Avoid undefined behaviour in short option parsing

The undefined behaviour lead to clashes / misinterpretation of short
options on some systems. Note that glibc for example uses the argument
of isalnum etc. as an index into a lookup table.

1 files changed, 5 insertions, 0 deletions

diff --git a/tools/tools.c b/tools/tools.c
index a14b9ca..e11c336 100644
--- a/tools/tools.c
+++ b/tools/tools.c
@@ -74,6 +74,11 @@ struct {
 static char
 short_option (int opt)
 {
+	// isalpha and isdigit require the argument to be representable as unsigned char
+	// if they're not representable as unsigned char, then we assume they're not printable -
+	// matching for example the enumerator definitions in computer.c
+	if (opt != (int)(unsigned char)opt)
+		return 0;
 	if (isalpha (opt) || isdigit (opt))
 		return (char)opt;
 	return 0;