libc: core: add return-value to printf output callbacks

lib/libc/core/stdio/printf.c

@@ -118,7 +118,7 @@
 #endif
 
 // output function type
-typedef void (
+typedef int (
 	*out_fct_type)(char character, void *buffer, size_t idx, size_t maxlen);
 
 // wrapper (used as buffer) for output function type
@@ -128,7 +128,7 @@ typedef struct {
 } out_fct_wrap_type;
 
 // internal buffer output
-static inline void _out_buffer(
+static inline int _out_buffer(
 	char character,
 	void *buffer,
 	size_t idx,
@@ -137,10 +137,12 @@ static inline void _out_buffer(
 	if (idx < maxlen) {
 		((char *)buffer)[idx] = character;
 	}
+
+	return 0;
 }
 
 // internal null output
-static inline void _out_null(
+static inline int _out_null(
 	char character,
 	void *buffer,
 	size_t idx,
@@ -150,10 +152,11 @@ static inline void _out_null(
 	(void)buffer;
 	(void)idx;
 	(void)maxlen;
+	return 0;
 }
 
 // internal output function wrapper
-static inline void _out_fct(
+static inline int _out_fct(
 	char character,
 	void *buffer,
 	size_t idx,
@@ -166,6 +169,8 @@ static inline void _out_fct(
 		((out_fct_wrap_type *)buffer)
 			->fct(character, ((out_fct_wrap_type *)buffer)->arg);
 	}
+
+	return 0;
 }
 
 // internal secure strlen