Added text on signed/unsigned chars

Author: rijnb

README.md

@@ -44,6 +44,14 @@ Check the `README.md` in directory `test` for more information.
 
 Also see http://www.mapcode.com for background and reference materials.
 
+## Compilers: Signed and Unsigned `char`
+
+The library expects the type `char` to be `signed char` by default. If your compiler
+uses `unsigned char` by default (such as for ARM or PowerPC), please add the correct
+compiler switch to change this behavior to `signed char`.
+
+The unit tests in `test` will test the correct settings of the compiler and will
+fail if you've compiled the library with incorrect settings.
 
 ## A Real-Life Example, The 'mapcode' Codec Tool: `utility/`
 

test/unittest.c

@@ -1465,6 +1465,16 @@ static int testEnvironment(void) {
     printf("sizeof(char)=%ld, sizeof(UWORD)=%ld, sizeof(int)=%ld, sizeof(long int)=%ld\n",
            sizeof(char), sizeof(UWORD), sizeof(int), sizeof(long int));
 
+    // Check signed/unsigned character.
+    char t = (char) (-1);
+    if ((int) t == -1) {
+        printf("char is signed\n");
+    } else {
+        printf("char is unsigned\n");
+        foundError();
+        printf("*** ERROR *** Compiled with 'unsigned char'; please recompile to use 'signed char' by default.\n");
+    }
+
     // Check size of UWORD.
     nrTests++;
     if (sizeOfWord != 2) {