fix logic of POSIX strerror_r (ITS#4292; refix ITS#4285)

diff --git a/include/ac/string.h b/include/ac/string.h
index f91f04035cd303252afda46d5629aa12c92de406..49075a2782a240d75434df2beef63844f265f1ac 100644 (file)
--- a/include/ac/string.h
+++ b/include/ac/string.h
@@ -106,14 +106,14 @@ void * memrchr(const void *b, int c, size_t len);
 #if defined( HAVE_NONPOSIX_STRERROR_R )
 #      define AC_STRERROR_R(e,b,l)             (strerror_r((e), (b), (l)))
 #elif defined( HAVE_STRERROR_R )
-#      define AC_STRERROR_R(e,b,l)             (strerror_r((e), (b), (l)) ? (b) : "")
+#      define AC_STRERROR_R(e,b,l)             (strerror_r((e), (b), (l)) == 0 ? (b) : "Unknown error")
 #elif defined( HAVE_SYS_ERRLIST )
 #      define AC_STRERROR_R(e,b,l)             ((e) > -1 && (e) < sys_nerr \
-                                                                               ? sys_errlist[(e)] : "" )
+                                                       ? sys_errlist[(e)] : "Unknown error" )
 #elif defined( HAVE_STRERROR )
-#      define AC_STRERROR_R(e,b,l)             (strerror(e))
+#      define AC_STRERROR_R(e,b,l)             (strerror(e))   /* NOTE: may be NULL */
 #else
-#      define AC_STRERROR_R(e,b,l)             ("")
+#      define AC_STRERROR_R(e,b,l)             ("Unknown error")
 #endif
 
 #endif /* _AC_STRING_H */