Fix write of 0 in '%c' (#6248) (#6249)

docs/CONTRIBUTORS

@@ -206,6 +206,7 @@ Richard Myers
 Risto Pejašinović
 Robert Balas
 Robin Heinemann
+Rodrigo Batista de Moraes
 Rupert Swarbrick
 Ryan Ziegler
 Ryszard Rozak

src/V3Number.cpp

@@ -746,10 +746,7 @@ string V3Number::displayed(FileLine* fl, const string& vformat) const VL_MT_STAB
         if (width() > 8)
             fl->v3warn(WIDTHTRUNC, "$display-like format of %c format of > 8 bit value");
         const unsigned int v = bitsValue(0, 8);
-        char strc[2];
-        strc[0] = v & 0xff;
-        strc[1] = '\0';
-        str = strc;
+        str.push_back(static_cast<char>(v));
         return str;
     }
     case 's': {

test_regress/t/t_sys_file_null.py

@@ -16,7 +16,7 @@ test.compile()
 test.execute()
 
 filename = test.obj_dir + "/zeros.log"
-if os.path.getsize(filename) != 16:
+if os.path.getsize(filename) != 20:
     test.error(filename + ": Wrong file size")
 
 test.passes()

test_regress/t/t_sys_file_null.v

@@ -10,6 +10,8 @@ module t;
    integer fd, cnt;
    initial begin
       fd = $fopen({`STRINGIFY(`TEST_OBJ_DIR),"/zeros.log"}, "w");
+      for (cnt = 0; cnt < 4; cnt = cnt + 1)
+        $fwrite(fd, "%c", 8'd0);
       for (cnt = 0; cnt < 16; cnt = cnt + 4)
         $fwrite(fd, "%u", 32'd0);
       $fclose(fd);