NSString stringWithCString:encoding: - not copying the Cstring?

Question

I need to compute a signature (a kind of a hash) for my object. The computation requires many per-character operations, so to speed this procedure up, my code operates on CString, and after that converts computed CString to final NSString. The stub code looks like this:

- (NSString *)signature
{
    NSString *signatureString = @"?";
    char *buffer;

    buffer = malloc(sizeof(char)*(self.hashLength+1));
    if ( buffer ) {
        // Code computing a signature into the buffer

        signatureString = [NSString stringWithCString:buffer
                                             encoding:NSASCIIStringEncoding];
        free(buffer);
    }

    return signatureString;
}

I expected that upon creation of NSString characters from source CString are copied into internal structure of NSString object. However in runtime I am getting the error:

malloc: *** error for object 0x8f734d4: incorrect checksum for freed object - object was probably modified after being freed.
*** set a breakpoint in malloc_error_break to debug

This looks like chars are not copied, but used from their original memory location. Is it really so? Does it mean that I need to not free allocated memory? Will the NSString deallocate that CString memory upon object destruction?

I can not find any documentation explaining that issue, but because I want to avoid memory leak obviously, I would prefer to be sure,

Answer 1

So the answer is:

Yes, [NSString stringWithCString:encoding:] does copy the source CString to it's internal structure. If the source CString was allocated on a memory heap, it can be freed immediately after NSString object creation.

The error message I got, was caused be a bug inside signature computing code. Due to a wrongly initialized index variable in a for loop, code was writing beyond the allocated memory.

Answer 2

Use a different encoding.

NSASCIIStringEncoding DOcs state:

Strict 7-bit ASCII encoding within 8-bit chars; ASCII values 0…127 only.

NSMacOSRomanStringEncoding is a good choice because it handles all 8-bit values.