diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 82974ce12..e46b92ea3 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -73,7 +73,7 @@ fedora-39-clang:
   image: $REGISTRY_HUB:fedora-39-clang
   variables:
     CONFIGURE_OPTS: "$CONFIGURE_MINI $CONFIGURE_FULL"
-    MAKE_FLAGS: "CFLAGS='-Werror -Wno-error=unused-but-set-variable' CXXFLAGS='-Werror -Wno-error=unused-but-set-variable'"
+    MAKE_FLAGS: "CFLAGS='-Werror -Wno-error=unused-but-set-variable' CXXFLAGS='-Werror'"
   <<: *compile_definition
 
 alpine-3.18-musl:
diff --git a/backend/bh.h b/backend/bh.h
index c96dc7918..1a4c7f14d 100644
--- a/backend/bh.h
+++ b/backend/bh.h
@@ -544,9 +544,8 @@ static SANE_String_Const paper_list[] =
   0
 };
 
-static /* inline */ int _is_host_little_endian(void);
 static /* inline */ int
-_is_host_little_endian()
+_is_host_little_endian(void)
 {
   SANE_Int val = 255;
   unsigned char *firstbyte = (unsigned char *) &val;
diff --git a/backend/dell1600n_net.c b/backend/dell1600n_net.c
index 9f2d48d43..45b0d2c76 100644
--- a/backend/dell1600n_net.c
+++ b/backend/dell1600n_net.c
@@ -907,7 +907,7 @@ sane_get_select_fd (SANE_Handle __sane_unused__ handle,
 
 /* Clears the contents of gKnownDevices and zeros it */
 void
-ClearKnownDevices ()
+ClearKnownDevices (void)
 {
 
   int i;
diff --git a/backend/genesys/genesys.cpp b/backend/genesys/genesys.cpp
index ab1367e9a..74ef74ecc 100644
--- a/backend/genesys/genesys.cpp
+++ b/backend/genesys/genesys.cpp
@@ -840,7 +840,8 @@ void scanner_move(Genesys_Device& dev, ScanMethod scan_method, unsigned steps, D
 
     // FIXME: should porbably wait for some timeout
     Status status;
-    for (unsigned i = 0;; ++i) {
+//    for (unsigned i = 0;; ++i) {
+    for(;;) {
         status = scanner_read_status(dev);
         if (status.is_feeding_finished || (
             direction == Direction::BACKWARD && status.is_at_home))
diff --git a/backend/hp-option.c b/backend/hp-option.c
index 6aed68006..99dad3115 100644
--- a/backend/hp-option.c
+++ b/backend/hp-option.c
@@ -3809,7 +3809,7 @@ hp_optset_isEnabled (HpOptSet this, HpData data, const char *name,
 SANE_Status
 sanei_hp_optset_download (HpOptSet this, HpData data, HpScsi scsi)
 {
-  int i, errcount = 0;
+  int i; //, errcount = 0;
 
   DBG(3, "Start downloading parameters to scanner\n");
 
@@ -3839,7 +3839,7 @@ sanei_hp_optset_download (HpOptSet this, HpData data, HpScsi scsi)
 
         if ( sanei_hp_scl_errcheck (scsi) != SANE_STATUS_GOOD )
         {
-          errcount++;
+          //errcount++;
           DBG(3, "Option %s generated scanner error\n",
               this->options[i]->descriptor->name);
 
diff --git a/backend/hp-scl.c b/backend/hp-scl.c
index 37a01e85d..8b6ada1ee 100644
--- a/backend/hp-scl.c
+++ b/backend/hp-scl.c
@@ -143,7 +143,7 @@ typedef struct
 
 /* Initialize structure where we remember out open file descriptors */
 void
-sanei_hp_init_openfd ()
+sanei_hp_init_openfd (void)
 {int iCount;
  memset (asHpOpenFd, 0, sizeof (asHpOpenFd));
 
diff --git a/backend/hp3900_config.c b/backend/hp3900_config.c
index c40920f00..9f64d3102 100644
--- a/backend/hp3900_config.c
+++ b/backend/hp3900_config.c
@@ -3813,7 +3813,7 @@ static SANE_Int cfg_timing_get(SANE_Int sensortype, SANE_Int tm, struct st_timin
 
