];
+  FAR struct passwd *result;
+  struct passwd pwd;
+  int ret;
+
+  ret = getpwuid_r(getuid(), &pwd, pwbuf, sizeof(pwbuf), &result);
+  if (ret != 0)
+    {
+      return ret;
+    }
+
+  if (result == NULL)
+    {
+      return ENXIO;
+    }
+
+  if (strlcpy(buf, pwd.pw_name, bufsize) >= bufsize)
+    {
+      return ERANGE;
+    }
+
+  return 0;
+}
+
+/****************************************************************************
+ * Name: getlogin
+ *
+ * Description:
+ *   Return the login name of the user.  See getlogin_r().  The string is
+ *   overwritten by the next call to getlogin().
+ *
+ * Returned Value:
+ *   A pointer to the name, or NULL with errno set if there is no name.
+ *
+ ****************************************************************************/
+
+FAR char *getlogin(void)
+{
+  static char name[NAME_MAX + 1];
+  int ret;
+
+  ret = getlogin_r(name, sizeof(name));
+  if (ret != 0)
+    {
+      set_errno(ret);
+      return NULL;
+    }
+
+  return name;
+}