diff --git a/src/tests/testformats.c b/src/tests/testformats.c
index d308875f..c53a6eaa 100644
--- a/src/tests/testformats.c
+++ b/src/tests/testformats.c
@@ -130,7 +130,7 @@ static void context_set_file_path(T_IO_Context * context, const char * filepath)
 /**
  * test the Test_* functions
  */
-int Test_Formats(void)
+int Test_Formats(char * errmsg)
 {
   T_IO_Context context;
   char path[256];
@@ -146,7 +146,7 @@ int Test_Formats(void)
     f = fopen(path, "rb");
     if (f == NULL)
     {
-      GFX2_Log(GFX2_ERROR, "error opening %s\n", path);
+      snprintf(errmsg, ERRMSG_LENGTH, "error opening %s", path);
       return 0;
     }
     context_set_file_path(&context, path);
@@ -155,7 +155,7 @@ int Test_Formats(void)
     fclose(f);
     if (File_error != 0)
     {
-      GFX2_Log(GFX2_ERROR, "Test_%s failed for file %s\n", formats[i].name, formats[i].sample);
+      snprintf(errmsg, ERRMSG_LENGTH, "Test_%s failed for file %s", formats[i].name, formats[i].sample);
       return 0;
     }
     // now check that all other samples are not recognized
@@ -170,7 +170,7 @@ int Test_Formats(void)
       f = fopen(path, "rb");
       if (f == NULL)
       {
-        GFX2_Log(GFX2_ERROR, "error opening %s\n", path);
+        snprintf(errmsg, ERRMSG_LENGTH, "error opening %s", path);
         return 0;
       }
       context_set_file_path(&context, path);
@@ -179,7 +179,7 @@ int Test_Formats(void)
       fclose(f);
       if (File_error == 0)
       {
-        GFX2_Log(GFX2_ERROR, "Test_%s failed for file %s\n", formats[i].name, formats[j].sample);
+        snprintf(errmsg, ERRMSG_LENGTH, "Test_%s failed for file %s", formats[i].name, formats[j].sample);
         return 0;
       }
     }
@@ -193,7 +193,7 @@ int Test_Formats(void)
 /**
  * test the Load_* functions
  */
-int Test_Load(void)
+int Test_Load(char * errmsg)
 {
   T_IO_Context context;
   char path[256];
@@ -211,7 +211,7 @@ int Test_Load(void)
     formats[i].Load(&context);
     if (File_error != 0)
     {
-      GFX2_Log(GFX2_ERROR, "Load_%s failed for file %s\n", formats[i].name, formats[i].sample);
+      snprintf(errmsg, ERRMSG_LENGTH, "Load_%s failed for file %s", formats[i].name, formats[i].sample);
       return 0;
     }
     if (context.Surface)
@@ -230,7 +230,7 @@ int Test_Load(void)
 /**
  * Test the Save_* functions
  */
-int Test_Save(void)
+int Test_Save(char * errmsg)
 {
   T_IO_Context context;
   char path[256];
@@ -394,7 +394,7 @@ ret:
   return ok;
 }
 
-int Test_C64_Formats(void)
+int Test_C64_Formats(char * errmsg)
 {
   int i, j;
   int ok = 0;
diff --git a/src/tests/testio.c b/src/tests/testio.c
index 0a237bd6..b7c9511f 100644
--- a/src/tests/testio.c
+++ b/src/tests/testio.c
@@ -41,7 +41,7 @@
 #define random (long)rand
 #endif
 
-int Test_Read_Write_byte(void)
+int Test_Read_Write_byte(char * errmsg)
 {
   char path[256];
   FILE * f;
@@ -51,25 +51,25 @@ int Test_Read_Write_byte(void)
   f = fopen(path, "w+b");
   if (f == NULL)
   {
-    GFX2_Log(GFX2_ERROR, "error opening %s\n", path);
+    snprintf(errmsg, ERRMSG_LENGTH, "error opening %s", path);
     return 0;
   }
   if (!Write_byte(f, 42))
   {
-    GFX2_Log(GFX2_ERROR, "error writing\n");
+    snprintf(errmsg, ERRMSG_LENGTH, "error writing");
     fclose(f);
     return 0;
   }
   rewind(f);
   if (!Read_byte(f, &b))
   {
-    GFX2_Log(GFX2_ERROR, "error reading\n");
+    snprintf(errmsg, ERRMSG_LENGTH, "error reading");
     fclose(f);
     return 0;
   }
   if (b != 42)
   {
-    GFX2_Log(GFX2_ERROR, "Byte value mismatch\n");
+    snprintf(errmsg, ERRMSG_LENGTH, "Byte value mismatch 0x%02x != 0x%02x", b, 42);
     fclose(f);
     return 0;
   }
@@ -78,7 +78,7 @@ int Test_Read_Write_byte(void)
   return 1;
 }
 