 /** SEC: Motor curves ---------- */
 
-static SANE_Int *bq5550_motor()
+static SANE_Int *bq5550_motor(void)
 {
 	SANE_Int *rst = NULL;
 	SANE_Int steps[]  =
@@ -3835,7 +3835,7 @@ static SANE_Int *bq5550_motor()
 	return rst;
 }
 
-static SANE_Int *hp4370_motor()
+static SANE_Int *hp4370_motor(void)
 {
 	SANE_Int *rst = NULL;
 	SANE_Int steps[]  =
@@ -3936,7 +3936,7 @@ static SANE_Int *hp4370_motor()
 	return rst;
 }
 
-static SANE_Int *hp3970_motor()
+static SANE_Int *hp3970_motor(void)
 {
 	SANE_Int *rst = NULL;
 	SANE_Int steps[]  =
@@ -4037,7 +4037,7 @@ static SANE_Int *hp3970_motor()
 	return rst;
 }
 
-static SANE_Int *hp3800_motor()
+static SANE_Int *hp3800_motor(void)
 {
 	SANE_Int *rst = NULL;
 	SANE_Int steps[]  =
@@ -4182,7 +4182,7 @@ static SANE_Int *hp3800_motor()
 	return rst;
 }
 
-static SANE_Int *cfg_motorcurve_get()
+static SANE_Int *cfg_motorcurve_get(void)
 {
 	/* returns motor setting buffer for a device */
 
diff --git a/backend/hp3900_rts8822.c b/backend/hp3900_rts8822.c
index f74d58689..5cdfafe6a 100644
--- a/backend/hp3900_rts8822.c
+++ b/backend/hp3900_rts8822.c
@@ -618,7 +618,7 @@ RTS_Free (struct st_device *dev)
 }
 
 static struct st_device *
-RTS_Alloc ()
+RTS_Alloc (void)
 {
   /* this function allocates space for device's variable */
 
@@ -6304,7 +6304,7 @@ Gamma_GetTables (struct st_device *dev, SANE_Byte * Gamma_buffer)
 }
 
 static void
-Gamma_FreeTables ()
+Gamma_FreeTables (void)
 {
   SANE_Int c;
 
@@ -11319,7 +11319,7 @@ Head_Relocate (struct st_device *dev, SANE_Int speed, SANE_Int direction,
 }
 
 static SANE_Int
-Calib_CreateFixedBuffers ()
+Calib_CreateFixedBuffers (void)
 {
   SANE_Byte channel;
   SANE_Int ret;
@@ -14002,7 +14002,7 @@ Free_Constrains (struct st_device *dev)
 }
 
 static void
-RTS_DebugInit ()
+RTS_DebugInit (void)
 {
   /* Default values */
   RTS_Debug->dev_model = HP3970;
diff --git a/backend/hp3900_sane.c b/backend/hp3900_sane.c
index 5face5e4d..daf04f1d8 100644
--- a/backend/hp3900_sane.c
+++ b/backend/hp3900_sane.c
@@ -1752,7 +1752,7 @@ sane_init (SANE_Int * version_code, SANE_Auth_Callback authorize)
   SANE_Char line[PATH_MAX];
   SANE_Char *str = NULL;
   SANE_String_Const proper_str;
-  SANE_Int nline = 0;
+  //SANE_Int nline = 0;
 
   /* Initialize debug */
   DBG_INIT ();
@@ -1771,7 +1771,7 @@ sane_init (SANE_Int * version_code, SANE_Auth_Callback authorize)
     {
       while (sanei_config_read (line, sizeof (line), conf_fp))
 	{
-	  nline++;
+	  //nline++;
 	  if (str)
 	    free (str);
 
diff --git a/backend/hp5400_internal.c b/backend/hp5400_internal.c
index 95866c4ed..e78e9e507 100644
--- a/backend/hp5400_internal.c
+++ b/backend/hp5400_internal.c
@@ -109,7 +109,7 @@ static TScannerModel Model_HP54xx =
 
 HP5400_SANE_STATIC
 int
-InitHp5400_internal() {
+InitHp5400_internal(void) {
 
 	MatchVersions = malloc( sizeof(versionString) * numVersions );
 	strcpy( MatchVersions[0].strVersion, "SilitekIBlizd C3 ScannerV0.84");
@@ -121,7 +121,7 @@ InitHp5400_internal() {
 
