Add str_format_v with va_list argument

So the string formatting can be reused in other places. Passing variadic arguments directly to another function is not possible.

The function must have a different name, as overloading `str_format` would cause the wrong function to be called when passing the `va_list`. The same naming with suffix `_v` is used for the logging functions.

src/base/system.cpp

@@ -2690,26 +2690,27 @@ int str_length(const char *str)
 	return (int)strlen(str);
 }
 
-int str_format(char *buffer, int buffer_size, const char *format, ...)
+int str_format_v(char *buffer, int buffer_size, const char *format, va_list args)
 {
 #if defined(CONF_FAMILY_WINDOWS)
-	va_list ap;
-	va_start(ap, format);
-	_vsprintf_p(buffer, buffer_size, format, ap);
-	va_end(ap);
-
+	_vsprintf_p(buffer, buffer_size, format, args);
 	buffer[buffer_size - 1] = 0; /* assure null termination */
 #else
-	va_list ap;
-	va_start(ap, format);
-	vsnprintf(buffer, buffer_size, format, ap);
-	va_end(ap);
-
+	vsnprintf(buffer, buffer_size, format, args);
 	/* null termination is assured by definition of vsnprintf */
 #endif
 	return str_utf8_fix_truncation(buffer);
 }
 
+int str_format(char *buffer, int buffer_size, const char *format, ...)
+{
+	va_list args;
+	va_start(args, format);
+	int length = str_format_v(buffer, buffer_size, format, args);
+	va_end(args);
+	return length;
+}
+
 const char *str_trim_words(const char *str, int words)
 {
 	while(*str && str_isspace(*str))

src/base/system.h

@@ -15,6 +15,7 @@
 #endif
 
 #include <cinttypes>
+#include <cstdarg>
 #include <cstdint>
 #include <ctime>
 
@@ -1248,13 +1249,32 @@ int str_length(const char *str);
  * @param buffer Pointer to the buffer to receive the formatted string.
  * @param buffer_size Size of the buffer.
  * @param format printf formatting string.
- * @param ... Parameters for the formatting.
+ * @param args The variable argument list.
  *
- * @return Length of written string, even if it has been truncated
+ * @return Length of written string, even if it has been truncated.
  *
  * @remark See the C manual for syntax for the printf formatting string.
  * @remark The strings are treated as zero-terminated strings.
- * @remark Guarantees that dst string will contain zero-termination.
+ * @remark Guarantees that buffer string will contain zero-termination.
+ */
+int str_format_v(char *buffer, int buffer_size, const char *format, va_list args)
+	GNUC_ATTRIBUTE((format(printf, 3, 0)));
+
+/**
+ * Performs printf formatting into a buffer.
+ *
+ * @ingroup Strings
+ *
+ * @param buffer Pointer to the buffer to receive the formatted string.
+ * @param buffer_size Size of the buffer.
+ * @param format printf formatting string.
+ * @param ... Parameters for the formatting.
+ *
+ * @return Length of written string, even if it has been truncated.
+ *
+ * @remark See the C manual for syntax for the printf formatting string.
+ * @remark The strings are treated as zero-terminated strings.
+ * @remark Guarantees that buffer string will contain zero-termination.
  */
 int str_format(char *buffer, int buffer_size, const char *format, ...)
 	GNUC_ATTRIBUTE((format(printf, 3, 4)));