First pass at fixing issue 99

A lot of code touched because low level functions needed to pass up
error messages reliably, and this had to propagate up the entire
driver hierarchy.

Tested OK *in English* on:
     > Linux
     > OSX 10.10.x
     > Windows VS2017
     > Windows mingw64
I have no way to test on Android, but it might work.

TODO: Needs testing in other languages to verify proper UTF8 error messages,
      esp. with Windows VS, due to complexities with FormatMessage() -- see get_ms_errmsg()

20 files changed, 336 insertions, 153 deletions

diff --git a/FL/Fl_File_Browser.H b/FL/Fl_File_Browser.H
index 67b0a9d81..40d328093 100644
--- a/FL/Fl_File_Browser.H
+++ b/FL/Fl_File_Browser.H
@@ -40,6 +40,7 @@ class FL_EXPORT Fl_File_Browser : public Fl_Browser {
   const char    *directory_;
   uchar         iconsize_;
   const char    *pattern_;
+  const char    *errmsg_;
 
   int           full_height() const;
   int           item_height(void *) const;
@@ -50,11 +51,8 @@ class FL_EXPORT Fl_File_Browser : public Fl_Browser {
 public:
   enum { FILES, DIRECTORIES };
 
-  /**
-    The constructor creates the Fl_File_Browser widget at the specified position and size.
-    The destructor destroys the widget and frees all memory that has been allocated.
-  */
   Fl_File_Browser(int, int, int, int, const char * = 0);
+  ~Fl_File_Browser();
 
   /**    Sets or gets the size of the icons. The default size is 20 pixels.  */
   uchar         iconsize() const { return (iconsize_); };
@@ -73,16 +71,7 @@ public:
     function in FLTK.
   */
   const char    *filter() const { return (pattern_); };
-
-  /**
-    Loads the specified directory into the browser. If icons have been
-    loaded then the correct icon is associated with each file in the list.
-
-    <P>The sort argument specifies a sort function to be used with
-    fl_filename_list().
-  */
   int           load(const char *directory, Fl_File_Sort_F *sort = fl_numericsort);
-
   Fl_Fontsize  textsize() const { return Fl_Browser::textsize(); };
   void          textsize(Fl_Fontsize s) { Fl_Browser::textsize(s); iconsize_ = (uchar)(3 * s / 2); };
 
@@ -100,6 +89,11 @@ public:
     shown.
   */
   void          filetype(int t) { filetype_ = t; };
+  void errmsg(const char *emsg);
+  /**
+    Returns OS error messages, or NULL if none. Use when advised.
+   */
+  const char* errmsg() const { return errmsg_; }
 };
 
 #endif // !_Fl_File_Browser_H_
diff --git a/FL/Fl_File_Chooser.H b/FL/Fl_File_Chooser.H
index 702c4ff9f..341b8e683 100644
--- a/FL/Fl_File_Chooser.H
+++ b/FL/Fl_File_Chooser.H
@@ -85,6 +85,7 @@ private:
   Fl_File_Browser *fileList;
   inline void cb_fileList_i(Fl_File_Browser*, void*);
   static void cb_fileList(Fl_File_Browser*, void*);
+  Fl_Box *errorBox;
   Fl_Box *previewBox;
 public:
   Fl_Check_Button *previewButton;
@@ -231,6 +232,8 @@ private:
   Fl_Widget* ext_group;
 public:
   Fl_Widget* add_extra(Fl_Widget* gr);
+protected:
+  void show_error_box(int val);
 };
 FL_EXPORT char *fl_dir_chooser(const char *message,const char *fname,int relative=0);
 FL_EXPORT char *fl_file_chooser(const char *message,const char *pat,const char *fname,int relative=0);
diff --git a/src/Fl_File_Browser.cxx b/src/Fl_File_Browser.cxx
index e7a7ea8d4..cb92428dd 100644
--- a/src/Fl_File_Browser.cxx
+++ b/src/Fl_File_Browser.cxx
@@ -354,10 +354,10 @@ Fl_File_Browser::item_draw(void *p,     // I - List item data
 }
 
 
-//
-// 'Fl_File_Browser::Fl_File_Browser()' - Create a Fl_File_Browser widget.
-//
-
+/**
+  The constructor creates the Fl_File_Browser widget at the specified position and size.
+  The destructor destroys the widget and frees all memory that has been allocated.
+*/
 Fl_File_Browser::Fl_File_Browser(int        X,  // I - Upper-lefthand X coordinate
                                  int        Y,  // I - Upper-lefthand Y coordinate
                                  int        W,  // I - Width in pixels
@@ -370,13 +370,40 @@ Fl_File_Browser::Fl_File_Browser(int        X,  // I - Upper-lefthand X coordina
   directory_ = "";
   iconsize_  = (uchar)(3 * textsize() / 2);
   filetype_  = FILES;
+  errmsg_    = NULL;
 }
 
 
-//
-// 'Fl_File_Browser::load()' - Load a directory into the browser.
-//
+// DTOR
+Fl_File_Browser::~Fl_File_Browser() {
+  errmsg(NULL);       // free()s prev errmsg, if any
+}
+
+
+/**
+  Sets OS error message to a string, which can be NULL.
+  Frees previous if any.
+  void errmsg(const char *emsg);
+ */
+void Fl_File_Browser::errmsg(const char* emsg) {
+  if ( errmsg_ ) { free((void*)errmsg_); errmsg_ = NULL; }
+  errmsg_ = emsg ? strdup(emsg) : NULL;
+}
+
+
+/**
+  Loads the specified directory into the browser. If icons have been
+  loaded then the correct icon is associated with each file in the list.
 
