diff --git a/Misc/NEWS.d/next/Library/2025-04-21-01-03-15.gh-issue-127081.WXRliX.rst b/Misc/NEWS.d/next/Library/2025-04-21-01-03-15.gh-issue-127081.WXRliX.rst
@@ -0,0 +1,2 @@
+Fix libc thread safety issues with :mod:`os` by replacing ``getlogin`` with
+``getlogin_r`` re-entrant version.
diff --git a/Modules/posixmodule.c b/Modules/posixmodule.c
@@ -9464,6 +9464,24 @@ os_getlogin_impl(PyObject *module)
     }
     else
         result = PyErr_SetFromWindowsErr(GetLastError());
+#elif defined (HAVE_GETLOGIN_R)
+# if defined (HAVE_MAXLOGNAME)
+    char name[MAXLOGNAME + 1];
+# elif defined (HAVE_UT_NAMESIZE)
+    char name[UT_NAMESIZE + 1];
+# else
+    char name[256];
+# endif
+    int err = getlogin_r(name, sizeof(name));
+    if (err) {
+        int old_errno = errno;
+        errno = -err;
+        posix_error();
+        errno = old_errno;
+    }
+    else {
+        result = PyUnicode_DecodeFSDefault(name);
+    }
 #else
     char *name;
     int old_errno = errno;

@@ -5116,7 +5116,7 @@ AC_CHECK_FUNCS([ \
   faccessat fchmod fchmodat fchown fchownat fdopendir fdwalk fexecve \
   fork fork1 fpathconf fstatat ftime ftruncate futimens futimes futimesat \
   gai_strerror getegid geteuid getgid getgrent getgrgid getgrgid_r \
-  getgrnam_r getgrouplist gethostname getitimer getloadavg getlogin \
+  getgrnam_r getgrouplist gethostname getitimer getloadavg getlogin getlogin_r \
   getpeername getpgid getpid getppid getpriority _getpty \
   getpwent getpwnam_r getpwuid getpwuid_r getresgid getresuid getrusage getsid getspent \
   getspnam getuid getwd grantpt if_nameindex initgroups kill killpg lchown linkat \
@@ -5400,6 +5400,18 @@ PY_CHECK_FUNC([setgroups], [
 #endif
 ])
 
+AC_CHECK_DECL([MAXLOGNAME],
+              [AC_DEFINE([HAVE_MAXLOGNAME], [1],
+                         [Define if you have the 'MAXLOGNAME' constant.])],
+              [],
+              [@%:@include <sys/params.h>])
+
+AC_CHECK_DECLS([UT_NAMESIZE],
+              [AC_DEFINE([HAVE_UT_NAMESIZE], [1],
+                         [Define if you have the 'HAVE_UT_NAMESIZE' constant.])],
+              [],
+              [@%:@include <utmp.h>])
+
 # check for openpty, login_tty, and forkpty
 
 AC_CHECK_FUNCS([openpty], [],

diff --git a/pyconfig.h.in b/pyconfig.h.in
@@ -258,6 +258,10 @@
    */
 #undef HAVE_DECL_TZNAME
 
+/* Define to 1 if you have the declaration of `UT_NAMESIZE', and to 0 if you
+   don't. */
+#undef HAVE_DECL_UT_NAMESIZE
+
 /* Define to 1 if you have the device macros. */
 #undef HAVE_DEVICE_MACROS
 
@@ -521,6 +525,9 @@
 /* Define to 1 if you have the `getlogin' function. */
 #undef HAVE_GETLOGIN
 
+/* Define to 1 if you have the `getlogin_r' function. */
+#undef HAVE_GETLOGIN_R
+
 /* Define to 1 if you have the `getnameinfo' function. */
 #undef HAVE_GETNAMEINFO
 
@@ -786,6 +793,9 @@
 /* Define this if you have the makedev macro. */
 #undef HAVE_MAKEDEV
 
+/* Define if you have the 'MAXLOGNAME' constant. */
+#undef HAVE_MAXLOGNAME
+
 /* Define to 1 if you have the `mbrtowc' function. */
 #undef HAVE_MBRTOWC
 
@@ -1539,6 +1549,9 @@
 /* Define to 1 if you have the <utmp.h> header file. */
 #undef HAVE_UTMP_H
 
+/* Define if you have the 'HAVE_UT_NAMESIZE' constant. */
+#undef HAVE_UT_NAMESIZE
+
 /* Define to 1 if you have the `uuid_create' function. */
 #undef HAVE_UUID_CREATE
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,2 @@
		Fix libc thread safety issues with :mod:`os` by replacing ``getlogin`` with
		``getlogin_r`` re-entrant version.