NSData's dataWithBytesNoCopy and writeToFile

Question

I have a category method that does somethign like this:

@implementation NSData (additions)
- (id)someFunc {

char *buffer = malloc(255);
NSUInteger length = 0;
while (true) {
    // . . . fill buffer[length++];
}

realloc(buffer, length);
return [NSData dataWithBytesNoCopy:buffer length:length freeWhenDone:NO];
}

I am then trying to write the returned data (well call this NSData *fileData):

NSError *error;
NSData fileData = [NSData someFunc];
[fileData writeToFile:somePath options:NSDataWritingAtomic error:&error];

I get an error:

Error Domain=NSCocoaErrorDomain Code=512 "The operation couldn’t be completed. (Cocoa error 512.)" UserInfo=0x20deffd0 {NSFilePath=/Users/user/Library/Application Support/iPhone Simulator/4.3/Applications/4C315580-153D-4FA7-9474-E17B58832515/Library/Caches/file.pdf, NSUnderlyingError=0x20de1fe0 "The operation couldn’t be completed. Bad address"}

The path exists and is valid. I think the issue is the returned NSData is just a light wrapper around an array allocated with malloc and that writeToFile does not know how to handle this. Does anyone see what I am doing wrong?

You need to save the address returned by realloc: buffer = realloc(buffer, length); — DarkDust
@DarkDust That was the problem, please make that an answer and I will accept it. — Tom Fobear

DarkDust DarkDust · Accepted Answer · 2011-06-24T15:24:05

When you call realloc you must save the returned pointer, since realloc may allocate a new buffer and copy the content of the old location to that new location. Afterwards the old location is freed and thus invalid. A different malloc call may overwrite that old location.

So you need to do:

buffer = realloc(buffer, length);

NSData's dataWithBytesNoCopy and writeToFile

2 Answers