+  If directory is "", all mount points (unix) or drive letters (Windows)
+  are listed.
+
+  The sort argument specifies a sort function to be used with
+  fl_filename_list().
+
+  Return value is the number of filename entries, or 0 if none.
+  On error, 0 is returned, and errmsg() has OS error string if non-NULL.
+*/
 int                                             // O - Number of files loaded
 Fl_File_Browser::load(const char     *directory,// I - Directory to load
                       Fl_File_Sort_F *sort)     // I - Sort function to use
@@ -387,6 +414,7 @@ Fl_File_Browser::load(const char     *directory,// I - Directory to load
   char          filename[4096];                 // Current file
   Fl_File_Icon  *icon;                          // Icon to use
 
+  errmsg(NULL); // clear errors first
 
 //  printf("Fl_File_Browser::load(\"%s\")\n", directory);
 
@@ -394,11 +422,12 @@ Fl_File_Browser::load(const char     *directory,// I - Directory to load
 
   directory_ = directory;
 
-  if (!directory)
-    return (0);
+  if (!directory) {
+    errmsg("NULL directory specified");
+    return 0;
+  }
 
-  if (directory_[0] == '\0')
-  {
+  if (directory_[0] == '\0') {
     //
     // No directory specified; for UNIX list all mount points.  For DOS
     // list all valid drive letters...
@@ -406,21 +435,25 @@ Fl_File_Browser::load(const char     *directory,// I - Directory to load
     if ((icon = Fl_File_Icon::find("any", Fl_File_Icon::DEVICE)) == NULL)
       icon = Fl_File_Icon::find("any", Fl_File_Icon::DIRECTORY);
     num_files = Fl::system_driver()->file_browser_load_filesystem(this, filename, (int)sizeof(filename), icon);
-  }
-  else
-  {
-    dirent      **files;        // Files in in directory
-    //
-    // Build the file list...
-    //
-    num_files = Fl::system_driver()->file_browser_load_directory(directory_, filename, sizeof(filename), &files, sort);
-    if (num_files <= 0)
-      return (0);
+  } else {
+    dirent **files;        // Files in in directory
+    char emsg[1024] = "";
+
+    // Build the file list, check for errors
+    num_files = Fl::system_driver()->file_browser_load_directory(directory_,
+                                                                 filename, sizeof(filename),
+                                                                 &files, sort,
+                                                                 emsg, sizeof(emsg));
+    // printf("Fl_File_Browser::load(dir='%s',filename='%s'): failed, emsg='%s'\n", directory_, filename, emsg);
+
+    if (num_files <= 0) {
+      errmsg(emsg);
+      return 0;
+    }
 
     for (i = 0, num_dirs = 0; i < num_files; i ++) {
       if (strcmp(files[i]->d_name, "./")) {
-        snprintf(filename, sizeof(filename), "%s/%s", directory_,
-                 files[i]->d_name);
+        fl_snprintf(filename, sizeof(filename), "%s/%s", directory_, files[i]->d_name);
 
         icon = Fl_File_Icon::find(filename);
         if ((icon && icon->type() == Fl_File_Icon::DIRECTORY) ||
diff --git a/src/Fl_File_Chooser.cxx b/src/Fl_File_Chooser.cxx
index 8036ffaed..22fadf57f 100644
--- a/src/Fl_File_Chooser.cxx
+++ b/src/Fl_File_Chooser.cxx
@@ -197,9 +197,18 @@ Fl_File_Chooser::Fl_File_Chooser(const char *d, const char *p, int t, const char
       o->callback((Fl_Callback*)cb_);
       { fileList = new Fl_File_Browser(10, 45, 295, 225);
         fileList->type(2);
+        fileList->box(FL_DOWN_BOX);
         fileList->callback((Fl_Callback*)cb_fileList);
         fileList->window()->hotspot(fileList);
       } // Fl_File_Browser* fileList
+      { errorBox = new Fl_Box(10, 45, 295, 225, "dynamic error display");
+        errorBox->box(FL_DOWN_BOX);
+        errorBox->color(FL_BACKGROUND2_COLOR);
+        errorBox->labelsize(18);
+        errorBox->labelcolor((Fl_Color)1);
+        errorBox->align(Fl_Align(133|FL_ALIGN_INSIDE));
+        errorBox->hide();
+      } // Fl_Box* errorBox
       { previewBox = new Fl_Box(305, 45, 175, 225, "?");
         previewBox->box(FL_DOWN_BOX);
         previewBox->labelsize(100);
@@ -471,3 +480,17 @@ Fl_Widget* Fl_File_Chooser::add_extra(Fl_Widget* gr) {
   }
   return ret;
 }
+
+/**
+ Show error box if val=1, hide if val=0
+*/
+void Fl_File_Chooser::show_error_box(int val) {
+  if ( val ) {
+    errorBox->color(fileList->color()); // inherit fileList's bg color
+    errorBox->show();
+    fileList->hide();
+  } else {
+    errorBox->hide();
+    fileList->show();
+  }
+}
diff --git a/src/Fl_File_Chooser.fl b/src/Fl_File_Chooser.fl
index d222bd1c8..7cf01e1c7 100644
--- a/src/Fl_File_Chooser.fl
+++ b/src/Fl_File_Chooser.fl
@@ -66,7 +66,8 @@ class FL_EXPORT Fl_File_Chooser {open
   }
   decl {void update_preview();} {private local
   }
-  Function {Fl_File_Chooser(const char *d, const char *p, int t, const char *title)} {} {
+  Function {Fl_File_Chooser(const char *d, const char *p, int t, const char *title)} {open
+  } {
     code {if (!prefs_) {
   prefs_ = new Fl_Preferences(Fl_Preferences::CORE_USER, "fltk.org", "filechooser");
 }} {}
@@ -77,11 +78,11 @@ class FL_EXPORT Fl_File_Chooser {open
 fileList->deselect();
 Fl::remove_timeout((Fl_Timeout_Handler)previewCB, this);
 window->hide();} open
-      private xywh {507 327 490 380} type Double hide resizable
+      private xywh {1171 438 490 380} type Double resizable
       code0 {if (title) window->label(title);}
       code1 {\#include <stdio.h>}
       code2 {\#include <stdlib.h>}
-      code3 {\#include <string.h>} modal
+      code3 {\#include <string.h>} modal visible
     } {
       Fl_Group {} {open
         private xywh {10 10 470 25}
@@ -106,14 +107,18 @@ window->hide();} open
         }
       }
       Fl_Tile {} {
-        callback {update_preview();}
+        callback {update_preview();} open
         private xywh {10 45 470 225} resizable
       } {
         Fl_File_Browser fileList {
           callback {fileListCB();}
-          private xywh {10 45 295 225} type Hold hotspot
+          private xywh {10 45 295 225} type Hold box DOWN_BOX hotspot
           code0 {\#include <FL/Fl_File_Browser.H>}
         }
+        Fl_Box errorBox {
+          label {dynamic error display} selected
+          private xywh {10 45 295 225} box DOWN_BOX color 7 labelsize 18 labelcolor 1 align 149 hide
+        }
         Fl_Box previewBox {
           label {?}
           private xywh {305 45 175 225} box DOWN_BOX labelsize 100 align 80
@@ -349,7 +354,7 @@ okButton->parent()->init_sizes();} {}
   } {
     code {fileList->textfont(f);} {}
   }
-  Function {textfont()} {selected return_type Fl_Font
+  Function {textfont()} {return_type Fl_Font
   } {
     code {return (fileList->textfont());} {}
   }
@@ -475,6 +480,18 @@ window->resizable(svres);} {}
     }
     code {return ret;} {}
   }
+  Function {show_error_box(int val)} {
+    comment {Show error box if val=1, hide if val=0} open protected return_type void
+  } {
+    code {if ( val ) {
+  errorBox->color(fileList->color()); // inherit fileList's bg color
+  errorBox->show();
+  fileList->hide();
+} else {
+  errorBox->hide();
+  fileList->show();
+}} {}
+  }
 }
 
