Not able to edit first byte of file using NSFileHandle - iphone

Question

In my app, I am using NSFileHandle to edit some file but it is not editing.
Below is the code: with comments & logs output
//Initialize file manager
NSFileManager *filemgr;
filemgr = [NSFileManager defaultManager];
//Initialize file handle
NSFileHandle *fileHandle = [NSFileHandle fileHandleForReadingAtPath:filePath];
//Check if file is writable
if ([filemgr isWritableFileAtPath:filePath] == YES)
NSLog (#"File is writable");
else
NSLog (#"File is read only");
//Read 1st byte of file
NSData *decryptData = [fileHandle readDataOfLength:1];
//Print first byte & length
NSLog(#"data1: %d %#",[decryptData length],decryptData); //data2: 1 <37>
//Replace 1st byte
NSData *zeroData = 0;
[fileHandle writeData:zeroData];
//Read 1st byte to check
decryptData = [fileHandle readDataOfLength:1];
//Print first byte
NSLog(#"data2: %d %#",[decryptData length],decryptData); //data2: 1 <00>
NSURL *fileUrl=[NSURL fileURLWithPath:filePath];
NSLog(#"fileUrl:%#",fileUrl);
[fileHandle closeFile];
Any Suggestions?

Answer 1

If you want to write using NSFileHandle you need to open the file for writing as well as reading:
NSFileHandle *fileHandle = [NSFileHandle fileHandleForUpdatingAtPath:filePath];
If you aren't sure if the file at the specified path is writable, you should check for the appropriate permissions before you open it and display an error to the user if they are insufficient for what you need to do.
Also, to write data you need to create an instance of NSData. The line of code
NSData *zeroData = 0;
is creating a nil object, not an object containing a zero byte. I think you want
int8_t zero = 0;
NSData *zeroData = [NSData dataWithBytes:&zero length:1];