 HP5400_SANE_STATIC
 int
-FreeHp5400_internal() {
+FreeHp5400_internal(void) {
 
 	free(MatchVersions);
 	MatchVersions = NULL;
diff --git a/backend/hpsj5s.c b/backend/hpsj5s.c
index 5ff506468..4c58dbfb4 100644
--- a/backend/hpsj5s.c
+++ b/backend/hpsj5s.c
@@ -648,7 +648,7 @@ DetectScanner (void)
 }
 
 static void
-StandByScanner ()
+StandByScanner (void)
 {
   WriteScannerRegister (0x74, 0x80);
   WriteScannerRegister (0x75, 0x0C);
@@ -678,7 +678,7 @@ SwitchHardwareState (SANE_Byte mask, SANE_Byte invert_mask)
 
 /*return value: 0 - no paper, 1 - paper loaded.*/
 static int
-CheckPaperPresent ()
+CheckPaperPresent (void)
 {
   if ((CallFunctionWithRetVal (0xB2) & 0x10) == 0)
     return 1;			/*Ok - paper present. */
@@ -686,7 +686,7 @@ CheckPaperPresent ()
 }
 
 static int
-ReleasePaper ()
+ReleasePaper (void)
 {
   int i;
 
@@ -874,7 +874,7 @@ TurnOnPaperPulling (enumColorDepth enColor, SANE_Word wResolution)
 }
 
 static void
-TurnOffPaperPulling ()
+TurnOffPaperPulling (void)
 {
   CallFunctionWithParameter (0x91, 0);
 }
@@ -884,7 +884,7 @@ TurnOffPaperPulling ()
         While paper not loaded this is base "white point".
 */
 static SANE_Byte
-GetCalibration ()
+GetCalibration (void)
 {
   int i;
   int Result;
@@ -1041,7 +1041,7 @@ PaperFeed (SANE_Word wLinesToFeed)
 
 /*For now we do no calibrate elements - just set maximum limits. FIX ME?*/
 static void
-CalibrateScanElements ()
+CalibrateScanElements (void)
 {
   /*Those arrays will be used in future for correct calibration. */
   /*Then we need to transfer UP brightness border, we use these registers */
@@ -1260,7 +1260,7 @@ CalibrateScanElements ()
 
 /*Returns 0 in case of fail and 1 in success.*/
 static int
-OutputCheck ()
+OutputCheck (void)
 {
   int i;
 
@@ -1277,7 +1277,7 @@ OutputCheck ()
 }
 
 static int
-InputCheck ()
+InputCheck (void)
 {
   int i;
   SANE_Byte Buffer[256];
@@ -1298,7 +1298,7 @@ InputCheck ()
 }
 
 static int
-CallCheck ()
+CallCheck (void)
 {
   int i;
   SANE_Byte Buffer[256];
@@ -1331,7 +1331,7 @@ CallCheck ()
 }
 
 static void
-LoadingPaletteToScanner ()
+LoadingPaletteToScanner (void)
 {
   /*For now we have statical gamma. */
   SANE_Byte Gamma[256];
@@ -1394,7 +1394,7 @@ CallFunctionWithRetVal (SANE_Byte Function)
 }
 
 static SANE_Byte
-ReadDataByte ()
+ReadDataByte (void)
 {
   SANE_Byte Result;
 
diff --git a/backend/lexmark_low.c b/backend/lexmark_low.c
index 246455f1f..a0237c7f6 100644
--- a/backend/lexmark_low.c
+++ b/backend/lexmark_low.c
@@ -2497,7 +2497,7 @@ sanei_lexmark_low_find_start_line (Lexmark_Device * dev)
    */
 
 
-  int blackLineCount = 0;
+//  int blackLineCount = 0;
   int whiteLineCount = 0;
   int blackByteCounter = 0;
   unsigned char max_byte = 0;
@@ -2838,14 +2838,14 @@ sanei_lexmark_low_find_start_line (Lexmark_Device * dev)
       if (blackByteCounter > 0)
 	{
 	  /* This was a black line */
-	  blackLineCount++;
+//	  blackLineCount++;
 	  whiteLineCount = 0;
 	}
       else
 	{
 	  /* This is a white line */
 	  whiteLineCount++;
-	  blackLineCount = 0;
+//	  blackLineCount = 0;
 	}
     }				/* end for buffer */
 
diff --git a/backend/lexmark_x2600.c b/backend/lexmark_x2600.c
index 610064ed8..08a5e3b21 100644
--- a/backend/lexmark_x2600.c
+++ b/backend/lexmark_x2600.c
@@ -624,7 +624,7 @@ attach_one (SANE_String_Const devname)
 }
 