 decl {FL_EXPORT char *fl_dir_chooser(const char *message,const char *fname,int relative=0);} {public local
diff --git a/src/Fl_File_Chooser2.cxx b/src/Fl_File_Chooser2.cxx
index de2b7c160..ec04bd77c 100644
--- a/src/Fl_File_Chooser2.cxx
+++ b/src/Fl_File_Chooser2.cxx
@@ -1020,17 +1020,15 @@ void Fl_File_Chooser::preview(int e)
   Fl_Group *p = previewBox->parent();
   if (e) {
     int w = p->w() * 2 / 3;
-    fileList->resize(fileList->x(), fileList->y(),
-                     w, fileList->h());
-    previewBox->resize(fileList->x()+w, previewBox->y(),
-                       p->w()-w, previewBox->h());
+    fileList->resize(fileList->x(), fileList->y(), w, fileList->h());
+    errorBox->resize(errorBox->x(), errorBox->y(), w, errorBox->h());
+    previewBox->resize(fileList->x()+w, previewBox->y(), p->w()-w, previewBox->h());
     previewBox->show();
     update_preview();
   } else {
-    fileList->resize(fileList->x(), fileList->y(),
-                     p->w(), fileList->h());
-    previewBox->resize(p->x()+p->w(), previewBox->y(),
-                       0, previewBox->h());
+    fileList->resize(fileList->x(), fileList->y(), p->w(), fileList->h());
+    errorBox->resize(errorBox->x(), errorBox->y(), p->w(), errorBox->h());
+    previewBox->resize(p->x()+p->w(), previewBox->y(), 0, previewBox->h());
     previewBox->hide();
   }
   p->init_sizes();
@@ -1070,7 +1068,14 @@ Fl_File_Chooser::rescan()
     okButton->deactivate();
 
   // Build the file list...
-  fileList->load(directory_, sort);
+  if ( fileList->load(directory_, sort) <= 0 ) {
+    if ( fileList->errmsg() ) errorBox->label(fileList->errmsg());     // show OS errormsg when possible
+    else                      errorBox->label("No files found...");
+    show_error_box(1);
+  } else {
+    show_error_box(0);
+  }
+
   if (Fl::system_driver()->dot_file_hidden() && !showHiddenButton->value()) remove_hidden_files();
   // Update the preview box...
   update_preview();
@@ -1094,7 +1099,13 @@ void Fl_File_Chooser::rescan_keep_filename()
   strlcpy(pathname, fn, sizeof(pathname));
 
   // Build the file list...
-  fileList->load(directory_, sort);
+  if (fileList->load(directory_, sort) <= 0) {
+    if ( fileList->errmsg() ) errorBox->label(fileList->errmsg());     // show OS errormsg when possible
+    else                      errorBox->label("No files found...");
+    show_error_box(1);
+  } else {
+    show_error_box(0);
+  }
   if (Fl::system_driver()->dot_file_hidden() && !showHiddenButton->value()) remove_hidden_files();
   // Update the preview box...
   update_preview();
diff --git a/src/Fl_System_Driver.H b/src/Fl_System_Driver.H
index 0474154d4..77a58971f 100644
--- a/src/Fl_System_Driver.H
+++ b/src/Fl_System_Driver.H
@@ -128,7 +128,9 @@ public:
   virtual int event_key(int k) {return 0;}
   virtual int get_key(int k) {return 0;}
   // implement scandir-like function
-  virtual int filename_list(const char *d, dirent ***list, int (*sort)(struct dirent **, struct dirent **) ) {return -1;}
+  virtual int filename_list(const char *d, dirent ***list,
+                            int (*sort)(struct dirent **, struct dirent **),
+                            char *errmsg=NULL, int errmsg_sz=0) {return -1;}
   // the default implementation of filename_expand() may be enough
   virtual int filename_expand(char *to, int tolen, const char *from);
   // to implement
@@ -162,7 +164,9 @@ public:
   // implement to support Fl_File_Browser::load()
   virtual int file_browser_load_filesystem(Fl_File_Browser *browser, char *filename, int lname, Fl_File_Icon *icon) {return 0;}
   // the default implementation of file_browser_load_directory() should be enough
-  virtual int file_browser_load_directory(const char *directory, char *filename, size_t name_size, dirent ***pfiles, Fl_File_Sort_F *sort);
+  virtual int file_browser_load_directory(const char *directory, char *filename, size_t name_size,
+                                          dirent ***pfiles, Fl_File_Sort_F *sort,
+                                          char *errmsg=NULL, int errmsg_sz=0);
   // implement to support Fl_Preferences
   virtual void newUUID(char *uuidBuffer) { uuidBuffer[0] = 0; }
   // implement to support Fl_Preferences
diff --git a/src/Fl_System_Driver.cxx b/src/Fl_System_Driver.cxx
index f2b3c9584..a68d3b381 100644
--- a/src/Fl_System_Driver.cxx
+++ b/src/Fl_System_Driver.cxx
@@ -449,9 +449,11 @@ int Fl_System_Driver::filename_expand(char *to,int tolen, const char *from) {
 }
 