-int Test_Read_Write_word(void)
+int Test_Read_Write_word(char * errmsg)
 {
   char path[256];
   FILE * f;
@@ -89,26 +89,26 @@ int Test_Read_Write_word(void)
   f = fopen(path, "w+b");
   if (f == NULL)
   {
-    GFX2_Log(GFX2_ERROR, "error opening %s\n", path);
+    snprintf(errmsg, ERRMSG_LENGTH, "error opening %s", path);
     return 0;
   }
   // write bytes 12 34 56 78 90
   if (!Write_word_le(f, 0x3412) || !Write_word_be(f, 0x5678) || !Write_byte(f, 0x90))
   {
-    GFX2_Log(GFX2_ERROR, "error writing\n");
+    snprintf(errmsg, ERRMSG_LENGTH, "error writing");
     fclose(f);
     return 0;
   }
   rewind(f);
   if (!Read_byte(f, &b) || !Read_word_be(f, &w1) || !Read_word_le(f, &w2))
   {
-    GFX2_Log(GFX2_ERROR, "error reading\n");
+    snprintf(errmsg, ERRMSG_LENGTH, "error reading");
     fclose(f);
     return 0;
   }
   if ((b != 0x12) || (w1 != 0x3456) || (w2 != 0x9078))
   {
-    GFX2_Log(GFX2_ERROR, "word values mismatch\n");
+    snprintf(errmsg, ERRMSG_LENGTH, "word values mismatch b=0x%02x w1=0x%04x w2=0x%04x", b, w1, w2);
     fclose(f);
     return 0;
   }
@@ -117,7 +117,7 @@ int Test_Read_Write_word(void)
   return 1;
 }
 
-int Test_Read_Write_dword(void)
+int Test_Read_Write_dword(char * errmsg)
 {
   char path[256];
   FILE * f;
@@ -128,26 +128,26 @@ int Test_Read_Write_dword(void)
   f = fopen(path, "w+b");
   if (f == NULL)
   {
-    GFX2_Log(GFX2_ERROR, "error opening %s\n", path);
+    snprintf(errmsg, ERRMSG_LENGTH, "error opening %s", path);
     return 0;
   }
   // write bytes 01 02 03 04 05 06 07 08 09
   if (!Write_dword_le(f, 0x04030201) || !Write_dword_be(f, 0x05060708) || !Write_byte(f, 9))
   {
-    GFX2_Log(GFX2_ERROR, "error writing\n");
+    snprintf(errmsg, ERRMSG_LENGTH, "error writing");
     fclose(f);
     return 0;
   }
   rewind(f);
   if (!Read_byte(f, &b) || !Read_dword_be(f, &dw1) || !Read_dword_le(f, &dw2))
   {
-    GFX2_Log(GFX2_ERROR, "error reading\n");
+    snprintf(errmsg, ERRMSG_LENGTH, "error reading");
     fclose(f);
     return 0;
   }
   if ((b != 0x1) || (dw1 != 0x02030405) || (dw2 != 0x09080706))
   {
-    GFX2_Log(GFX2_ERROR, "word values mismatch\n");
+    snprintf(errmsg, ERRMSG_LENGTH, "word values mismatch");
     fclose(f);
     return 0;
   }
@@ -156,7 +156,7 @@ int Test_Read_Write_dword(void)
   return 1;
 }
 
