improve `SWAP' macro

src/include/ngspice/macros.h

@@ -36,7 +36,12 @@
 #define ABS(a)    ((a) < 0.0 ? -(a) : (a))
 #define SGN(a)    ((a) < 0.0 ? -(1.0) : (1.0))
 #define SIGN(a,b) ( b >= 0 ? (a >= 0 ? a : - a) : (a >= 0 ? - a : a))
-#define  SWAP(type, a, b)   {type swapx; swapx = a; a = b; b = swapx;}
+#define SWAP(type, a, b)                        \
+    do {                                        \
+        type SWAP_macro_local = a;              \
+        a = b;                                  \
+        b = SWAP_macro_local;                   \
+    } while(0)
  
  
 #define ABORT()                                 \

src/maths/sparse/spdefs.h

@@ -78,7 +78,12 @@
 #define  SQR(a)             ((a)*(a))
 
 /* Macro procedure that swaps two entities. */
-#define  SWAP(type, a, b)   {type swapx; swapx = a; a = b; b = swapx;}
+#define SWAP(type, a, b)                        \
+    do {                                        \
+        type SWAP_macro_local = a;              \
+        a = b;                                  \
+        b = SWAP_macro_local;                   \
+    } while(0)