打印C字符串时的NSLog（）vs printf（）（UTF-8）

Question

I have noticed that if I try to print the byte array containing the representation of a string in UTF-8, using the format specifier "%s", printf() gets it right but NSLog() gets it garbled (ie, each byte printed as-is, so for example "¥" gets printed as the 2 characters: "¬•"). This is curious, because I always thought that NSLog() is just printf() , plus:

1. The first parameter (the 'format') is an Objective-C string, not a C string (hence the "@").
2. The timestamp and app name prepended.
3. The newline automatically added at the end.
4. The ability to print Objective-C objects (using the format "%@").

My code:

NSString* string; 

// (...fill string with unicode string...)

const char* stringBytes = [string cStringUsingEncoding:NSUTF8Encoding];

NSUInteger stringByteLength = [string lengthOfBytesUsingEncoding:NSUTF8Encoding];
stringByteLength += 1; // add room for '\0' terminator

char* buffer = calloc(sizeof(char), stringByteLength);

memcpy(buffer, stringBytes, stringByteLength);

NSLog(@"Buffer after copy: %s", buffer);
// (renders ascii, no matter what)

printf("Buffer after copy: %s\n", buffer);
// (renders correctly, e.g. japanese text)

Somehow, it looks as if printf() is "smarter" than NSLog() . Does anyone know the underlying cause, and if this feature is documented anywhere? (Couldn't find)

Answer 1

NSLog() and stringWithFormat: seem to expect the string for %s in the "system encoding" (for example "Mac Roman" on my computer):

NSString *string = @"¥";
NSStringEncoding enc = CFStringConvertEncodingToNSStringEncoding(CFStringGetSystemEncoding());
const char* stringBytes = [string cStringUsingEncoding:enc];
NSString *log = [NSString stringWithFormat:@"%s", stringBytes];
NSLog(@"%@", log);

// Output: ¥

Of course this will fail if some characters are not representable in the system encoding. I could not find an official documentation for this behavior, but one can see that using %s in stringWithFormat: or NSLog() does not reliably work with arbitrary UTF-8 strings.

If you want to check the contents of a char buffer containing an UTF-8 string, then this would work with arbitrary characters (using the boxed expression syntax to create an NSString from a UTF-8 string):

NSLog(@"%@", @(utf8Buffer));