Found the following significant warnings:
  redefine.c:147:40: warning: passing 'char *' to parameter of type 'unsigned char *' converts between pointers to integer types where one is of the unique plain 'char' type and the other is not [-Wpointer-sign]
  redefine.c:147:10: warning: returning 'unsigned char *' from a function with result type 'char *' converts between pointers to integer types where one is of the unique plain 'char' type and the other is not [-Wpointer-sign]
  redefine.c:155:45: warning: passing 'const char *' to parameter of type 'const unsigned char *' converts between pointers to integer types where one is of the unique plain 'char' type and the other is not [-Wpointer-sign]
  redefine.c:166:40: warning: passing 'const char *' to parameter of type 'const unsigned char *' converts between pointers to integer types where one is of the unique plain 'char' type and the other is not [-Wpointer-sign]
  rulebasedmodels.c:239:12: warning: returning 'unsigned char *' from a function with result type 'char *' converts between pointers to integer types where one is of the unique plain 'char' type and the other is not [-Wpointer-sign]
  strbuf.c:206:22: warning: passing 'unsigned char *' to parameter of type 'char *' converts between pointers to integer types where one is of the unique plain 'char' type and the other is not [-Wpointer-sign]
  strbuf.c:206:45: warning: passing 'const unsigned char *' to parameter of type 'const char *' converts between pointers to integer types where one is of the unique plain 'char' type and the other is not [-Wpointer-sign]
  strbuf.c:241:19: warning: passing 'unsigned char *' to parameter of type 'char *' converts between pointers to integer types where one is of the unique plain 'char' type and the other is not [-Wpointer-sign]
  strbuf.c:241:42: warning: passing 'const unsigned char *' to parameter of type 'const char *' converts between pointers to integer types where one is of the unique plain 'char' type and the other is not [-Wpointer-sign]
  strbuf.c:268:37: warning: passing 'const unsigned char *' to parameter of type 'const char *' converts between pointers to integer types where one is of the unique plain 'char' type and the other is not [-Wpointer-sign]
  top.c:49:41: warning: passing 'char *' to parameter of type 'unsigned char *' converts between pointers to integer types where one is of the unique plain 'char' type and the other is not [-Wpointer-sign]
  top.c:57:41: warning: passing 'char *' to parameter of type 'unsigned char *' converts between pointers to integer types where one is of the unique plain 'char' type and the other is not [-Wpointer-sign]
  top.c:67:43: warning: passing 'char *' to parameter of type 'unsigned char *' converts between pointers to integer types where one is of the unique plain 'char' type and the other is not [-Wpointer-sign]
  top.c:92:15: warning: initializing 'char *' with an expression of type 'unsigned char *' converts between pointers to integer types where one is of the unique plain 'char' type and the other is not [-Wpointer-sign]
  top.c:106:15: warning: initializing 'char *' with an expression of type 'unsigned char *' converts between pointers to integer types where one is of the unique plain 'char' type and the other is not [-Wpointer-sign]
  top.c:153:41: warning: passing 'char *' to parameter of type 'unsigned char *' converts between pointers to integer types where one is of the unique plain 'char' type and the other is not [-Wpointer-sign]
  top.c:158:41: warning: passing 'char *' to parameter of type 'unsigned char *' converts between pointers to integer types where one is of the unique plain 'char' type and the other is not [-Wpointer-sign]
  top.c:164:43: warning: passing 'char *' to parameter of type 'unsigned char *' converts between pointers to integer types where one is of the unique plain 'char' type and the other is not [-Wpointer-sign]
  top.c:170:44: warning: passing 'char *' to parameter of type 'unsigned char *' converts between pointers to integer types where one is of the unique plain 'char' type and the other is not [-Wpointer-sign]
  top.c:183:43: warning: passing 'char *' to parameter of type 'unsigned char *' converts between pointers to integer types where one is of the unique plain 'char' type and the other is not [-Wpointer-sign]
See '/home/hornik/tmp/R.check/r-devel-clang/Work/PKGS/C50.Rcheck/00install.out' for details.