A sample command-line OSX app for switching the endianness of an input file.

gistfile1.m

#import <Foundation/Foundation.h>

int main(int argc, const char * argv[]) {

    @autoreleasepool {

        // Declare some config values; the input and output file paths and the
        // number of bytes per word
        NSString *inputFile = @"/Users/simon/Desktop/input.dat";
        NSString *outputFile = @"/Users/simon/Desktop/output.dat";
        NSUInteger bytesPerWord = 2;

        // First, read in the data. I'm using NSMutableData so that I can write
        // back to the same object later
        NSMutableData *data = [NSMutableData dataWithContentsOfFile:inputFile];

        // Next, work out how many words are in the file. That'll be the length
        // (in bytes) divided by the number of bytes per word.
        NSUInteger numberOfWords = [data length] / bytesPerWord;

        // Create a temporary array to store the bytes from each word while we
        // reverse them
        char *tempByteArray = calloc(bytesPerWord, 1);
        
        // Now step through the data a word at a time
        for (NSUInteger wordIndex = 0; wordIndex < numberOfWords; wordIndex++) {
            // NSData lets you deal with ranges of bytes, so start by creating
            // an NSRange that describes the range of bytes for the current word
            NSRange wordRange = NSMakeRange(wordIndex * bytesPerWord, bytesPerWord);
            
            // Load the bytes from that range into our buffer
            [data getBytes:tempByteArray range:wordRange];
            
            // Now reverse the bytes in this word. Work through the first half of
            // the byte array, swapping each byte with the corresponding byte from
            // the far end of the array. So for example, with 64-bit words we'd
            // swap indices 0 and 7, then 1 and 6, then 2 and 5, then 3 and 4.
            for (NSUInteger byteIndex = 0; byteIndex < bytesPerWord / 2; byteIndex++) {
                NSUInteger byteToSwapIndex = bytesPerWord - 1 - byteIndex;

                // Swap the values at indices byteIndex and byteToSwapIndex
                char temp = tempByteArray[byteIndex];
                tempByteArray[byteIndex] = tempByteArray[byteToSwapIndex];
                tempByteArray[byteToSwapIndex] = temp;
            }
            
            // Finally, write the newly-reversed word back to the original
            // data object
            [data replaceBytesInRange:wordRange withBytes:tempByteArray];
            
        }

        // Release the memory allocated with calloc earlier.
        free(tempByteArray);
        
        // We've reversed endianness, Captain! Write to our output file.
        [data writeToFile:outputFile atomically:NO];
        
    }
    return 0;
}