string-util: explicitly cast character to unsigned

yuwata · poettering · commit 3d56acef7f73 · 2021-05-08T14:24:51.000+02:00
This also adds comment why we cast to unsigned. Follow-up for 7971f90. Addresses the comment systemd#19544 (comment).
diff --git a/src/basic/string-util.h b/src/basic/string-util.h
@@ -130,7 +130,12 @@ static inline bool _pure_ in_charset(const char *s, const char* charset) {
 }
 
 static inline bool char_is_cc(char p) {
-        return (uint8_t) p < ' ' || p == 127;
+        /* char is unsigned on some architectures, e.g. aarch64. So, compiler may warn the condition
+         * p >= 0 is always true. See #19543. Hence, let's cast to unsigned before the comparison. Note
+         * that the cast in the right hand side is redundant, as according to the C standard, compilers
+         * automatically cast a signed value to unsigned when comparing with an unsigned variable. Just
+         * for safety and readability. */
+        return (uint8_t) p < (uint8_t) ' ' || p == 127;
 }
 bool string_has_cc(const char *p, const char *ok) _pure_;
 

Original file line number	Diff line number	Diff line change
`@@ -130,7 +130,12 @@ static inline bool _pure_ in_charset(const char s, const char charset) {`
`130`	`130`	`}`
`131`	`131`
`132`	`132`	`static inline bool char_is_cc(char p) {`
`133`		`- return (uint8_t) p < ' ' \|\| p == 127;`
	`133`	`+ /* char is unsigned on some architectures, e.g. aarch64. So, compiler may warn the condition`
	`134`	`+ * p >= 0 is always true. See #19543. Hence, let's cast to unsigned before the comparison. Note`
	`135`	`+ * that the cast in the right hand side is redundant, as according to the C standard, compilers`
	`136`	`+ * automatically cast a signed value to unsigned when comparing with an unsigned variable. Just`
	`137`	`+ * for safety and readability. */`
	`138`	`+ return (uint8_t) p < (uint8_t) ' ' \|\| p == 127;`
`134`	`139`	`}`
`135`	`140`	`bool string_has_cc(const char p, const char ok) _pure_;`
`136`	`141`