diff --git a/cfg/posix.cfg b/cfg/posix.cfg
index 73197fad4c1..1a90eea0963 100644
--- a/cfg/posix.cfg
+++ b/cfg/posix.cfg
@@ -4179,6 +4179,30 @@ The function 'mktemp' is considered to be dangerous due to race conditions and s
       <valid>0:</valid>
     </arg>
   </function>
+  <!-- https://pubs.opengroup.org/onlinepubs/9699919799/functions/strxfrm_l.html -->
+  <!-- size_t strxfrm_l(char *restrict s1, const char *restrict s2, size_t n, locale_t locale) -->
+  <function name="strxfrm_l">
+    <returnValue type="size_t"/>
+    <noreturn>false</noreturn>
+    <leak-ignore/>
+    <!-- In case the 3rd argument is 0, the 1st argument is permitted to be a null pointer. (#6306) -->
+    <arg nr="1" direction="out">
+      <minsize type="argvalue" arg="3"/>
+    </arg>
+    <arg nr="2" direction="in">
+      <not-null/>
+      <not-uninit/>
+      <strz/>
+      <minsize type="argvalue" arg="3"/>
+    </arg>
+    <arg nr="3" direction="in">
+      <not-uninit/>
+      <valid>0:</valid>
+    </arg>
+    <arg nr="4" direction="in">
+      <not-uninit/>
+    </arg>
+  </function>  
   <!-- https://pubs.opengroup.org/onlinepubs/9699919799/functions/asctime.html -->
   <!-- char *asctime_r(const struct tm *tm, char *buf); -->
   <function name="asctime_r">
diff --git a/test/cfg/posix.c b/test/cfg/posix.c
index 27f296de988..1e6de23ca83 100644
--- a/test/cfg/posix.c
+++ b/test/cfg/posix.c
@@ -31,14 +31,18 @@
 #include <ctype.h>
 #include <stdbool.h>
 #include <time.h>
+#include <string.h>
+#include <strings.h>
+#if defined(__APPLE__)
+#include <xlocale.h>
+#endif
 #if !(defined(__APPLE__) && defined(__MACH__))
 #include <mqueue.h>
 #endif
 #include <stdlib.h>
 #include <unistd.h>
 #include <wchar.h>
-#include <string.h>
-#include <strings.h>
+
 
 #if !(defined(__APPLE__) && defined(__MACH__))
 void nullPointer_mq_timedsend(mqd_t mqdes, const char* msg_ptr, size_t msg_len, unsigned msg_prio, const struct timespec* abs_timeout) {
@@ -102,6 +106,17 @@ int nullPointer_posix_trace_getnext_event(trace_id_t trid, struct posix_trace_ev
 }
 #endif // __TRACE_H__
 
+size_t nullPointer_strxfrm_l(char *restrict dest, const char *restrict src, size_t count, locale_t locale)
+{
+    (void)strxfrm_l(dest, src, count, locale);
+    // In case the 3rd argument is 0, the 1st argument is permitted to be a null pointer. (#6306)
+    (void)strxfrm_l(NULL, src, 0, locale);
+    (void)strxfrm_l(NULL, src, 1, locale);
+    (void)strxfrm_l(NULL, src, count, locale);
+    // cppcheck-suppress nullPointer
+    return strxfrm_l(dest, NULL, count, locale);
+}
+
 void nullPointer_pthread_attr_getstack(const pthread_attr_t *attr, void *stackaddr, size_t stacksize) {
     // cppcheck-suppress nullPointer
     (void) pthread_attr_getstack(NULL, &stackaddr, &stacksize);