How to solve Variadic Arguments issue (using C)

First of all, what are Variadic Arguments (VA)? It's a way to NOT define the number of arguments a function (or method) takes. You only define the args you definitely going to be given on every call. All additional arguments are then optional, but can be used if given.

While porting C-code from Linux to AS/400 I ran into a problem. This article is going to explain the issue and the solution.

The Issue

I have some code that I have been using for years in macOS and Linux projects. It's purpose is to log messages with the source's filename, line number and some custom message, depending on the severity/log level.

This is how it's used in the code:

Say(0, "Error: Socket creation failed with %d\n", errNum);

This is how the define adds the filename and linenumber:

#if (kDebug)
   #define Say(logLevel, parameter, ...) logWithMethodName(__FUNCTION__, __LINE__, logLevel, (parameter), ##__VA_ARGS__);
#else
   #define Say(logLevel, parameter, ...) //#define KeinLoggingHeute
#endif

This is the actual logging code (you can skip that, it's not relevant, except for the parameters):

void	logWithMethodName(const char* methodName, int lineNumber, int logLevel, char *format, ...)
{
		if (gLogLevel>=logLevel)
		{
			/* Variables */
			char msg1[kMaxMsgLen];
			char msg2[kMaxMsgLen];
			char lMethodName[kMaxMethodLen];
			
			/* Get the arguments */
			va_list argp;
			va_start(argp, format); // format being the last argument before '...'

			/* Combine format and arguments! */
			vsprintf(msg2, format, argp);

				if (strlen(methodName) > kMaxMethodLen)
				{
					memcpy(lMethodName, methodName, kMaxMethodLen-1);
					lMethodName[kMaxMethodLen]='\0';
				}
				else
				{
					strcpy(lMethodName, methodName);
				}

				if (strlen(format) > kMaxFormatLen) format[kMaxFormatLen]='\0';
				
				sprintf(msg1, "%s (%d): ", lMethodName, lineNumber);
				strcat(msg1, msg2);

				/* Output to stdout */
				if (1)
				{
					printf(msg1);
				}


				/* Write to syslog */
				if (0)
				{
					//logger(msg1);
				}
			
			va_end (argp);
		}
	
	return;
}

Looks fine, you say? I'd agree. This code worked for me for years. Then I compiled it on our AS/400... CRTCMOD MODULE(CTI/NTWRKNG) SRCFILE(CTI/SOURCES) SRCMBR(NETWORKING) OPTION(*LOGMSG) OUTPUT(*PRINT)<ref>Note the OUTPUT-Option! If not given, you will get only Module NTWRKNG is not created because statement errors occurred. info, and no further indication of what went wrong. With OUTPUT(*PRINT) given, you will receive detailed information in your print queue.

and ran into these error messages:

The Solution

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• Links
  • https://gcc.gnu.org/onlinedocs/cpp/Variadic-Macros.html GNU.org explaining VA