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()

2 files changed, 22 insertions, 4 deletions

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;