 int Fl_System_Driver::file_browser_load_directory(const char *directory, char *filename,
-                                                  size_t name_size, dirent ***pfiles, Fl_File_Sort_F *sort)
+                                                  size_t name_size, dirent ***pfiles,
+                                                  Fl_File_Sort_F *sort,
+                                                  char *errmsg, int errmsg_sz)
 {
-  return filename_list(directory, pfiles, sort);
+  return filename_list(directory, pfiles, sort, errmsg, errmsg_sz);
 }
 
 int Fl_System_Driver::file_type(const char *filename)
diff --git a/src/drivers/Android/Fl_Android_System_Driver.H b/src/drivers/Android/Fl_Android_System_Driver.H
index 2a8682ddb..198d7a2b3 100644
--- a/src/drivers/Android/Fl_Android_System_Driver.H
+++ b/src/drivers/Android/Fl_Android_System_Driver.H
@@ -70,7 +70,9 @@ public:
   // these 2 are in Fl_get_key_win32.cxx
   virtual int event_key(int k);
   virtual int get_key(int k);
-  virtual int filename_list(const char *d, dirent ***list, int (*sort)(struct dirent **, struct dirent **) );
+  virtual int filename_list(const char *d, dirent ***list,
+                            int (*sort)(struct dirent **, struct dirent **),
+                            char *errmsg, int errmsg_sz);
   virtual int filename_expand(char *to,int tolen, const char *from);
   virtual int filename_relative(char *to, int tolen, const char *from, const char *base);
   virtual int filename_absolute(char *to, int tolen, const char *from);
@@ -80,7 +82,9 @@ public:
   virtual int open_uri(const char *uri, char *msg, int msglen);
   virtual int use_recent_tooltip_fix() {return 1;}
   virtual int file_browser_load_filesystem(Fl_File_Browser *browser, char *filename, int lname, Fl_File_Icon *icon);
-  virtual int file_browser_load_directory(const char *directory, char *filename, size_t name_size, dirent ***pfiles, Fl_File_Sort_F *sort);
+  virtual int file_browser_load_directory(const char *directory, char *filename, size_t name_size,
+                                          dirent ***pfiles, Fl_File_Sort_F *sort,
+                                          char *errmsg=NULL, int errmsg_sz=0);
   virtual void newUUID(char *uuidBuffer);
   virtual char *preference_rootnode(Fl_Preferences *prefs, Fl_Preferences::Root root, const char *vendor,
                                     const char *application);
diff --git a/src/drivers/Android/Fl_Android_System_Driver.cxx b/src/drivers/Android/Fl_Android_System_Driver.cxx
index e6aa970e4..0fa9671c5 100644
--- a/src/drivers/Android/Fl_Android_System_Driver.cxx
+++ b/src/drivers/Android/Fl_Android_System_Driver.cxx
@@ -486,11 +486,13 @@ int Fl_WinAPI_System_Driver::clocale_printf(FILE *output, const char *format, va
   return retval;
 }
 
-int Fl_WinAPI_System_Driver::filename_list(const char *d, dirent ***list, int (*sort)(struct dirent **, struct dirent **) ) {
+int Fl_WinAPI_System_Driver::filename_list(const char *d, dirent ***list,
+                                           int (*sort)(struct dirent **, struct dirent **),
+                                           char *errmsg, int errmsg_sz ) {
   // For Windows we have a special scandir implementation that uses
   // the Win32 "wide" functions for lookup, avoiding the code page mess
   // entirely. It also fixes up the trailing '/'.
-  return fl_scandir(d, list, 0, sort);
+  return fl_scandir(d, list, 0, sort, errmsg, errmsg_sz);
 }
 
 int Fl_WinAPI_System_Driver::filename_expand(char *to, int tolen, const char *from) {
@@ -747,7 +749,9 @@ int Fl_WinAPI_System_Driver::file_browser_load_filesystem(Fl_File_Browser *brows
 }
 
 int Fl_WinAPI_System_Driver::file_browser_load_directory(const char *directory, char *filename,
-                                                         size_t name_size, dirent ***pfiles, Fl_File_Sort_F *sort)
+                                                         size_t name_size, dirent ***pfiles,
+                                                         Fl_File_Sort_F *sort,
+                                                         char *errmsg, int errmsg_sz)
 {
   strlcpy(filename, directory, name_size);
   int i = (int) (strlen(filename) - 1);
@@ -756,7 +760,7 @@ int Fl_WinAPI_System_Driver::file_browser_load_directory(const char *directory,
     filename[2] = '/';
   else if (filename[i] != '/' && filename[i] != '\\')
     strlcat(filename, "/", name_size);
-  return filename_list(filename, pfiles, sort);
+  return filename_list(filename, pfiles, sort, errmsg, errmsg_sz);
 }
 
