NSFileHandle seekToEndOfFile

gistfile1.m

NSFileHandle *aFileHandle;
NSString *aFile;

aFile = [NSString stringWithString:@"Your File Path"]; //setting the file to write to
aFile = [logFile stringByExpandingTildeInPath];

aFileHandle = [NSFileHandle fileHandleForWritingAtPath:aFile]; //telling aFilehandle what file write to
[aFileHandle truncateFileAtOffset:[aFileHandle seekToEndOfFile]]; //setting aFileHandle to write at the end of the file

[aFileHandle writeData:[toBeWritten dataUsingEncoding:nil]]; //actually write the data

For more Information see Documentation---Foundation---NSFileHandle