NSFileHandle

NSFileHandle *inFile, *outFile;

// open
inFile = [NSFileHandle fileHandleForReadingAtPath:szPath];
outFile = [NSFileHandle fileHandleForWritingAtPath:szPath];

// read, write
[outFile truncateFileAtOffset:lOffset];     // file size to offset
[outFile writeData:[NSData dataWithBytes:[szMsg UTF8String] length:[szMsg length]]];
szMsg = [[NSString alloc] initWithData:[inFile readDataToEndOfFile]
                                                       encoding:NSUTF8StringEncoding]];

// close
[inFile closeFile];
[outFile closeFile];