-int Test_Read_Write_bytes(void)
+int Test_Read_Write_bytes(char * errmsg)
 {
   char path[256];
   FILE * f;
@@ -168,7 +168,7 @@ int Test_Read_Write_bytes(void)
   f = fopen(path, "w+b");
   if (f == NULL)
   {
-    GFX2_Log(GFX2_ERROR, "error opening %s\n", path);
+    snprintf(errmsg, ERRMSG_LENGTH, "error opening %s", path);
     return 0;
   }
   b = (byte)random();
@@ -184,7 +184,7 @@ int Test_Read_Write_bytes(void)
   // write bytes
   if (!Write_bytes(f, buffer, len))
   {
-    GFX2_Log(GFX2_ERROR, "error writing\n");
+    snprintf(errmsg, ERRMSG_LENGTH, "error writing");
     free(buffer);
     fclose(f);
     return 0;
@@ -193,14 +193,14 @@ int Test_Read_Write_bytes(void)
   memset(buffer, 0, len);
   if (!Read_bytes(f, buffer, len))
   {
-    GFX2_Log(GFX2_ERROR, "error reading\n");
+    snprintf(errmsg, ERRMSG_LENGTH, "error reading");
     free(buffer);
     fclose(f);
     return 0;
   }
   if (!GFX2_is_mem_filled_with(buffer, b, len))
   {
-    GFX2_Log(GFX2_ERROR, "byte values mismatch\n");
+    snprintf(errmsg, ERRMSG_LENGTH, "byte values mismatch");
     free(buffer);
     fclose(f);
     return 0;
@@ -208,7 +208,7 @@ int Test_Read_Write_bytes(void)
   free(buffer);
   if (File_length_file(f) != len)
   {
-    GFX2_Log(GFX2_ERROR, "File_length_file() returned %lu (should be %lu)\n",
+    snprintf(errmsg, ERRMSG_LENGTH, "File_length_file() returned %lu (should be %lu)",
              File_length_file(f), len);
     fclose(f);
     return 0;
@@ -216,7 +216,7 @@ int Test_Read_Write_bytes(void)
   fclose(f);
   if (File_length(path) != len)
   {
-    GFX2_Log(GFX2_ERROR, "File_length() returned %lu (should be %lu)\n",
+    snprintf(errmsg, ERRMSG_LENGTH, "File_length() returned %lu (should be %lu)",
              File_length(path), len);
     return 0;
   }
@@ -249,7 +249,7 @@ static void directory_callback(void * pdata, const char * filename, const word *
     data->result = 1;
 }
 
-int Test_File_exists(void)
+int Test_File_exists(char * errmsg)
 {
   struct dir_callback_data data;
   char path[256];
@@ -257,40 +257,40 @@ int Test_File_exists(void)
 
   if (!Directory_exists(tmpdir))
   {
-    GFX2_Log(GFX2_ERROR, "Directory_exists(\"%s\") FALSE\n", tmpdir);
+    snprintf(errmsg, ERRMSG_LENGTH, "Directory_exists(\"%s\") FALSE", tmpdir);
     return 0;
   }
   snprintf(path, sizeof(path), "%s%sX%07x.tmp", tmpdir, PATH_SEPARATOR, (unsigned)(random() & 0x0fffffff));
   if (File_exists(path))
   {
-    GFX2_Log(GFX2_ERROR, "File_exists(\"%s\") TRUE before creation\n", path);
+    snprintf(errmsg, ERRMSG_LENGTH, "File_exists(\"%s\") TRUE before creation", path);
     return 0;
   }
   f = fopen(path, "w");
   if (f == NULL)
   {
-    GFX2_Log(GFX2_ERROR, "fopen(\"%s\", \"wb\") FAILED\n", path);
+    snprintf(errmsg, ERRMSG_LENGTH, "fopen(\"%s\", \"wb\") FAILED", path);
     return 0;
   }
   fputs("test\n", f);
   fclose(f);
   if (!File_exists(path))
   {
-    GFX2_Log(GFX2_ERROR, "File_exists(\"%s\") FALSE after create\n", path);
+    snprintf(errmsg, ERRMSG_LENGTH, "File_exists(\"%s\") FALSE after create", path);
     return 0;
   }
   data.result = 0;
   data.filename = Extract_filename(NULL, path);
   if (data.filename == NULL)
   {
-    GFX2_Log(GFX2_ERROR, "Extract_filename(NULL, \"%s\") FAILED\n", path);
+    snprintf(errmsg, ERRMSG_LENGTH, "Extract_filename(NULL, \"%s\") FAILED", path);
     return 0;
   }
   GFX2_Log(GFX2_DEBUG, "Listing directory with For_each_directory_entry(\"%s\"):\n", tmpdir);
   For_each_directory_entry(tmpdir, &data, directory_callback);
   if (!data.result)
   {
-    GFX2_Log(GFX2_ERROR, "Failed to find file \"%s\" in directory \"%s\"\n", data.filename, tmpdir);
+    snprintf(errmsg, ERRMSG_LENGTH, "Failed to find file \"%s\" in directory \"%s\"", data.filename, tmpdir);
     free(data.filename);
     return 0;
   }
@@ -298,20 +298,20 @@ int Test_File_exists(void)
   Remove_path(path);
   if (File_exists(path))
   {
-    GFX2_Log(GFX2_ERROR, "File_exists(\"%s\") TRUE after Remove\n", path);
+    snprintf(errmsg, ERRMSG_LENGTH, "File_exists(\"%s\") TRUE after Remove", path);
     return 0;
   }
   return 1;
 }
 
