From 2bf7b7919c22446bb71fd0fa19fd9e11b0e29ded Mon Sep 17 00:00:00 2001
From: Samuel Gomes <samuel_gomes@outlook.com>
Date: Thu, 4 Jan 2024 04:46:14 +0530
Subject: [PATCH] Make _FILES$() throw an error if it called with an argument
 the first time

---
 internal/c/libqb/src/filesystem.cpp | 22 ++++++++++++++++++----
 1 file changed, 18 insertions(+), 4 deletions(-)

diff --git a/internal/c/libqb/src/filesystem.cpp b/internal/c/libqb/src/filesystem.cpp
index 4c6b1edfc..3ac184b6b 100644
--- a/internal/c/libqb/src/filesystem.cpp
+++ b/internal/c/libqb/src/filesystem.cpp
@@ -625,10 +625,13 @@ qbs *func__files(qbs *qbsFileSpec, int32_t passed) {
     if (passed) {
         std::string fileSpec(reinterpret_cast<char *>(qbsFileSpec->chr), qbsFileSpec->len);
 
-        if (FS_DirectoryExists(filepath_fix_directory(fileSpec)))
+        if (FS_DirectoryExists(filepath_fix_directory(fileSpec))) {
             directory = fileSpec;
-        else
+        } else {
             filepath_split(fileSpec, directory, pathName); // split the file path
+            if (directory.empty())
+                directory = "./";
+        }
 
         entry = FS_GetDirectoryEntryName(fileSpec.c_str());
 
@@ -639,7 +642,15 @@ qbs *func__files(qbs *qbsFileSpec, int32_t passed) {
             return qbsFinal;
         }
     } else {
-        entry = FS_GetDirectoryEntryName(nullptr);
+        // Check if we've been called the first time without a filespec
+        if (directory.empty()) {
+            // This is per MS BASIC PDS 7.1 and VBDOS 1.0 behavior
+            qbsFinal = qbs_new(0, 1);
+            error(QB_ERROR_ILLEGAL_FUNCTION_CALL);
+            return qbsFinal;
+        }
+
+        entry = FS_GetDirectoryEntryName(nullptr); // get the next entry
     }
 
     filepath_join(pathName, directory, entry);
@@ -655,6 +666,9 @@ qbs *func__files(qbs *qbsFileSpec, int32_t passed) {
         memcpy(qbsFinal->chr, entry, size);
     }
 
+    if (!size)
+        directory.clear(); // clear the directory string since we are done with the session
+
     return qbsFinal;
 }
 
@@ -785,7 +799,7 @@ void sub_files(qbs *str, int32_t passed) {
         directory = FS_GetFQN(fileSpec.c_str());
     }
 
-    if (!directory.size()) {
+    if (directory.empty()) {
         // Invalid filespec
         error(QB_ERROR_ILLEGAL_FUNCTION_CALL);
         return;