Apparently char can be an unsigned data type?

a.c

#include <stdio.h>
 
int
main(int argc, char* argv[])
{
    fprintf(stderr, "%d\r\n", (char) -1);
    fprintf(stderr, "%lu\r\n", sizeof(char));
}

darwin.txt

$ uname -a
Darwin prism.local 13.3.0 Darwin Kernel Version 13.3.0: Tue Jun  3 21:27:35 PDT 2014; root:xnu-2422.110.17~1/RELEASE_X86_64 x86_64
$ ./a.out 
-1
1

ppc64el.txt

$ uname -a
Linux powerprism 3.13.0-24-generic #46-Ubuntu SMP Thu Apr 10 19:09:21 UTC 2014 ppc64le ppc64le ppc64le GNU/Linux
$ ./a.out
255
1