diff --git a/extlib/libdxfrw b/extlib/libdxfrw
index e389be8..8f95895 160000
--- a/extlib/libdxfrw
+++ b/extlib/libdxfrw
@@ -1 +1 @@
-Subproject commit e389be82d060ee923b17b9bc574438f3272ef2ad
+Subproject commit 8f958955f54668c142ded760dc951ffd16d9c71b
diff --git a/src/exportvector.cpp b/src/exportvector.cpp
index 1b76b8e..c4f70bd 100644
--- a/src/exportvector.cpp
+++ b/src/exportvector.cpp
@@ -384,7 +384,7 @@ public:
                         Vector dna = norm.Cross(da).WithMagnitude(1.0);
 
                         double thetaf = acos(da.DirectionCosineWith(db));
-                        
+
                         // Calculate median
                         Vector m = da.WithMagnitude(1.0).ScaledBy(cos(thetaf/2)).Plus(
                                    dna.ScaledBy(sin(thetaf/2)));
@@ -684,12 +684,19 @@ void DxfFileWriter::Bezier(SBezier *sb) {
     paths.back().beziers.push_back(sb);
 }
 
-void DxfFileWriter::FinishAndCloseFile(void) {
-    dxfRW dxf(filename.c_str());
+void DxfFileWriter::FinishAndCloseFile() {
+    dxfRW dxf;
+
     DxfWriteInterface interface(this, &dxf);
-    dxf.write(&interface, DRW::AC1021, false);
+    std::stringstream stream;
+    dxf.write(stream, &interface, DRW::AC1021, /*bin=*/false);
     paths.clear();
     constraint = NULL;
+
+    if(!WriteFile(filename, stream.str())) {
+        Error("Couldn't write to '%s'", filename.c_str());
+        return;
+    }
 }
 
 bool DxfFileWriter::NeedToOutput(Constraint *c) {
diff --git a/src/importdxf.cpp b/src/importdxf.cpp
index 5b7d6bd..dbd4e47 100644
--- a/src/importdxf.cpp
+++ b/src/importdxf.cpp
@@ -902,13 +902,21 @@ public:
 };
 
 void ImportDxf(const std::string &filename) {
-    SS.UndoRemember();
-    dxfRW dxf(filename.c_str());
     DxfReadInterface interface;
     interface.clearBlockTransform();
-    if(!dxf.read(&interface, false)) {
-        Error("Corrupted DXF file!");
+
+    std::string data;
+    if(!ReadFile(filename, &data)) {
+        Error("Couldn't read from '%s'", filename.c_str());
+        return;
     }
+
+    SS.UndoRemember();
+    std::stringstream stream(data);
+    if(!dxfRW().read(stream, &interface, /*ext=*/false)) {
+        Error("Corrupted DXF file.");
+    }
+
     if(interface.unknownEntities > 0) {
         Message(ssprintf("%u DXF entities of unknown type were ignored.",
                          interface.unknownEntities).c_str());
@@ -916,13 +924,21 @@ void ImportDxf(const std::string &filename) {
 }
 
 void ImportDwg(const std::string &filename) {
-    SS.UndoRemember();
-    dwgR dwg(filename.c_str());
     DxfReadInterface interface;
     interface.clearBlockTransform();
-    if(!dwg.read(&interface, false)) {
-        Error("Corrupted DWG file!");
+
+    std::string data;
+    if(!ReadFile(filename, &data)) {
+        Error("Couldn't read from '%s'", filename.c_str());
+        return;
     }
+
+    SS.UndoRemember();
+    std::stringstream stream(data);
+    if(!dwgR().read(stream, &interface, /*ext=*/false)) {
+        Error("Corrupted DWG file.");
+    }
+
     if(interface.unknownEntities > 0) {
         Message(ssprintf("%u DWG entities of unknown type were ignored.",
                          interface.unknownEntities).c_str());
diff --git a/src/solvespace.h b/src/solvespace.h
index 46debb9..a543d5e 100644
--- a/src/solvespace.h
+++ b/src/solvespace.h
@@ -24,6 +24,7 @@
 #include <unordered_map>
 #include <map>
 #include <set>
+#include <sstream>
 #ifdef WIN32
 #   include <windows.h> // required by GL headers
 #endif
@@ -135,6 +136,7 @@ class RgbaColor;
 #endif
 
 FILE *ssfopen(const std::string &filename, const char *mode);
+std::fstream ssfstream(const std::string &filename, std::ios_base::openmode mode);
 void ssremove(const std::string &filename);
 
 #define MAX_RECENT 8
@@ -374,6 +376,8 @@ void MakeMatrix(double *mat, double a11, double a12, double a13, double a14,
                              double a41, double a42, double a43, double a44);
 std::string MakeAcceleratorLabel(int accel);
 bool FilenameHasExtension(const std::string &str, const char *ext);
+bool ReadFile(const std::string &filename, std::string *data);
+bool WriteFile(const std::string &filename, const std::string &data);
 void Message(const char *str, ...);
 void Error(const char *str, ...);
 void CnfFreezeBool(bool v, const std::string &name);
diff --git a/src/util.cpp b/src/util.cpp
index f1c50e4..847f378 100644
--- a/src/util.cpp
+++ b/src/util.cpp
@@ -61,6 +61,33 @@ bool SolveSpace::FilenameHasExtension(const std::string &str, const char *ext)
     return true;
 }
 
+bool SolveSpace::ReadFile(const std::string &filename, std::string *data)
+{
+    FILE *f = ssfopen(filename.c_str(), "rb");
+    if(f == NULL)
+        return false;
+
+    fseek(f, 0, SEEK_END);
+    data->resize(ftell(f));
+    fseek(f, 0, SEEK_SET);
+    fread(&(*data)[0], 1, data->size(), f);
+    fclose(f);
+
+    return true;
+}
+
+bool SolveSpace::WriteFile(const std::string &filename, const std::string &data)
+{
+    FILE *f = ssfopen(filename.c_str(), "wb");
+    if(f == NULL)
+        return false;
+
+    fwrite(&data[0], 1, data.size(), f);
+    fclose(f);
+
+    return true;
+}
+
 void SolveSpace::MakeMatrix(double *mat,
                             double a11, double a12, double a13, double a14,
                             double a21, double a22, double a23, double a24,
diff --git a/src/win32/w32util.cpp b/src/win32/w32util.cpp
index 20f3ec6..cba9c78 100644
--- a/src/win32/w32util.cpp
+++ b/src/win32/w32util.cpp
@@ -67,7 +67,7 @@ std::wstring Widen(const std::string &in)
     return out;
 }
 
-FILE *ssfopen(const std::string &filename, const char *mode)
+static std::string MakeUNCFilename(const std::string &filename)
 {
     // Prepend \\?\ UNC prefix unless already an UNC path.
     // We never try to fopen paths that are not absolute or
@@ -76,9 +76,13 @@ FILE *ssfopen(const std::string &filename, const char *mode)
     std::string uncFilename = filename;
     if(uncFilename.substr(0, 2) != "\\\\")
         uncFilename = "\\\\?\\" + uncFilename;
+    return uncFilename;
+}
 
+FILE *ssfopen(const std::string &filename, const char *mode)
+{
     if(filename.length() != strlen(filename.c_str())) oops();
-    return _wfopen(Widen(uncFilename).c_str(), Widen(mode).c_str());
+    return _wfopen(Widen(MakeUNCFilename(filename)).c_str(), Widen(mode).c_str());
 }
 
 void ssremove(const std::string &filename)