 SANE_Status
-scan_devices(){
+scan_devices(void){
   DBG (2, "scan_devices\n");
   SANE_Char config_line[PATH_MAX];
   FILE *fp;
diff --git a/backend/mustek_scsi_pp.c b/backend/mustek_scsi_pp.c
index 98e3f20bd..860fca388 100644
--- a/backend/mustek_scsi_pp.c
+++ b/backend/mustek_scsi_pp.c
@@ -78,7 +78,7 @@ static int mustek_scsi_pp_timeout = 5000;
 
 /* FIXME: use same method as mustek.c ? */
 static int
-mustek_scsi_pp_get_time ()
+mustek_scsi_pp_get_time (void)
 {
   struct timeval tv;
   int retval;
diff --git a/backend/mustek_usb2.c b/backend/mustek_usb2.c
index cb511b9f4..785bdb363 100644
--- a/backend/mustek_usb2.c
+++ b/backend/mustek_usb2.c
@@ -537,7 +537,7 @@ Return value:
 	return FALSE
 ***********************************************************************/
 static SANE_Bool
-GetDeviceStatus ()
+GetDeviceStatus (void)
 {
   DBG (DBG_FUNC, "GetDeviceStatus: start\n");
   return MustScanner_GetScannerState ();
@@ -576,7 +576,7 @@ Return value:
 	return FALSE
 ***********************************************************************/
 static SANE_Bool
-CarriageHome ()
+CarriageHome (void)
 {
   DBG (DBG_FUNC, "CarriageHome: start\n");
   return MustScanner_BackHome ();
@@ -914,7 +914,7 @@ Return value:
 	return FALSE
 ***********************************************************************/
 static SANE_Bool
-StartScan ()
+StartScan (void)
 {
   DBG (DBG_FUNC, "StartScan: start\n");
   if (ST_Reflective == g_ScanType)
@@ -1099,7 +1099,7 @@ Return value:
 	return FALSE
 ***********************************************************************/
 static SANE_Bool
-StopScan ()
+StopScan (void)
 {
   SANE_Bool rt;
   int i;
@@ -1159,7 +1159,7 @@ Return value:
 	return FALSE
 ***********************************************************************/
 static SANE_Bool
-IsTAConnected ()
+IsTAConnected (void)
 {
   SANE_Bool hasTA;
 
diff --git a/backend/mustek_usb2_high.c b/backend/mustek_usb2_high.c
index e8b067f5f..cf0157bed 100644
--- a/backend/mustek_usb2_high.c
+++ b/backend/mustek_usb2_high.c
@@ -199,7 +199,7 @@ Return value:
 	return FALSE
 ***********************************************************************/
 static SANE_Bool
-MustScanner_Init ()
+MustScanner_Init (void)
 {
   DBG (DBG_FUNC, "MustScanner_Init: Call in\n");
 
@@ -260,7 +260,7 @@ Return value:
 	return FASLE
 ***********************************************************************/
 static SANE_Bool
-MustScanner_GetScannerState ()
+MustScanner_GetScannerState (void)
 {
 
   if (SANE_STATUS_GOOD != Asic_Open (&g_chip, g_pDeviceFile))
@@ -343,7 +343,7 @@ Return value:
 	return FALSE
 ***********************************************************************/
 static SANE_Bool
-MustScanner_BackHome ()
+MustScanner_BackHome (void)
 {
   DBG (DBG_FUNC, "MustScanner_BackHome: call in \n");
 
@@ -3081,7 +3081,7 @@ Return value:
 	the lines of scanned
 ***********************************************************************/
 static unsigned int
-GetScannedLines ()
+GetScannedLines (void)
 {
   unsigned int dwScannedLines = 0;
 
@@ -3103,7 +3103,7 @@ Return value:
 	the lines which pass to superstratum
 ***********************************************************************/
 static unsigned int
-GetReadyLines ()
+GetReadyLines (void)
 {
   unsigned int dwReadyLines = 0;
 