 void Fl_WinAPI_System_Driver::newUUID(char *uuidBuffer)
diff --git a/src/drivers/Darwin/Fl_Darwin_System_Driver.H b/src/drivers/Darwin/Fl_Darwin_System_Driver.H
index ee7ab30ea..548277901 100644
--- a/src/drivers/Darwin/Fl_Darwin_System_Driver.H
+++ b/src/drivers/Darwin/Fl_Darwin_System_Driver.H
@@ -52,7 +52,9 @@ public:
   // these 2 are in Fl_get_key_mac.cxx
   virtual int event_key(int k);
   virtual int get_key(int k);
-  virtual int filename_list(const char *d, dirent ***list, int (*sort)(struct dirent **, struct dirent **) );
+  virtual int filename_list(const char *d, dirent ***list,
+                            int (*sort)(struct dirent **, struct dirent **),
+                            char *errmsg=NULL, int errmsg_sz=0);
   virtual int open_uri(const char *uri, char *msg, int msglen);
   virtual int need_test_shortcut_extra() {return 1;}
   virtual int file_browser_load_filesystem(Fl_File_Browser *browser, char *filename, int lname, Fl_File_Icon *icon);
diff --git a/src/drivers/Darwin/Fl_Darwin_System_Driver.cxx b/src/drivers/Darwin/Fl_Darwin_System_Driver.cxx
index 51a416b87..0c1a9de9e 100644
--- a/src/drivers/Darwin/Fl_Darwin_System_Driver.cxx
+++ b/src/drivers/Darwin/Fl_Darwin_System_Driver.cxx
@@ -15,6 +15,7 @@
 //
 
 #include "Fl_Darwin_System_Driver.H"
+#include <src/flstring.h>
 #include <FL/platform.H>
 #include <FL/Fl.H>
 #include <FL/Fl_File_Browser.H>
@@ -139,7 +140,9 @@ void *Fl_Darwin_System_Driver::get_carbon_function(const char *function_name) {
   return (carbon ? dlsym(carbon, function_name) : NULL);
 }
 
-int Fl_Darwin_System_Driver::filename_list(const char *d, dirent ***list, int (*sort)(struct dirent **, struct dirent **) ) {
+int Fl_Darwin_System_Driver::filename_list(const char *d, dirent ***list,
+                                           int (*sort)(struct dirent **, struct dirent **),
+                                           char *errmsg, int errmsg_sz) {
   int dirlen;
   char *dirloc;
   // Assume that locale encoding is no less dense than UTF-8
@@ -150,6 +153,10 @@ int Fl_Darwin_System_Driver::filename_list(const char *d, dirent ***list, int (*
 # else
   int n = scandir(dirloc, list, 0, (int(*)(const void*,const void*))sort);
 # endif
+  if (n==-1) {
+    if (errmsg) fl_snprintf(errmsg, errmsg_sz, "%s", strerror(errno));
+    return -1;
+  }
   // convert every filename to UTF-8, and append a '/' to all
   // filenames that are directories
   int i;
diff --git a/src/drivers/WinAPI/Fl_WinAPI_System_Driver.H b/src/drivers/WinAPI/Fl_WinAPI_System_Driver.H
index e6446923d..91b230d5f 100644
--- a/src/drivers/WinAPI/Fl_WinAPI_System_Driver.H
+++ b/src/drivers/WinAPI/Fl_WinAPI_System_Driver.H
@@ -68,7 +68,9 @@ public:
   // these 2 are in Fl_get_key_win32.cxx
   virtual int event_key(int k);
   virtual int get_key(int k);
-  virtual int filename_list(const char *d, dirent ***list, int (*sort)(struct dirent **, struct dirent **) );
+  virtual int filename_list(const char *d, dirent ***list,
+                            int (*sort)(struct dirent **, struct dirent **),
+                            char *errmsg=NULL, int errmsg_sz=0);
   virtual int filename_expand(char *to,int tolen, const char *from);
   virtual int filename_relative(char *to, int tolen, const char *from, const char *base);
   virtual int filename_absolute(char *to, int tolen, const char *from);
@@ -78,7 +80,9 @@ public:
   virtual int open_uri(const char *uri, char *msg, int msglen);
   virtual int use_recent_tooltip_fix() {return 1;}
   virtual int file_browser_load_filesystem(Fl_File_Browser *browser, char *filename, int lname, Fl_File_Icon *icon);
-  virtual int file_browser_load_directory(const char *directory, char *filename, size_t name_size, dirent ***pfiles, Fl_File_Sort_F *sort);
+  virtual int file_browser_load_directory(const char *directory, char *filename, size_t name_size,
+                                          dirent ***pfiles, Fl_File_Sort_F *sort,
+                                          char *errmsg=NULL, int errmsg_sz=0);
   virtual void newUUID(char *uuidBuffer);
   virtual char *preference_rootnode(Fl_Preferences *prefs, Fl_Preferences::Root root, const char *vendor,
                                     const char *application);
diff --git a/src/drivers/WinAPI/Fl_WinAPI_System_Driver.cxx b/src/drivers/WinAPI/Fl_WinAPI_System_Driver.cxx
index ab20cfe32..592e192e3 100644
--- a/src/drivers/WinAPI/Fl_WinAPI_System_Driver.cxx
+++ b/src/drivers/WinAPI/Fl_WinAPI_System_Driver.cxx
@@ -61,7 +61,8 @@ static wchar_t *wbuf1 = NULL;
 extern "C" {
   int fl_scandir(const char *dirname, struct dirent ***namelist,
                  int (*select)(struct dirent *),
-                 int (*compar)(struct dirent **, struct dirent **));
+                 int (*compar)(struct dirent **, struct dirent **),
+                 char *errmsg, int errmsg_len);
 }
 
 /*
@@ -488,11 +489,13 @@ int Fl_WinAPI_System_Driver::clocale_printf(FILE *output, const char *format, va
   return retval;
 }
 
