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/syscalls.cc
+++ b/libinterp/corefcn/syscalls.cc
@@ -190,10 +190,10 @@
 
   if (nargin == 1 || nargin == 2)
     {
-      std::string exec_file = args(0).string_value ();
+      if (args(0).is_string ())
+        {
+          std::string exec_file = args(0).string_value ();
 
-      if (! error_state)
-        {
           string_vector exec_args;
 
           if (nargin == 2)
@@ -212,7 +212,7 @@
                     exec_args[i+1] = tmp[i];
                 }
               else
-                error ("exec: arguments must be character strings");
+                error ("exec: all arguments must be strings");
             }
           else
             {
@@ -300,10 +300,10 @@
 
   if (nargin >= 1 && nargin <= 3)
     {
-      std::string exec_file = args(0).string_value ();
+      if (args(0).is_string ())
+        {
+          std::string exec_file = args(0).string_value ();
 
-      if (! error_state)
-        {
           string_vector arg_list;
 
           if (nargin >= 2)
@@ -322,7 +322,7 @@
                     arg_list[i+1] = tmp[i];
                 }
               else
-                error ("popen2: arguments must be character strings");
+                error ("popen2: all arguments must be strings");
             }
           else
             {
@@ -367,7 +367,7 @@
                 }
             }
           else
-            error ("popen2: arguments must be character strings");
+            error ("popen2: all arguments must be strings");
         }
       else
         error ("popen2: COMMAND argument must be a string");
@@ -1625,10 +1625,9 @@
 
   if (args.length () == 1)
     {
-      std::string name = args(0).string_value ();
-
-      if (! error_state)
+      if (args(0).is_string ())
         {
+          std::string name = args(0).string_value ();
           std::string msg;
 
           std::string result = octave_canonicalize_file_name (name, msg);
@@ -1638,7 +1637,7 @@
           retval(0) = result;
         }
       else
-        error ("canonicalize_file_name: NAME must be a character string");
+        error ("canonicalize_file_name: NAME must be a string");
     }
   else
     print_usage ();