@@ -3143,7 +3143,7 @@ Return value:
 	none
 ***********************************************************************/
 static void
-AddReadyLines ()
+AddReadyLines (void)
 {
   pthread_mutex_lock (&g_readyLinesMutex);
   g_wtheReadyLines++;
diff --git a/backend/mustek_usb2_reflective.c b/backend/mustek_usb2_reflective.c
index ed3729676..e0b0d1031 100644
--- a/backend/mustek_usb2_reflective.c
+++ b/backend/mustek_usb2_reflective.c
@@ -72,7 +72,7 @@ Return value:
 	return FALSE
 ***********************************************************************/
 static SANE_Bool
-Reflective_Reset ()
+Reflective_Reset (void)
 {
   DBG (DBG_FUNC, "Reflective_Reset: call in\n");
 
@@ -576,7 +576,7 @@ Return value:
 	return FALSE
 ***********************************************************************/
 static SANE_Bool
-Reflective_AdjustAD ()
+Reflective_AdjustAD (void)
 {
   SANE_Byte * lpCalData;
   unsigned short wCalWidth;
@@ -1293,7 +1293,7 @@ Return value:
 	return FALSE
 ***********************************************************************/
 static SANE_Bool
-Reflective_StopScan ()
+Reflective_StopScan (void)
 {
   DBG (DBG_FUNC, "Reflective_StopScan: call in\n");
   if (!g_bOpened)
@@ -1338,7 +1338,7 @@ Return value:
 	return FALSE
 ***********************************************************************/
 static SANE_Bool
-Reflective_LineCalibration16Bits ()
+Reflective_LineCalibration16Bits (void)
 {
   SANE_Status status;
   SANE_Byte * lpWhiteData;
@@ -1758,7 +1758,7 @@ Return value:
 	return FALSE
 ***********************************************************************/
 static SANE_Bool
-Reflective_PrepareScan ()
+Reflective_PrepareScan (void)
 {
   g_wScanLinesPerBlock = g_dwBufferSize / g_BytesPerRow;
   g_wMaxScanLines = g_dwImageBufferSize / g_BytesPerRow;
diff --git a/backend/mustek_usb2_transparent.c b/backend/mustek_usb2_transparent.c
index 1c4b91b88..04b9eda19 100644
--- a/backend/mustek_usb2_transparent.c
+++ b/backend/mustek_usb2_transparent.c
@@ -73,7 +73,7 @@ Return value:
 	return FALSE
 ***********************************************************************/
 static SANE_Bool
-Transparent_Reset ()
+Transparent_Reset (void)
 {
   DBG (DBG_FUNC, "Transparent_Reset: call in\n");
 
@@ -502,7 +502,7 @@ Return value:
 	return FALSE
 ***********************************************************************/
 static SANE_Bool
-Transparent_StopScan ()
+Transparent_StopScan (void)
 {
   DBG (DBG_FUNC, "Transparent_StopScan: call in\n");
 
@@ -613,7 +613,7 @@ Return value:
 	return FALSE
 ***********************************************************************/
 static SANE_Bool
-Transparent_AdjustAD ()
+Transparent_AdjustAD (void)
 {
   SANE_Byte * lpCalData;
   unsigned short wCalWidth;
@@ -1659,7 +1659,7 @@ Return value:
 	return FALSE
 ***********************************************************************/
 static SANE_Bool
-Transparent_PrepareScan ()
+Transparent_PrepareScan (void)
 {
   DBG (DBG_FUNC, "Transparent_PrepareScan: call in\n");
 
diff --git a/backend/net.c b/backend/net.c
index 7b1ea05b8..bea22cccc 100644
--- a/backend/net.c
+++ b/backend/net.c
@@ -313,7 +313,7 @@ add_device (const char *name, Net_Device ** ndp)
 #endif /* NET_USES_AF_INDEP */
 
 /* Calls getpwuid_r(). The return value must be freed by the caller. */
-char* get_current_username()
+static char* get_current_username(void)
 {
   long bufsize = sysconf(_SC_GETPW_R_SIZE_MAX);
   if (bufsize == -1)
diff --git a/backend/plustek-pp_io.c b/backend/plustek-pp_io.c
index 45ffe8ba8..d979c8bde 100644
--- a/backend/plustek-pp_io.c
+++ b/backend/plustek-pp_io.c
@@ -422,7 +422,7 @@ static Bool ioP98OpenScanPath( pScanData ps )
 {
     Byte  tmp;
     ULong dw;
-	ULong dwTime = 1;
+//	ULong dwTime = 1;	// Doesn't seem to be used. [RL]
 
 	if( 0 == ps->IO.bOpenCount ) {
 
@@ -457,7 +457,7 @@ static Bool ioP98OpenScanPath( pScanData ps )
 				ps->IO.bOpenCount = 0;
 			}
 