-int Test_Realpath(void)
+int Test_Realpath(char * errmsg)
 {
   char * path;
 
   path = Realpath(tmpdir, NULL);
   if (path == NULL)
   {
-    GFX2_Log(GFX2_ERROR, "Realpath(\"%s\") returned NULL\n", tmpdir);
+    snprintf(errmsg, ERRMSG_LENGTH, "Realpath(\"%s\") returned NULL", tmpdir);
     return 0;
   }
   GFX2_Log(GFX2_DEBUG, "Realpath(\"%s\") returned \"%s\"\n", tmpdir, path);
diff --git a/src/tests/testmain.c b/src/tests/testmain.c
index 13c46cfe..85967e16 100644
--- a/src/tests/testmain.c
+++ b/src/tests/testmain.c
@@ -99,7 +99,7 @@ dword Key;
 char tmpdir[256];
 
 static const struct {
-  int (*test_func)(void);
+  int (*test_func)(char * errmsg);
   const char * test_name;
 } tests[] = {
 #define TEST(func) { Test_ ## func, # func },
@@ -174,6 +174,7 @@ int main(int argc, char * * argv)
   int fail_early = 0;
   const char * xml_path = "test-report.xml";
   FILE * xml; // see https://llg.cubic.org/docs/junit/
+  char errmsg[ERRMSG_LENGTH];
 
   for (i = 1; i < argc; i++)
   {
@@ -209,17 +210,26 @@ int main(int argc, char * * argv)
   }
   fprintf(xml, "<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n");
   fprintf(xml, "<testsuite name=\"GrafX2\" tests=\"%lu\">\n", TEST_COUNT);
-  for (i = 0; i < (int)TEST_COUNT && !(fail_early && fail > 0) ; i++)
+  for (i = 0; i < (int)TEST_COUNT; i++)
   {
-    printf("Testing %s :\n", tests[i].test_name);
     fprintf(xml, "  <testcase name=\"%s\" classname=\"%s\">\n", tests[i].test_name, "GrafX2");
-    r[i] = tests[i].test_func();
-    if (r[i])
-      printf(ESC_GREEN "OK" ESC_RESET "\n");
-    else {
-      printf(ESC_RED "FAILED" ESC_RESET "\n");
-      fail++;
-      fprintf(xml, "    <failure message=\"test failure\">failure details</failure>\n");
+    if (fail_early && fail > 0)
+      fprintf(xml, "    <skipped />\n");
+    else
+    {
+      printf("Testing %s :\n", tests[i].test_name);
+      errmsg[0] = '\0';
+      r[i] = tests[i].test_func(errmsg);
+      if (r[i])
+        printf(ESC_GREEN "OK" ESC_RESET "\n");
+      else
+      {
+        fprintf(stderr, ESC_RED "%s" ESC_RESET "\n", errmsg);
+        printf(ESC_RED "FAILED" ESC_RESET "\n");
+        fail++;
+        fprintf(xml, "    <failure message='%s'><!-- failure details --></failure>\n",
+                errmsg);
+      }
     }
     fprintf(xml, "  </testcase>\n");
   }
diff --git a/src/tests/testop_c.c b/src/tests/testop_c.c
index eb61bc81..a68e62ba 100644
--- a/src/tests/testop_c.c
+++ b/src/tests/testop_c.c
@@ -28,10 +28,11 @@
 ///
 #include <stdio.h>
 #include <string.h>
+#include "tests.h"
 #include "../op_c.h"
 #include "../gfx2log.h"
 