-int Fl_WinAPI_System_Driver::filename_list(const char *d, dirent ***list, int (*sort)(struct dirent **, struct dirent **) ) {
+int Fl_WinAPI_System_Driver::filename_list(const char *d, dirent ***list,
+                                           int (*sort)(struct dirent **, struct dirent **),
+                                           char *errmsg, int errmsg_sz) {
   // For Windows we have a special scandir implementation that uses
   // the Win32 "wide" functions for lookup, avoiding the code page mess
   // entirely. It also fixes up the trailing '/'.
-  return fl_scandir(d, list, 0, sort);
+  return fl_scandir(d, list, 0, sort, errmsg, errmsg_sz);
 }
 
 int Fl_WinAPI_System_Driver::filename_expand(char *to, int tolen, const char *from) {
@@ -749,7 +752,9 @@ int Fl_WinAPI_System_Driver::file_browser_load_filesystem(Fl_File_Browser *brows
 }
 
 int Fl_WinAPI_System_Driver::file_browser_load_directory(const char *directory, char *filename,
-                                                         size_t name_size, dirent ***pfiles, Fl_File_Sort_F *sort)
+                                                         size_t name_size, dirent ***pfiles,
+                                                         Fl_File_Sort_F *sort,
+                                                         char *errmsg, int errmsg_sz)
 {
   strlcpy(filename, directory, name_size);
   int i = (int) (strlen(filename) - 1);
@@ -758,7 +763,7 @@ int Fl_WinAPI_System_Driver::file_browser_load_directory(const char *directory,
     filename[2] = '/';
   else if (filename[i] != '/' && filename[i] != '\\')
     strlcat(filename, "/", name_size);
-  return filename_list(filename, pfiles, sort);
+  return filename_list(filename, pfiles, sort, errmsg, errmsg_sz);
 }
 
 void Fl_WinAPI_System_Driver::newUUID(char *uuidBuffer)
diff --git a/src/drivers/X11/Fl_X11_System_Driver.H b/src/drivers/X11/Fl_X11_System_Driver.H
index 7c2306a28..f8fddc741 100644
--- a/src/drivers/X11/Fl_X11_System_Driver.H
+++ b/src/drivers/X11/Fl_X11_System_Driver.H
@@ -34,7 +34,9 @@ public:
   // these 2 are in Fl_get_key.cxx
   virtual int event_key(int k);
   virtual int get_key(int k);
-  virtual int filename_list(const char *d, dirent ***list, int (*sort)(struct dirent **, struct dirent **) );
+  virtual int filename_list(const char *d, dirent ***list,
+                            int (*sort)(struct dirent **, struct dirent **),
+                            char *errmsg=NULL, int errmsg_sz=0);
   virtual int need_menu_handle_part1_extra() {return 1;}
   virtual int open_uri(const char *uri, char *msg, int msglen);
   virtual int use_tooltip_timeout_condition() {return 1;}
diff --git a/src/drivers/X11/Fl_X11_System_Driver.cxx b/src/drivers/X11/Fl_X11_System_Driver.cxx
index 0714ff753..28827bcfc 100644
--- a/src/drivers/X11/Fl_X11_System_Driver.cxx
+++ b/src/drivers/X11/Fl_X11_System_Driver.cxx
@@ -25,7 +25,8 @@
 #include <unistd.h>
 #include <sys/types.h>
 #include <pwd.h>
-
+#include <string.h>     // strerror(errno)
+#include <errno.h>      // errno
 
 #if defined(_AIX)
 extern "C" {
@@ -56,7 +57,8 @@ extern "C" {
 extern "C" {
   int fl_scandir(const char *dirname, struct dirent ***namelist,
                  int (*select)(struct dirent *),
-                 int (*compar)(struct dirent **, struct dirent **));
+                 int (*compar)(struct dirent **, struct dirent **),
+                 char *errmsg, int errmsg_sz);
 }
 #endif
 
@@ -444,10 +446,19 @@ int Fl_X11_System_Driver::XParseGeometry(const char* string, int* x, int* y,
   return ::XParseGeometry(string, x, y, width, height);
 }
 
-int Fl_X11_System_Driver::filename_list(const char *d, dirent ***list, int (*sort)(struct dirent **, struct dirent **) ) {
+//
+// Needs some docs
+// Returns -1 on error, errmsg will contain OS error if non-NULL.
+//
+int Fl_X11_System_Driver::filename_list(const char *d,
+                                        dirent ***list,
+                                        int (*sort)(struct dirent **, struct dirent **),
+                                        char *errmsg, int errmsg_sz) {
   int dirlen;
   char *dirloc;
 
+  if (errmsg && errmsg_sz>0) errmsg[0] = '\0';
+
   // Assume that locale encoding is no less dense than UTF-8
   dirlen = strlen(d);
   dirloc = (char *)malloc(dirlen + 1);
@@ -476,6 +487,11 @@ int Fl_X11_System_Driver::filename_list(const char *d, dirent ***list, int (*sor
 
   free(dirloc);
 
+  if (n==-1) {
+    if (errmsg) fl_snprintf(errmsg, errmsg_sz, "%s", strerror(errno));
+    return -1;
+  }
+
   // convert every filename to UTF-8, and append a '/' to all
   // filenames that are directories
   int i;
diff --git a/src/filename_isdir.cxx b/src/filename_isdir.cxx
index fad66b3c7..47e1de58f 100644
--- a/src/filename_isdir.cxx
+++ b/src/filename_isdir.cxx
@@ -57,6 +57,7 @@ int Fl_System_Driver::filename_isdir_quick(const char* n) {
 }
 
 
+// TODO: This should probably handle errors better (like permission denied) -erco
 int Fl_System_Driver::filename_isdir(const char* n) {
   struct stat   s;
   char          fn[FL_PATH_MAX];
diff --git a/src/filename_list.cxx b/src/filename_list.cxx
index 2ad240996..4c8204fe7 100644
--- a/src/filename_list.cxx
+++ b/src/filename_list.cxx
@@ -63,9 +63,10 @@ int fl_casealphasort(struct dirent **a, struct dirent **b) {
       to put unpadded numbers in consecutive order; upper and lowercase letters are compared
       according to their ASCII ordering - uppercase before lowercase.
   \return the number of entries if no error, a negative value otherwise.
+  \todo should support returning OS error messages
 */
 int fl_filename_list(const char *d, dirent ***list, Fl_File_Sort_F *sort) {
-  return Fl::system_driver()->filename_list(d, list, sort);
+  return Fl::system_driver()->filename_list(d, list, sort, NULL, 0);
 }
 
