diff --git a/calma/calma.h b/calma/calma.h
index d5f6363e..422cd344 100644
--- a/calma/calma.h
+++ b/calma/calma.h
@@ -65,7 +65,7 @@ extern bool CalmaWrite(CellDef *rootDef, FILE *f);
 extern void CalmaReadFile(FILETYPE file, char *filename);
 extern void CalmaTechInit(void);
 extern bool CalmaGenerateArray(FILE *f, TileType type, int llx, int lly, int pitch, int cols, int rows);
-extern void CalmaReadError(char *format, ...);
+extern void CalmaReadError(char *format, ...) ATTR_FORMAT_PRINTF_1;
 
 /* C99 compat */
 extern void calmaDelContacts(void);
diff --git a/cif/CIFread.h b/cif/CIFread.h
index c5400391..39fdb0e3 100644
--- a/cif/CIFread.h
+++ b/cif/CIFread.h
@@ -188,8 +188,8 @@ extern int CIFEdgeDirection(CIFPath *first, CIFPath *last);
 
 /* Variable argument procedures require complete prototype */
 
-extern void CIFReadError(char *format, ...);
-extern void CIFReadWarning(char *format, ...);
+extern void CIFReadError(char *format, ...) ATTR_FORMAT_PRINTF_1;
+extern void CIFReadWarning(char *format, ...) ATTR_FORMAT_PRINTF_1;
 
 /* Variables shared by the CIF-reading modules, see CIFreadutils.c
  * for more details:
diff --git a/lef/lefInt.h b/lef/lefInt.h
index 21e2c88b..c91e8533 100644
--- a/lef/lefInt.h
+++ b/lef/lefInt.h
@@ -162,8 +162,8 @@ void LefReadLayerSection();
 
 LefMapping *defMakeInverseLayerMap();
 
-void LefError(int, char *, ...);	/* Variable argument procedure requires */
-					/* parameter list.			*/
+/* Variable argument procedure requires parameter list. */
+void LefError(int, char *, ...) ATTR_FORMAT_PRINTF_2;
 
 /* C99 compat */
 extern void LefRead();
diff --git a/textio/textio.h b/textio/textio.h
index aa3e5fff..1158b7d9 100644
--- a/textio/textio.h
+++ b/textio/textio.h
@@ -68,10 +68,10 @@ extern void TxUseMore();
 extern void TxStopMore();
 
 /* printing procedures with variable arguments lists */
-extern void TxError(char *, ...);
+extern void TxError(char *, ...) ATTR_FORMAT_PRINTF_1;
 extern void TxErrorV(char *, va_list args);
-extern void TxPrintf(char *, ...);
-extern char *TxPrintString(char *, ...);
+extern void TxPrintf(char *, ...) ATTR_FORMAT_PRINTF_1;
+extern char *TxPrintString(char *, ...) ATTR_FORMAT_PRINTF_1;
 
 /* input procedures */
 extern char *TxGetLinePrompt();
diff --git a/textio/textioInt.h b/textio/textioInt.h
index 83de8c62..7075a979 100644
--- a/textio/textioInt.h
+++ b/textio/textioInt.h
@@ -52,7 +52,7 @@ extern bool TxGetInputEvent();
 
 /* Routines with variable argument lists */
 
-extern void txFprintfBasic(FILE *, ...);
+extern void txFprintfBasic(FILE *, const char *fmt, ...) ATTR_FORMAT_PRINTF_2;
 
 /* C99 compat */
 void txCommandsInit();
diff --git a/textio/txOutput.c b/textio/txOutput.c
index 7aec132e..d1195ede 100644
--- a/textio/txOutput.c
+++ b/textio/txOutput.c
@@ -76,13 +76,11 @@ static bool txPrintFlag = TRUE;
  */
 
 void
-txFprintfBasic(FILE *f, ...)
+txFprintfBasic(FILE *f, const char *fmt, ...)
 {
     va_list args;
-    char *fmt;
 
-    va_start(args, f);
-    fmt = va_arg(args, char *);
+    va_start(args, fmt);
     Vfprintf(f, fmt, args);
     va_end(args);
 }
diff --git a/utils/magic.h b/utils/magic.h
index 0dcda8e9..8f905edf 100644
--- a/utils/magic.h
+++ b/utils/magic.h
@@ -168,6 +168,16 @@ extern char AbortMessage[];
     #define PaLockZOpen  PaLockOpen
 #endif
 
+/* ------------ modern compiler support -------------------------------- */
+
+#if defined(__GNUC__) || defined(__clang__)
+ #define ATTR_FORMAT_PRINTF_1 __attribute__((format (printf,1,2)))
+ #define ATTR_FORMAT_PRINTF_2 __attribute__((format (printf,2,3)))
+#else
+ #define ATTR_FORMAT_PRINTF_1 /* */
+ #define ATTR_FORMAT_PRINTF_2 /* */
+#endif
+
 /* ---------------- Start of Machine Configuration Section ----------------- */
 
     /* ------- Configuration:  Handle Missing Routines/Definitions ------- */
diff --git a/utils/tech.h b/utils/tech.h
index e2cd8ff0..3bb3f9f3 100644
--- a/utils/tech.h
+++ b/utils/tech.h
@@ -51,7 +51,7 @@ extern bool TechOverridesDefault; /* Set TRUE if technology was specified on
 
 /* ----------------- Exported procedures ---------------- */
 
-extern void TechError(char *, ...);
+extern void TechError(char *, ...) ATTR_FORMAT_PRINTF_1;
 extern void TechAddClient();
 extern void TechAddAlias();