Using va_list and getting EXC_BAD_ACCESS - iphone

Question

Similar to how NSLog takes variable argument list, I want to create my own method.
I have my method declared like this but when I try to access the 'args' variable, I get an EXEC_BAD_ACCESS. What is that I'm not doing correctly here?
- (void)info:(NSString *)formatString, ...
{
va_list args;
va_start(args, formatString);
NSLog(#"formatString value: %#", formatString);
// The following line causes the EXEC_BAD_ACCESS
NSLog(#"args value: %#", args);
// This is what I'm trying to do:
NSLog(formatString, args);
va_end(args);
}
I was following the 'va_list in Cocoa' section from this blog:
http://cocoawithlove.com/2009/05/variable-argument-lists-in-cocoa.html

Answer 1

There are a couple of mistakes in your code. Firstly, args cannot be printed directly as is. It is a reference to several arguments, and trying to print it using NSLog("%#") will not work. What you can do is use NSLogv() to print it (e.g. NSLogv(format, args);) instead.
Or you can do what I do, and use this function:
void print (NSString *format, ...) {
va_list args;
va_start(args, format);
fputs([[[[NSString alloc] initWithFormat:format arguments:args] autorelease] UTF8String], stdout);
va_end(args);
}

Answer 2

The "%#" format directive takes an argument, interprets it as an Objective-C object and sends it the "description" selector. That needs to return a NSString which is printed.
So you code is trying to execute ‘[args description]‘, but args is not a Objective-C object, it's of type ‘va_list‘. Hence the exception.
See your link, the implementation of "setContentByAppendingStrings:" shows how do get the arguments out of your va_list.

Answer 3

Morning lads,
I just have been confronted with the very similar issue. Here is what I was doing:
+ (void) l:(D3LogLevel)p_logLevel s:(NSString *)p_format, ...
{
if (p_logLevel >= logLevel) {
va_list v_args;
va_start(v_args, p_format);
NSLog(#"[%d] %#", p_logLevel, [NSString stringWithFormat:p_format, v_args]);
va_end(v_args);
}
}
Which, as DarkDust here has described accurately, uses v_args as an Objective-C object when it is not. Here is the culprit call:
[NSString stringWithFormat:p_format, v_args]
Hence the modification to take a va_list:
[[NSString alloc] initWithFormat:p_format arguments:v_args]
Making use of the appropriate method initWithFormat which signature is:
- (id)initWithFormat:(NSString *)format arguments:(va_list)argList
We can see that the type is right and everything becomes crystal clear. The full rewritten method is then:
+ (void) l:(D3LogLevel)p_logLevel s:(NSString *)p_format, ...
{
if (p_logLevel >= logLevel) {
va_list v_args;
va_start(v_args, p_format);
NSLog(#"[%d] %#", p_logLevel, [[NSString alloc] initWithFormat:p_format arguments:v_args]);
va_end(v_args);
}
}
It works like a charm!