Use stricter input validation when looking for a string as input (bug #42651). * data.cc (get_sort_mode_option, Fissorted): Use is_string() to check string input. * debug.cc (Fdbstep): use "string" rather than "character string" in error messages. * error.cc (Flasterr, Flastwarn): use "string" rather than "character string" in error messages. * file-io.cc (do_stream_open, do_fread, do_fwrite, Fpopen, Ftempname, Fmkstemp): Use is_string() to check string input. * graphics.cc (Fgraphics_toolkit): Use is_string() to check string input. Rephrase error message. * help.cc (F__list_functions): Use is_string() to check string input. * input.cc (Fyes_or_no): Use is_string() to check string input. Rephrase error message. * input.cc (Fadd_input_event_hook): Rephrase error message. * load-path.cc (Fgenpath, Faddpath): Rephrase error message. * matrix_type.cc (Fmatrix_type): Use is_string() to check string input. * qz.cc (Fqz): Follow Octave coding convention for space after '!'. * regexp.cc (parse_options): Use is_string() to check string input. Rephrase error message. * schur.cc (Fschur): Use is_string() to check string input. * strfns.cc (Flist_in_columns): Use is_string() to check string input. Rephrase error message. * symtab.cc (Fignore_function_time_stamp): Use is_string() to check string input. Rephrase error message. * syscalls.cc (Fexec, Fpopen2, Fcanonicalize_file_name): Use is_string() to check string input. Rephrase error message. * sysdep.cc (Fsetenv): Use is_string() to check string input. * time.cc (Fstrftime, Fstrptime): Use is_string() to check string input. * toplev.cc (Fsystem, Fatexit): Use is_string() to check string input. * urlwrite.cc (Furlwrite, Furlread): Rephrase error message. * utils.cc (Ffile_in_path): Use is_string() to check string input. Rephrase error message. * variables.cc (extract_function): Add FIXME about potentially using is_string. * variables.cc (do_isglobal, Fmunlock, Fmislocked): Use is_string() to check string input. * variables.cc (set_internal_variable): Rephrase error message. * ov-base.cc (make_idx_args): Rephrase error message. * ov-class.cc (octave_class::all_strings, Fclass): Rephrase error message. * ov-fcn-handle.cc (Fstr2func): Use is_string() to check string input * ov-java.cc (FjavaObject, FjavaMethod, F__java_get__, F__java_set__): Use is_string() to check string input. * ov.cc (Fdecode_subscripts): Use is_string() to check string input. Rephrase error message. * pt-idx.cc (tree_index_expression::get_struct_index): Rephrase error message. * io.tst: Change %!warning test to %!error test to match stricter checking. * system.tst: Change %!warning test for setenv to %!error test to match stricter checking.

--- a/libinterp/corefcn/variables.cc
+++ b/libinterp/corefcn/variables.cc
@@ -150,6 +150,8 @@
 
   if (! retval)
     {
+      // FIXME: Should is_string () be used instead which will warn more
+      //        broadly about incorrect input?
       std::string s = arg.string_value ();
 
       std::string cmd = header;
@@ -341,14 +343,14 @@
       return retval;
     }
 
-  std::string name = args(0).string_value ();
-
-  if (error_state)
+  if (! args(0).is_string ())
     {
       error ("isglobal: NAME must be a string");
       return retval;
     }
 
+  std::string name = args(0).string_value ();
+
   return symbol_table::is_global (name);
 }
 
@@ -946,7 +948,7 @@
             error ("%s: value must not be empty", nm);
         }
       else
-        error ("%s: expecting arg to be a character string", nm);
+        error ("%s: expecting arg to be a string", nm);
     }
   else if (nargin > 1)
     print_usage ();
@@ -994,7 +996,7 @@
             error ("%s: value not allowed (\"%s\")", nm, sval.c_str ());
         }
       else
-        error ("%s: expecting arg to be a character string", nm);
+        error ("%s: expecting arg to be a string", nm);
     }
   else if (nargin > 1)
     print_usage ();
@@ -2076,10 +2078,11 @@
 
   if (args.length () == 1)
     {
-      std::string name = args(0).string_value ();
-
-      if (! error_state)
-        munlock (name);
+      if (args(0).is_string ())
+        {
+          std::string name = args(0).string_value ();
+          munlock (name);
+        }
       else
         error ("munlock: FCN must be a string");
     }
@@ -2112,10 +2115,11 @@
 
   if (args.length () == 1)
     {
-      std::string name = args(0).string_value ();
-
-      if (! error_state)
-        retval = mislocked (name);
+      if (args(0).is_string ())
+        {
+          std::string name = args(0).string_value ();
+          retval = mislocked (name);
+        }
       else
         error ("mislocked: FCN must be a string");
     }