-int Test_Convert_24b_bitmap_to_256(void)
+int Test_Convert_24b_bitmap_to_256(char * msg)
 {
   T_Palette palette;
   byte dest[256];
diff --git a/src/tests/tests.c b/src/tests/tests.c
index 1360ddbd..2cf208e8 100644
--- a/src/tests/tests.c
+++ b/src/tests/tests.c
@@ -48,7 +48,7 @@ unsigned int MOTO_MAP_pack(byte * packed, const byte * unpacked, unsigned int un
 /**
  * Tests for MOTO_MAP_pack()
  */
-int Test_MOTO_MAP_pack(void)
+int Test_MOTO_MAP_pack(char * errmsg)
 {
   unsigned int i, j;
   byte buffer[1024];
@@ -89,7 +89,8 @@ int Test_MOTO_MAP_pack(void)
     }
     if (unpacked_len != original_len || 0 != memcmp(tests[i], buffer2, original_len))
     {
-      GFX2_Log(GFX2_ERROR, "*** %u %s != %u %s ***\n", original_len, tests[i], unpacked_len, buffer2);
+      snprintf(errmsg, ERRMSG_LENGTH, "(un)pack error : %u %s != %u %s",
+               original_len, tests[i], unpacked_len, buffer2);
       return 0;  // test failed
     }
   }
@@ -99,7 +100,7 @@ int Test_MOTO_MAP_pack(void)
 /**
  * Test for Test_CPC_compare_colors()
  */
-int Test_CPC_compare_colors(void)
+int Test_CPC_compare_colors(char * errmsg)
 {
   unsigned int r, g, b;
   T_Components c1, c2;
@@ -113,13 +114,16 @@ int Test_CPC_compare_colors(void)
         c1.G = g * 0x11;
         c1.B = b * 0x11;
         if (!CPC_compare_colors(&c1, &c1))
-          return 0; // same colors should be recognized as identical !!!
+        {
+          snprintf(errmsg, ERRMSG_LENGTH, "same colors should be recognized as identical");
+          return 0;
+        }
         c2.R = ((r + 6) & 15) * 0x11;
         c2.G = c1.G;
         c2.B = c1.B;
         if (CPC_compare_colors(&c1, &c2))
         {
-          GFX2_Log(GFX2_ERROR, "#%02x%02x%02x <> #%02x%02x%02x\n",
+          snprintf(errmsg, ERRMSG_LENGTH, "#%02x%02x%02x <> #%02x%02x%02x",
                    c1.R, c1.G, c1.B, c2.R, c2.G, c2.B);
           return 0; // Should be differents !
         }
@@ -127,7 +131,7 @@ int Test_CPC_compare_colors(void)
         c2.G = ((g + 6) & 15) * 0x11;
         if (CPC_compare_colors(&c1, &c2))
         {
-          GFX2_Log(GFX2_ERROR, "#%02x%02x%02x <> #%02x%02x%02x\n",
+          snprintf(errmsg, ERRMSG_LENGTH, "#%02x%02x%02x <> #%02x%02x%02x",
                    c1.R, c1.G, c1.B, c2.R, c2.G, c2.B);
           return 0; // Should be differents !
         }
@@ -135,7 +139,7 @@ int Test_CPC_compare_colors(void)
         c2.B = ((b + 6) & 15) * 0x11;
         if (CPC_compare_colors(&c1, &c2))
         {
-          GFX2_Log(GFX2_ERROR, "#%02x%02x%02x <> #%02x%02x%02x\n",
+          snprintf(errmsg, ERRMSG_LENGTH, "#%02x%02x%02x <> #%02x%02x%02x",
                    c1.R, c1.G, c1.B, c2.R, c2.G, c2.B);
           return 0; // Should be differents !
         }
@@ -149,7 +153,7 @@ int Test_CPC_compare_colors(void)
  * Tests for the packbits compression used in IFF ILBM, etc.
  * see http://fileformats.archiveteam.org/wiki/PackBits
  */
-int Test_Packbits(void)
+int Test_Packbits(char * errmsg)
 {
   char tempfilename[256];
   FILE * f;
@@ -187,7 +191,7 @@ int Test_Packbits(void)
   f = fopen(tempfilename, "wb");
   if (f == NULL)
   {
-    GFX2_Log(GFX2_ERROR, "Failed to open %s for writing\n", tempfilename);
+    snprintf(errmsg, ERRMSG_LENGTH, "Failed to open %s for writing", tempfilename);
     return 0;
   }
 
