Hooking a variadic function with Cydia Substrate

gistfile1.c

//
//  LibC.m
//
//  Created by Alban Diquet on 5/14/14.
//  Copyright (c) 2014 Alban Diquet. All rights reserved.
//

#import <CydiaSubstrate.h>
#import "LibC.h"

// Hook open() to catch the path of all files being accessed
static int (*original_open)(const char *path, int oflag, ...);
static int replaced_open(const char *path, int oflag, ...) {
    int result = 0;
    
    // Handle the optional third argument
    if (oflag & O_CREAT) {
        mode_t mode;
        va_list args;
        
        va_start(args, oflag);
        mode = (mode_t)va_arg(args, int);
        va_end(args);
        result = original_open(path, oflag, mode);
    }
    else {
        result = original_open(path, oflag);
    }
    NSLog(@"OPEN: %s", path);
    return result;
}

void hookLibC(void) {
    MSHookFunction(open, replaced_open, (void **) &original_open);
}

how to hook NSString’s + (instancetype)stringWithFormat:(NSString *)format, ... ?

With a variable number of args passed to the variadic without a "count" parameter, the hooking becomes impossible without using ASM.
(For future reference for anyone hitting here)