 /**
diff --git a/src/scandir_posix.c b/src/scandir_posix.c
index cf58c4195..b311c72e1 100644
--- a/src/scandir_posix.c
+++ b/src/scandir_posix.c
@@ -128,11 +128,16 @@ readentry(DIR *dirp, struct dirent **entryp, size_t *len)
 }
 
 
-/* ========================================================================== */
+/*
+ * This could use some docs.
+ *
+ * Returns -1 on error, errmsg returns error string (if non-NULL)
+ */
 int
 fl_scandir(const char *dir, struct dirent ***namelist,
            int (*sel)(struct dirent *),
-           int (*compar)(struct dirent **, struct dirent **))
+           int (*compar)(struct dirent **, struct dirent **),
+           char *errmsg, int errmsg_sz) {
 {
   int result = -1;
   DIR *dirp;
@@ -140,64 +145,62 @@ fl_scandir(const char *dir, struct dirent ***namelist,
   struct dirent *entryp, **entries, **p;
 
   entries = (struct dirent **) malloc(sizeof(*entries) * max);
-  if (NULL != entries)
+  if (NULL == entries) {
+    if (errmsg) fl_snprintf(errmsg, errmsg_sz, "out of memory");
+    return -1;
+  }
+
+  /* Open directory 'dir' (and verify that it really is a directory) */
+  dirp = opendir(dir);
+  if (NULL == dirp) {
+    if (errmsg) fl_snprintf(errmsg, errmsg_sz, "%s", strerror(errno));
+    return -1;
+  }
+
+  /* Read next directory entry */
+  while (!readentry(dirp, &entryp, &len))
   {
-    /* Open directory 'dir' (and verify that it really is a directory) */
-    dirp = opendir(dir);
-    if (NULL != dirp)
+    if (NULL == entryp)
     {
-      /* Read next directory entry */
-      while (!readentry(dirp, &entryp, &len))
-      {
-        if (NULL == entryp)
-        {
-          /* EOD => Return number of directory entries */
-          result = (int) num;
-          break;
-        }
-        /* Apply select function if there is one provided */
-        if (NULL != sel)  { if (!sel(entryp))  continue; }
-        entries[num++] = entryp;
-        if (num >= max)
-        {
-          /* Allocate exponentially increasing sized memory chunks */
-          if (INT_MAX / 2 >= (int) max)  { max *= (size_t) 2; }
-          else
-          {
-            errno = ENOMEM;
-            break;
-          }
-          p = (struct dirent **) realloc((void *) entries,
-                                         sizeof(*entries) * max);
-          if (NULL != p)  { entries = p; }
-          else  break;
-        }
-      }
-      closedir(dirp);
-      /*
-       * A standard compliant 'closedir()' is allowed to fail with 'EINTR',
-       * but the state of the directory structure is undefined in this case.
-       * Therefore we ignore the return value because we can't call 'closedir()'
-       * again and must hope that the system has released all resources.
-       */
+      /* EOD => Return number of directory entries */
+      result = (int) num;
+      break;
     }
-    /* Sort entries in array if there is a compare function provided */
-    if (NULL != compar)
-    {
-      qsort((void *) entries, num, sizeof(*entries),
-            (int (*)(const void *, const void *)) compar);
+    /* Apply select function if there is one provided */
+    if (NULL != sel)  { if (!sel(entryp))  continue; }
+    entries[num++] = entryp;
+    if (num >= max) {
+      /* Allocate exponentially increasing sized memory chunks */
+      if (INT_MAX / 2 >= (int) max)  { max *= (size_t) 2; }
+      else {
+        errno = ENOMEM;
+        break;
+      }
+      p = (struct dirent **) realloc((void *)entries, sizeof(*entries)*max);
+      if (NULL != p) { entries = p; }
+      else  break;
     }
-    *namelist = entries;
   }
-
+  closedir(dirp);
+  /*
+   * A standard compliant 'closedir()' is allowed to fail with 'EINTR',
+   * but the state of the directory structure is undefined in this case.
+   * Therefore we ignore the return value because we can't call 'closedir()'
+   * again and must hope that the system has released all resources.
+   */
+
+  /* Sort entries in array if there is a compare function provided */
+  if (NULL != compar) {
+    qsort((void *) entries, num, sizeof(*entries),
+          (int (*)(const void *, const void *)) compar);
+  }
+  *namelist = entries;
   /* Check for error */
-  if (-1 == result)
-  {
+  if (-1 == result) {
     /* Free all memory we have allocated */
     while (num--)  { free(entries[num]); }
     free(entries);
   }
-
   return result;
 }
 
diff --git a/src/scandir_win32.c b/src/scandir_win32.c
index 2663e30dc..822770234 100644
--- a/src/scandir_win32.c
+++ b/src/scandir_win32.c
@@ -15,16 +15,59 @@
  */
 
 #ifndef __CYGWIN__
-/* Emulation of POSIX scandir() call */
+/* Emulation of POSIX scandir() call with error messages */
 #include <FL/platform_types.h>
 #include <FL/fl_utf8.h>
 #include "flstring.h"
 #include <windows.h>
 #include <stdlib.h>
 
