InitWithBytesNoCopy:length:encoding:freeWhenDone: (NSString)Edit

Created 8/4/2006, updated 3/12/2020

An experiment with some leaking strings:

#import <Foundation/Foundation.h>

int main (int argc, const char * argv[]) {
    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];

    // Question: how does initWithBytesNoCopy:length:encoding:freeWhenDone: handle NULL bytes?

    const char *str = "hello"; // 5 letters plus NULL terminating byte
    NSLog(@"str \"%s\" has length %d", str, strlen(str));   // str "hello" has length 5

    NSString *string = [[NSString alloc] initWithBytesNoCopy:(void *)str
                                                      length:5
                                                    encoding:NSUTF8StringEncoding
                                                freeWhenDone:NO];

    NSLog(@"string created with length 5: %@ (%d)", string, [string length]);       // string created with length 5: hello (5)
    NSLog(@"roundtrip back to string -> length %d", strlen([string UTF8String]));   // roundtrip back to string -> length 5

    string = [[NSString alloc] initWithBytesNoCopy:(void *)str
                                            length:6
                                          encoding:NSUTF8StringEncoding
                                      freeWhenDone:NO];

    // note that in this line echoing the string causes the NULL byte to be echoed, cutting off the rest of the NSLog
    NSLog(@"string created with length 6: %@ (%d)", string, [string length]);       // string created with length 6: hello
    NSLog(@"length %d", [string length]);                                           // length 6
    NSLog(@"roundtrip back to string -> length %d", strlen([string UTF8String]));   // roundtrip back to string -> length 5

    string = [NSString stringWithUTF8String:str];
    NSLog(@"string created with stringWithUTF8String: %@ (%d)", string, [string length]);   // string created with stringWithUTF8String: hello (5)
    NSLog(@"roundtrip back to string -> length %d", strlen([string UTF8String]));           // roundtrip back to string -> length 5

    // char array with NULL byte in the middle
    char tricky[10] = { 'h', 'e', 'l', 'l', 'o', 0, 'r', 'o', 'b', 0 };
    string = [NSString stringWithUTF8String:tricky];
    NSLog(@"string: %@ (%d)", string, [string length]);                             // string: hello (5)
    NSLog(@"roundtrip back to string -> length %d", strlen([string UTF8String]));   // roundtrip back to string -> length 5

    string = [[NSString alloc] initWithBytesNoCopy:(void *)tricky
                                            length:9
                                          encoding:NSUTF8StringEncoding
                                      freeWhenDone:NO];

    NSLog(@"string created with length 9: %@ (%d)", string, [string length]);       // string created with length 9: hello
    NSLog(@"length %d", [string length]);                                           // length 9
    NSLog(@"roundtrip back to string -> length %d", strlen([string UTF8String]));   // roundtrip back to string -> length 5

    string = [[NSString alloc] initWithBytesNoCopy:(void *)tricky
                                            length:10
                                          encoding:NSUTF8StringEncoding
                                      freeWhenDone:NO];

    NSLog(@"string created with length 10: %@ (%d)", string, [string length]);      // string created with length 10: hello
    NSLog(@"length %d", [string length]);                                           // length 10
    NSLog(@"roundtrip back to string -> length %d", strlen([string UTF8String]));   // roundtrip back to string -> length 5

    // are the characters after the NULL byte really there?
    int i;
    for (i = 0; i < 10; i++)
        NSLog(@"%d: %C", i, [string characterAtIndex:i]);

    // 0: h
    // 1: e
    // 2: l
    // 3: l
    // 4: o
    // 5:
    // 6: r
    // 7: o
    // 8: b
    // 9:

    [pool release];
    return 0;
}

Conclusions

The value you pass for length need not include space for a NULL terminating byte.
You can pass a buffer containing mid-string NULL bytes and provided the length is set correctly the string won't be truncated.
If you try to print such a string containing a mid-string NULL byte using NSLog output will be incorrectly truncated at the NULL byte.
Even so, the actual characters really are present in the created string.

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