tgt-vvp: inform vvp about undriven constant values.

This causes tgt-vvp to use a lower case 'c' instead of an upper case
'C' as the prefix for constant values used to initialise undriven nets.

For use by the following commit.

tgt-vvp/draw_net_input.c

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2001-2022 Stephen Williams (steve@icarus.com)
+ * Copyright (c) 2001-2024 Stephen Williams (steve@icarus.com)
  *
  *    This source code is free software; you can redistribute it
  *    and/or modify it in source code form under the terms of the GNU
@@ -651,18 +651,20 @@ static void draw_net_input_x(ivl_nexus_t nex,
       nex_data->flags |= nex_flags;
 
 	/* If the nexus has no drivers, then send a constant HiZ or
-	   0.0 into the net. */
+	   0.0 into the net. Use a lower case 'c' prefix for the
+	   constant to inform vvp that this is an undriven value. */
       if (ndrivers == 0) {
 	    unsigned wid = width_of_nexus(nex);
 	    int pull = (res == IVL_SIT_TRI0) || (res == IVL_SIT_TRI1);
 	      /* For real nets put 0.0. */
 	    if (signal_data_type_of_nexus(nex) == IVL_VT_REAL) {
 		  nex_private = draw_Cr_to_string(0.0);
+		  nex_private[0] = 'c';
 	    } else {
 		  unsigned jdx;
 		  char*tmp = malloc((pull ? 3 : 1) * wid + 5);
 		  nex_private = tmp;
-		  strcpy(tmp, pull ? "C8<" : "C4<");
+		  strcpy(tmp, pull ? "c8<" : "c4<");
 		  tmp += strlen(tmp);
 		  switch (res) {
 		      case IVL_SIT_TRI:

vvp/compile.cc

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2001-2021 Stephen Williams (steve@icarus.com)
+ * Copyright (c) 2001-2024 Stephen Williams (steve@icarus.com)
  *
  *    This source code is free software; you can redistribute it
  *    and/or modify it in source code form under the terms of the GNU
@@ -891,9 +891,20 @@ void compile_vpi_time_precision(long pre)
  *
  * The real value is sign * (mant ** exp).
  */
+static bool crstring_header_test(const char*str)
+{
+      if ((str[0] != 'C') && (str[0] != 'c'))
+	    return false;
+      if ((str[1] != 'r') || (str[2] != '<'))
+	    return false;
+
+      return true;
+}
+
 bool crstring_test(const char*str)
 {
-      if (strncmp(str, "Cr<", 3) != 0) return false;
+      if (!crstring_header_test(str))
+	    return false;
       const char*tp = strchr(str, '>');
       if (tp == 0) return false;
       if (tp[1] != 0) return false;
@@ -906,6 +917,8 @@ bool crstring_test(const char*str)
 
 double crstring_to_double(const char*label)
 {
+      assert(crstring_header_test(label));
+
       const char*cp = label+3;
       assert(*cp == 'm');
       cp += 1;

vvp/lexor.lex

@@ -4,7 +4,7 @@
 
 %{
 /*
- * Copyright (c) 2001-2018 Stephen Williams (steve@icarus.com)
+ * Copyright (c) 2001-2024 Stephen Williams (steve@icarus.com)
  *
  *    This source code is free software; you can redistribute it
  *    and/or modify it in source code form under the terms of the GNU
@@ -263,17 +263,17 @@ static char* strdupnew(char const *str)
 
   /* Handle some specialized constant/literals as symbols. */
 
-"C4<"[01xz]*">" {
+[Cc]"4<"[01xz]*">" {
       yylval.text = strdup(yytext);
       assert(yylval.text);
       return T_SYMBOL; }
 
-"C8<"[01234567xz]*">" {
+[Cc]"8<"[01234567xz]*">" {
       yylval.text = strdup(yytext);
       assert(yylval.text);
       return T_SYMBOL; }
 
-"Cr<m"[a-f0-9]*"g"[a-f0-9]*">" {
+[Cc]"r<m"[a-f0-9]*"g"[a-f0-9]*">" {
       yylval.text = strdup(yytext);
       assert(yylval.text);
       return T_SYMBOL; }

vvp/vvp_net.cc

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2004-2023 Stephen Williams (steve@icarus.com)
+ * Copyright (c) 2004-2024 Stephen Williams (steve@icarus.com)
  *
  *    This source code is free software; you can redistribute it
  *    and/or modify it in source code form under the terms of the GNU
@@ -2999,9 +2999,18 @@ vvp_vector4_t vector2_to_vector4(const vvp_vector2_t&that, unsigned wid)
       return res;
 }
 
+static bool c4string_header_test(const char*str)
+{
+      if ((str[0] != 'C') && (str[0] != 'c'))
+	    return false;
+      if ((str[1] != '4') || (str[2] != '<'))
+	    return false;
+      return true;
+}
+
 bool c4string_test(const char*str)
 {
-      if (strncmp(str, "C4<", 3) != 0)
+      if (!c4string_header_test(str))
 	    return false;
       size_t value_size = strspn(str+3, "01xz");
       if (str[3+value_size] != '>')
@@ -3014,7 +3023,7 @@ bool c4string_test(const char*str)
 
 vvp_vector4_t c4string_to_vector4(const char*str)
 {
-      assert((str[0]=='C') && (str[1]=='4') && (str[2]=='<'));
+      assert(c4string_header_test(str));
 
       str += 3;
       const char*tp = str + strspn(str,"01xz");
@@ -3186,14 +3195,21 @@ vvp_vector8_t part_expand(const vvp_vector8_t&that, unsigned wid, unsigned off)
       return tmp;
 }
 
+static bool c8string_header_test(const char*str)
+{
+      if ((str[0] != 'C') && (str[0] != 'c'))
+	    return false;
+      if ((str[1] != '8') || (str[2] != '<'))
+	    return false;
+      return true;
+}
+
 bool c8string_test(const char*str)
 {
-      const char*cp;
-      if (str[0] != 'C') return false;
-      if (str[1] != '8') return false;
-      if (str[2] != '<') return false;
+      if (!c8string_header_test(str))
+	    return false;
 
-      cp = str+3;
+      const char*cp = str+3;
       for (;; cp += 1) {
 	    if (cp[0] == '>' && cp[1] == 0) return true;
 	    if (cp[0] >= '0' && cp[0] <= '9') continue;
@@ -3210,6 +3226,8 @@ bool c8string_test(const char*str)
  */
 vvp_vector8_t c8string_to_vector8(const char*str)
 {
+      assert(c8string_header_test(str));
+
       size_t vsize = strlen(str)-4;
       assert(vsize%3 == 0);
       vsize /= 3;