-			dwTime++;
+//			dwTime++;	// Doesn't seem to be used. [RL]
 		}
 		DBG( DBG_IO, "ioP98OpenScanPath() failed!\n" );
 		return _FALSE;
diff --git a/backend/snapscan-usb.c b/backend/snapscan-usb.c
index 49cca1ad0..6e6751f0f 100644
--- a/backend/snapscan-usb.c
+++ b/backend/snapscan-usb.c
@@ -457,7 +457,7 @@ static int enqueue_bq(int fd,const void *src, size_t src_size)
     return 0;
 }
 
-static void dequeue_bq()
+static void dequeue_bq(void)
 {
     static const char me[] = "dequeue_bq";
     struct usb_busy_queue *tbqe;
diff --git a/backend/test.c b/backend/test.c
index 4663a16e4..bb2b64333 100644
--- a/backend/test.c
+++ b/backend/test.c
@@ -1255,7 +1255,7 @@ fail:
 }
 
 static void
-cleanup_initial_string_values ()
+cleanup_initial_string_values (void)
 {
   // Cleanup backing memory for initial values of string options.
   free (init_mode);
diff --git a/backend/umax_pp_low.c b/backend/umax_pp_low.c
index d1f3c16de..6d80d43b5 100644
--- a/backend/umax_pp_low.c
+++ b/backend/umax_pp_low.c
@@ -1496,7 +1496,7 @@ ECPFifoMode (void)
 /* wait for ack bit */
 /* return 1 on success, 0 on error */
 static int
-waitAck ()
+waitAck (void)
 {
   unsigned char breg = 0;
   int i = 0;
diff --git a/backend/umax_pp_mid.c b/backend/umax_pp_mid.c
index ef608ac1c..b5266b399 100644
--- a/backend/umax_pp_mid.c
+++ b/backend/umax_pp_mid.c
@@ -449,7 +449,7 @@ sanei_umax_pp_status (void)
 }
 
 int
-sanei_umax_pp_close ()
+sanei_umax_pp_close (void)
 {
 #ifdef HAVE_LINUX_PPDEV_H
   int fd;
diff --git a/backend/xerox_mfp-usb.c b/backend/xerox_mfp-usb.c
index 519aba611..d08323832 100644
--- a/backend/xerox_mfp-usb.c
+++ b/backend/xerox_mfp-usb.c
@@ -22,7 +22,7 @@
 extern int sanei_debug_xerox_mfp;
 
 static int
-xerox_need_clear_halt()
+xerox_need_clear_halt(void)
 {
     char *env;
     int workaround = 0;
diff --git a/include/sane/sanei_usb.h b/include/sane/sanei_usb.h
index 81032db60..f6597c828 100644
--- a/include/sane/sanei_usb.h
+++ b/include/sane/sanei_usb.h
@@ -217,12 +217,12 @@ extern SANE_Status sanei_usb_testing_enable_record(SANE_String_Const path,
  * Returns backend name for the file registered in sanei_usb_testing_enable.
  * The caller is responsible for freeing it.
  */
-extern SANE_String sanei_usb_testing_get_backend();
+extern SANE_String sanei_usb_testing_get_backend(void);
 
 /** Returns SANE_TRUE if replay testing mode is enabled, i.e. whether we are working with fake
  * scan data.
  */
-extern SANE_Bool sanei_usb_is_replay_mode_enabled();
+extern SANE_Bool sanei_usb_is_replay_mode_enabled(void);
 
 /** Clears currently recorded data.
 