+/* Get error message string for last failed WIN32 operation
+ * in 'errmsg' (if non-NULL), string size limited to errmsg_sz.
+ *
+ * NOTE: Copied from: fluid/ExternalCodeEditor_WIN32.cxx
+ *
+ * TODO: Verify works in different languages, with utf8 strings.
+ * TODO: This should be made available globally to the FLTK internals, in case
+ *       other parts of FLTK need OS error messages..
+ */
+static void get_ms_errmsg(char *errmsg, int errmsg_sz) {
+  DWORD lastErr = GetLastError();
+  DWORD flags = FORMAT_MESSAGE_ALLOCATE_BUFFER |
+                FORMAT_MESSAGE_IGNORE_INSERTS  |
+                FORMAT_MESSAGE_FROM_SYSTEM;
+  DWORD langid = MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT);
+  LPSTR mbuf = 0;
+
+  // Early exit if parent doesn't want an errmsg
+  if (!errmsg || errmsg_sz<=0 ) return;
+  // Get error message from Windows
+  DWORD size = FormatMessageA(flags, 0, lastErr, langid, (LPSTR)&mbuf, 0, NULL);
+  if ( size == 0 ) {
+    fl_snprintf(errmsg, errmsg_sz, "Error #%lu", (unsigned long)lastErr);
+  } else {
+    int cnt = 0;
+    /* Copy mbuf -> errmsg, remove '\r's -- they screw up fl_alert()) */
+    for ( char *src=mbuf, *dst=errmsg; 1; src++ ) {
+      if ( *src == '\0' ) { *dst = '\0'; break; }
+      if ( *src != '\r' ) {
+        if ( ++cnt >= errmsg_sz ) { *dst = '\0'; break; } // trunc on overflow
+        *dst++ = *src;
+      }
+    }
+    LocalFree(mbuf);    /* Free the buffer allocated by the system */
+  }
+}
+
+/*
+ * This could use some docs.
+ *
+ * Returns -1 on error, errmsg returns error string (if non-NULL)
+ */
 int fl_scandir(const char *dirname, struct dirent ***namelist,
                int (*select)(struct dirent *),
-               int (*compar)(struct dirent **, struct dirent **)) {
+               int (*compar)(struct dirent **, struct dirent **),
+               char *errmsg, int errmsg_sz) {
   int len;
   char *findIn, *d, is_dir = 0;
   WIN32_FIND_DATAW findw;
@@ -33,9 +76,14 @@ int fl_scandir(const char *dirname, struct dirent ***namelist,
   struct dirent **dir = 0, *selectDir;
   unsigned long ret;
 
+  if (errmsg && errmsg_sz>0) errmsg[0] = '\0';
   len    = (int) strlen(dirname);
   findIn = (char *)malloc((size_t)(len+10));
-  if (!findIn) return -1;
+  if (!findIn) {
+    /* win32 malloc() docs: "malloc sets errno to ENOMEM if allocation fails" */
+    if (errmsg) fl_snprintf(errmsg, errmsg_sz, "%s", strerror(errno));
+    return -1;
+  }
   strcpy(findIn, dirname);
 
   /* #if defined(__GNUC__) */
@@ -49,7 +97,7 @@ int fl_scandir(const char *dirname, struct dirent ***namelist,
   if ((len>1) && (d[-1]=='.') && (d[-2]=='\\')) { d[-1] = '*'; is_dir = 1; }
   if (!is_dir) { /* this file may still be a directory that we need to list */
     DWORD attr = GetFileAttributes(findIn);
-    if (attr&FILE_ATTRIBUTE_DIRECTORY)
+    if (attr != INVALID_FILE_ATTRIBUTES && (attr & FILE_ATTRIBUTE_DIRECTORY) )
       strcpy(d, "\\*");
   }
   { /* Create a block to limit the scope while we find the initial "wide" filename */
@@ -64,55 +112,54 @@ int fl_scandir(const char *dirname, struct dirent ***namelist,
         h = FindFirstFileW(wbuf, &findw); /* get a handle to the first filename in the search */
         free(wbuf); /* release the "wide" buffer before the pointer goes out of scope */
   }
+
   if (h==INVALID_HANDLE_VALUE) {
     free(findIn);
     ret = GetLastError();
     if (ret != ERROR_NO_MORE_FILES) {
       nDir = -1;
+      get_ms_errmsg(errmsg, errmsg_sz);         /* return OS error msg */
     }
     *namelist = dir;
     return nDir;
   }
   do {
-        int l = (int) wcslen(findw.cFileName);
-        int dstlen = l * 5 + 1;
-        selectDir=(struct dirent*)malloc(sizeof(struct dirent)+dstlen);
+    int l = (int) wcslen(findw.cFileName);
+    int dstlen = l * 5 + 1;
+    selectDir=(struct dirent*)malloc(sizeof(struct dirent)+dstlen);
 
-     /* l = fl_unicode2utf(findw.cFileName, l, selectDir->d_name); */
-        l = fl_utf8fromwc(selectDir->d_name, dstlen, findw.cFileName, l);
+ /* l = fl_unicode2utf(findw.cFileName, l, selectDir->d_name); */
+    l = fl_utf8fromwc(selectDir->d_name, dstlen, findw.cFileName, l);
 
-        selectDir->d_name[l] = 0;
-        if (findw.dwFileAttributes & FILE_ATTRIBUTE_DIRECTORY) {
-                /* Append a trailing slash to directory names... */
-                strcat(selectDir->d_name, "/");
-        }
-        if (!select || (*select)(selectDir)) {
-                if (nDir==NDir) {
+    selectDir->d_name[l] = 0;
+    if (findw.dwFileAttributes & FILE_ATTRIBUTE_DIRECTORY) {
+      /* Append a trailing slash to directory names... */
+      strcat(selectDir->d_name, "/");
+    }
+    if (!select || (*select)(selectDir)) {
+      if (nDir==NDir) {
         struct dirent **tempDir = (struct dirent **)calloc(sizeof(struct dirent*), (size_t)(NDir+33));
         if (NDir) memcpy(tempDir, dir, sizeof(struct dirent*)*NDir);
         if (dir) free(dir);
         dir = tempDir;
         NDir += 32;
-                }
-                dir[nDir] = selectDir;
-                nDir++;
-                dir[nDir] = 0;
-        } else {
-                free(selectDir);
-        }
-   } while (FindNextFileW(h, &findw));
+      }
+      dir[nDir] = selectDir;
+      nDir++;
+      dir[nDir] = 0;
+    } else {
+      free(selectDir);
+    }
+  } while (FindNextFileW(h, &findw));
   ret = GetLastError();
   if (ret != ERROR_NO_MORE_FILES) {
     /* don't return an error code, because the dir list may still be valid
        up to this point */
   }
   FindClose(h);
-
   free (findIn);
-
   if (compar) qsort(dir, (size_t)nDir, sizeof(*dir),
                     (int(*)(const void*, const void*))compar);
-
   *namelist = dir;
   return nDir;
 }