@@ -199,14 +203,14 @@ int Test_Packbits(void)
     {
       if (PackBits_pack_add(&pb_data, (byte)tests[i][j]) < 0)
       {
-        GFX2_Log(GFX2_ERROR, "PackBits_pack_add() failed\n");
+        snprintf(errmsg, ERRMSG_LENGTH, "PackBits_pack_add() failed");
         return 0;
       }
       unpacked++;
     }
     if (PackBits_pack_flush(&pb_data) < 0)
     {
-      GFX2_Log(GFX2_ERROR, "PackBits_pack_flush() failed\n");
+      snprintf(errmsg, ERRMSG_LENGTH, "PackBits_pack_flush() failed");
       return 0;
     }
   }
@@ -214,7 +218,7 @@ int Test_Packbits(void)
   fclose(f);
   GFX2_Log(GFX2_DEBUG, "Compressed %ld bytes to %ld\n", unpacked, packed);
   if (packed > best_packed) {
-    GFX2_Log(GFX2_ERROR, "*** Packbits less efficient as expected (%ld > %ld bytes) ***\n",
+    snprintf(errmsg, ERRMSG_LENGTH, "Packbits less efficient than expected (%ld > %ld bytes)",
              packed, best_packed);
     return 0;
   }
@@ -223,7 +227,7 @@ int Test_Packbits(void)
   f = fopen(tempfilename, "rb");
   if (f == NULL)
   {
-    GFX2_Log(GFX2_ERROR, "Failed to open %s for reading\n", tempfilename);
+    snprintf(errmsg, ERRMSG_LENGTH, "Failed to open %s for reading", tempfilename);
     return 0;
   }
   for (i = 0; tests[i]; i++)
@@ -232,7 +236,7 @@ int Test_Packbits(void)
     memset(buffer, 0x80, len);
     if (PackBits_unpack_from_file(f, buffer, len) != PACKBITS_UNPACK_OK)
     {
-      GFX2_Log(GFX2_ERROR, "PackBits_unpack_from_file() failed\n");
+      snprintf(errmsg, ERRMSG_LENGTH, "PackBits_unpack_from_file() failed");
       return 0;
     }
     if (memcmp(buffer, tests[i], len) != 0)
@@ -240,6 +244,7 @@ int Test_Packbits(void)
       GFX2_Log(GFX2_ERROR, "uncompressed stream mismatch !\n");
       GFX2_LogHexDump(GFX2_ERROR, "original ", (const byte *)tests[i], 0, len);
       GFX2_LogHexDump(GFX2_ERROR, "unpacked ", buffer, 0, len);
+      snprintf(errmsg, ERRMSG_LENGTH, "uncompressed stream mismatch");
       return 0;
     }
   }
diff --git a/src/tests/tests.h b/src/tests/tests.h
index edf4174a..088a9480 100644
--- a/src/tests/tests.h
+++ b/src/tests/tests.h
@@ -22,7 +22,9 @@
 #ifndef TESTS_H_INCLUDED
 #define TESTS_H_INCLUDED
 
-#define TEST(func) int Test_ ## func (void);
+#define ERRMSG_LENGTH (256)
+
+#define TEST(func) int Test_ ## func (char *);
 #include "testlist.h"
 #undef TEST