writeToFile fails with error = null

I have an app running in the XCode simulator (v6.4); this is the pertinent code:

            NSString *documentsPath = [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) objectAtIndex:0];

        //  read the file back into databuffer...
        NSFileHandle *readFile = [NSFileHandle fileHandleForReadingAtPath:[documentsPath stringByAppendingPathComponent: @"Backup.txt"]];
        NSData *databuffer = [readFile readDataToEndOfFile];
        [readFile closeFile];

        // compress the file
        NSData *compressedData = [databuffer gzippedData] ;

        // Write to disk
        NSString *outputPath = [NSString stringWithFormat:@"%@/%@%@.zip", documentsPath, venueName, strDate];
        _BackupFilename = fileName;  //  save for upload

        NSFileHandle *outputFile = [NSFileHandle fileHandleForWritingAtPath:outputPath];
        NSError *error = nil;

        //  write the data for the backup file
        BOOL success = [compressedData writeToFile: outputPath options: NSDataWritingAtomic error: &error];

        if (error == nil && success == YES) {
            NSLog(@"Success at: %@",outputPath);
        }
        else {
            NSLog(@"Failed to store. Error: %@",error);
        }

        [outputFile closeFile];

I'm trying to create a backup of a file by taking the file, compressing it and then writing it out. I get an error Failed to store. Error: (null)); why is it failing without returning the error code?

Solution

There are quite a few things wrong here. To start. change your if statement to:

if (success) {

Never explicitly compare a BOOL value to YES or NO.

You also never use outputFile so remove that code. It may interfere with the call to writeToFile:.

And there's no point in using the file handle to read the data. Just use NSData dataWithContentsOfFile:.

And don't build paths using stringWithFormat:.

Overall, I would write your code as:

NSString *documentsPath = [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) objectAtIndex:0];

//  read the file back into databuffer...
NSString *dataPath = [documentsPath stringByAppendingPathComponent:@"Backup.txt"]];
NSData *databuffer = [NSData dataWithContentsOfFile:dataPath];

// compress the file
NSData *compressedData = [databuffer gzippedData];

// Write to disk
NSString *outputName = [NSString stringWithFormat:@"%@%@.zip", venueName, strDate];
NSString *outputPath = [documentsPath stringByAppendingPathComponent:outputName];

//  write the data for the backup file
NSError *error = nil;
BOOL success = [compressedData writeToFile:outputPath options:NSDataWritingAtomic error:&error];

if (success) {
    NSLog(@"Success at: %@",outputPath);
} else {
    NSLog(@"Failed to store. Error: %@",error);
}

Since success is still NO and error is still nil, then most likely this means that compressedData is nil. This probably means that databuffer is nil which means that there is no file named Backup.txt (case matters) in the Documents folder.