bug#76876: logname output is often wrong when linked with glibc

[Bruno Haible via GNU coreutils Bug Reports]

I wrote:
> Thus, on Linux systems, a correct implementation of getlogin() can not
> distinguish different users with the same uid (with reasonable effort).
> This applies to both glibc and the new code in gnulib.

Let me document this limitation.


2025-03-10  Bruno Haible  <br...@clisp.org>

        getlogin, getlogin_r: Document limitation.
        Reported by Nicolas Boos <nicolas.b...@wanadoo.fr> in
        <https://lists.gnu.org/archive/html/bug-gnulib/2025-03/msg00033.html>.
        * doc/posix-functions/getlogin.texi: Mention the "different user names
        with same uid" limitation.
        * doc/posix-functions/getlogin_r.texi: Likewise.

diff --git a/doc/posix-functions/getlogin.texi 
b/doc/posix-functions/getlogin.texi
index 0ccce336ce..b0b8da73ae 100644
--- a/doc/posix-functions/getlogin.texi
+++ b/doc/posix-functions/getlogin.texi
@@ -16,7 +16,8 @@
 This function is not declared unless @code{_POSIX} is defined on some 
platforms:
 mingw.
 @item
-This function returns the value of the @env{LOGNAME} environment variable:
+This function returns the value of the @env{LOGNAME} environment variable
+and this therefore arbitrarily fakeable:
 musl libc 1.2.5.
 @end itemize
 
@@ -26,4 +27,9 @@
 This function returns an empty string even when standard input is a tty
 on some platforms:
 HP-UX 11.11.
+@item
+When there are several user names with the same user ID,
+this function may return, instead of the user name that logged in,
+another user name that has the same user ID, on some platforms:
+Linux and others.
 @end itemize
diff --git a/doc/posix-functions/getlogin_r.texi 
b/doc/posix-functions/getlogin_r.texi
index df145ecfe2..0e3b7968db 100644
--- a/doc/posix-functions/getlogin_r.texi
+++ b/doc/posix-functions/getlogin_r.texi
@@ -21,7 +21,8 @@
 @code{ERANGE}, when the buffer is not large enough, on some platforms:
 macOS 14.
 @item
-This function returns the value of the @env{LOGNAME} environment variable:
+This function returns the value of the @env{LOGNAME} environment variable
+and this therefore arbitrarily fakeable:
 musl libc 1.2.5.
 @end itemize
 
@@ -42,4 +43,9 @@
 some platforms:
 @c https://dev.haiku-os.org/ticket/18349
 Haiku.
+@item
+When there are several user names with the same user ID,
+this function may return, instead of the user name that logged in,
+another user name that has the same user ID, on some platforms:
+Linux and others.
 @end itemize