@@ -235,7 +235,7 @@ extern SANE_Bool sanei_usb_is_replay_mode_enabled();
     This function may only be called when no USB devices are open, otherwise the behavior is
     unpredictable.
  */
-extern void sanei_usb_testing_record_clear();
+extern void sanei_usb_testing_record_clear(void);
 
 /** Records a debug message in the captured USB data if testing mode is enabled. If testing mode
  * is not enabled, this function does nothing.
diff --git a/sanei/sanei_config.c b/sanei/sanei_config.c
index 45f380337..fc883eee5 100644
--- a/sanei/sanei_config.c
+++ b/sanei/sanei_config.c
@@ -74,7 +74,7 @@
 static char *dir_list;
 
 const char *
-sanei_config_get_paths ()
+sanei_config_get_paths (void)
 {
 #ifdef __BEOS__
   char result[PATH_MAX];
diff --git a/sanei/sanei_pa4s2.c b/sanei/sanei_pa4s2.c
index 92fcf7f41..7de0aa65b 100644
--- a/sanei/sanei_pa4s2.c
+++ b/sanei/sanei_pa4s2.c
@@ -876,7 +876,7 @@ pa4s2_close (int fd, SANE_Status * status)
 }
 
 const char **
-sanei_pa4s2_devices()
+sanei_pa4s2_devices(void)
 {
 
   SANE_Status status;
diff --git a/sanei/sanei_scsi.c b/sanei/sanei_scsi.c
index 6eef9988d..d0aa167c4 100644
--- a/sanei/sanei_scsi.c
+++ b/sanei/sanei_scsi.c
@@ -2053,7 +2053,7 @@ issue (struct req *req)
     fdp->sane_qhead = fdp->sane_qtail = 0;
   }
 
-  void sanei_scsi_req_flush_all ()
+  void sanei_scsi_req_flush_all (void)
   {
     int fd, i, j = 0;
 
diff --git a/sanei/sanei_usb.c b/sanei/sanei_usb.c
index 850c3b369..f91cb793f 100644
--- a/sanei/sanei_usb.c
+++ b/sanei/sanei_usb.c
@@ -529,7 +529,7 @@ SANE_Status sanei_usb_testing_enable_replay(SANE_String_Const path,
     fail_test();                                                               \
   } while (0)
 
-void fail_test()
+static void fail_test(void)
 {
 }
 
@@ -671,12 +671,12 @@ static int sanei_xml_is_known_commands_end(xmlNode* node)
   return xmlStrcmp(node->name, (const xmlChar*)"known_commands_end") == 0;
 }
 
-static xmlNode* sanei_xml_peek_next_tx_node()
+static xmlNode* sanei_xml_peek_next_tx_node(void)
 {
   return testing_xml_next_tx_node;
 }
 
-static xmlNode* sanei_xml_get_next_tx_node()
+static xmlNode* sanei_xml_get_next_tx_node(void)
 {
   xmlNode* next = testing_xml_next_tx_node;
 
@@ -906,7 +906,7 @@ static void sanei_xml_record_seq(xmlNode* node)
     testing_last_known_seq = seq;
 }
 
-static void sanei_xml_break()
+static void sanei_xml_break(void)
 {
 }
 
@@ -1030,7 +1030,7 @@ static int sanei_usb_check_data_equal(xmlNode* node,
   return 0;
 }
 
-SANE_String sanei_usb_testing_get_backend()
+SANE_String sanei_usb_testing_get_backend(void)
 {
   if (testing_xml_doc == NULL)
     return NULL;
@@ -1054,7 +1054,7 @@ SANE_String sanei_usb_testing_get_backend()
   return ret;
 }
 
-SANE_Bool sanei_usb_is_replay_mode_enabled()
+SANE_Bool sanei_usb_is_replay_mode_enabled(void)
 {
   if (testing_mode == sanei_usb_testing_mode_replay)
     return SANE_TRUE;
@@ -1123,7 +1123,7 @@ static void sanei_usb_replay_debug_msg(SANE_String_Const message)
     }
 }
 
-extern void sanei_usb_testing_record_clear()
+extern void sanei_usb_testing_record_clear(void)
 {
   if (testing_mode != sanei_usb_testing_mode_record)
     return;
@@ -1153,7 +1153,7 @@ static void sanei_usb_add_endpoint(device_list_type* device,
                                    SANE_Int ep_address,
                                    SANE_Int ep_direction);
 
-static SANE_Status sanei_usb_testing_init()
+static SANE_Status sanei_usb_testing_init(void)
 {
   DBG_INIT();
 
@@ -1311,7 +1311,7 @@ static SANE_Status sanei_usb_testing_init()
   return SANE_STATUS_GOOD;
 }
 
-static void sanei_usb_testing_exit()
+static void sanei_usb_testing_exit(void)
 {
   if (testing_development_mode || testing_mode == sanei_usb_testing_mode_record)
     {