可变参数函数缓存上次调用的参数列表

Question

I wrote a Helper class with c functions for an iOS Library with the following pattern. There are 2 wrapping (variadic) functions, which finally call the same function, with slightly different parameter. Idea is to have "default" properties being set.

__attribute__((overloadable)) void func1(NSString* _Nonnull format, ...);
__attribute__((overloadable)) void func1(int param1, NSString* _Nonnull format, ...);

Both will then call the following function:

void prefixAndArguments(int param1, NSString* _Nonnull format, va_list arguments);

Implementation as followed:

__attribute__((overloadable)) void func1(NSString* _Nonnull format, ...)
{
    va_list argList;
    va_start(argList, format);
    prefixAndArguments(0, format, argList);
    va_end(argList);
}

__attribute__((overloadable)) void func1(int param1, NSString* _Nonnull format, ...)
{
    va_list argList;
    va_start(argList, format);
    prefixAndArguments(param1, format, argList);
    va_end(argList);
}


void prefixAndArguments(NMXLogLevelType logLevel, NSString* _Nullable logPrefix, __strong NSString* _Nonnull format, va_list arguments)
{
    // Evaluate input parameters
    if (format != nil && [format isKindOfClass:[NSString class]])
    {
        // Get a reference to the arguments that follow the format parameter
        va_list argList;
        va_copy(argList, arguments);

        int argCount = 0;
        NSLog(@"%d",argCount);
        while (va_arg(argList, NSObject *))
        {
            argCount += 1;
        }
        NSLog(@"%d",argCount);
        va_end(argList);

        NSMutableString *s;
        if (numSpecifiers > argCount)
        {
            // Perform format string argument substitution, reinstate %% escapes, then print
            NSString *debugOutput = [[NSString alloc] initWithFormat:@"Error occured when logging: amount of arguments does not for to the defined format. Callstack:\n%@\n", [NSThread callStackSymbols]];
            printf("%s\n", [debugOutput UTF8String]);
            s = [[NSMutableString alloc] initWithString:format];
        }
        else
        {
            // Perform format string argument substitution, reinstate %% escapes, then print
            va_copy(argList, arguments);

            // This is were the EXC_BAD_ACCESS will occur!
            // Error: Thread 1: EXC_BAD_ACCESS (code=EXC_I386_GPFLT)
            s = [[NSMutableString alloc] initWithFormat:format arguments:argList];
            [s replaceOccurrencesOfString:@"%%"
                               withString:@"%%%%"
                                  options:0
                                    range:NSMakeRange(0, [s length])];
            NSLog(@"%@",s);
            va_end(argList);
        }
    ...
}

My Unit Tests for the function look the following (order is important).

// .. some previous cases, I commented out
XCTAssertNoThrow(NMXLog(@"Simple string output"));
XCTAssertNoThrow(NMXLog(@"2 Placeholders. 0 Vars %@ --- %@"));

The crash happens when I want to use the arguments and the format (making format strong did not solve the problem, and does not seem being part of the problem, see below):

s = [[NSMutableString alloc] initWithFormat:format arguments:argList];

Here is the Log:

xctest[28082:1424378] 0
xctest[28082:1424378] --> 1
xctest[28082:1424378] Simple string output
xctest[28082:1424378] 0
xctest[28082:1424378] --> 4

Of course we won't see the desired string "2 Placeholders. 0 Vars %@ --- %@" as the crash happened before.

So, the question is now: Why is the amount of arguments now being 4 instead of 0? As none being passed in the second call, are the arguments being collected when the function is being called immediately again?

So, I started to call the function "again" to make sure the argument's list is being cleared, although va_end was being called:

__attribute__((overloadable)) void func1(NSString* _Nonnull format, ...)
{
    va_list argList;
    va_start(argList, format);
    prefixAndArguments(none, nil, format, argList);
    va_end(argList);
    NSString *obj = nil;
    prefixAndArguments(none, nil, obj, nil);
}

This does work now like a charm (argument's list is being cleared and the desired output is being received):

xctest[28411:1453508] 0
xctest[28411:1453508] --> 1
xctest[28411:1453508] Simple string output
xctest[28411:1453508] 0
xctest[28411:1453508] --> 1
Error occured when logging: amount of arguments does not for to the defined format. Callstack: ....
xctest[28411:1453508] 2 Placeholders. 0 Vars %@ --- %@

Here is finally my question:

What is the reason for this behavior and how can I avoid it? Is there a better way to solve the issue than "stupidly" calling the function a second time with "no" arguments to clear the them? Ps I tried not to use macros, because I consider them as more error prone than c functions. See this thread: Macro vs Function in C

Answer 1

You appear to have some misconceptions about variadic functions, exemplified by this approach to counting the variable arguments:

  while (va_arg(argList, NSObject *)) { argCount += 1; } 

That code assumes that the variable arguments have at least one member, that all of them are of type NSObject * , and that the list will be terminated by a null pointer of that type. None of those is guaranteed by the system, and if those assumptions are not satisfied then the behavior of one or more va_arg() invocations will be undefined.

In practice, you can probably get away with actual arguments that are pointers of other types (though formally, the behavior will still be undefined in that case). If the arguments may have non-pointer types, however, then that approach to counting them is completely broken. More importantly, your test cases appear to assume that the system will provide a trailing NULL argument, but that is in no way guaranteed.

If your function relies on the end of the variable argument list being signaled by a NULL argument, then it is relying on the caller to provide one. It is very likely the absence of null termination in your argument lists that gives rise to the behavior you are asking about.