Re: file programming in the Gnustep framework

[Richard Frith-Macdonald]

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On 1 Jan 2008, at 15:13, Arno R. Schleich wrote:

> Hi,
>
> I am just starting to use the framework. I am looking for some  
> tutorial
> like stuff on file I/O in GNUStep. Is it advisable not to use "lower
> level" constructs such as fprintf etc at all ? From reading the online
> documentation for file handle/ file manager I could not figure out how
> to perform a trivial task like reading a variable length file of real
> numbers. Any hints where I would find some beginner's
> examples/instructions regarding file I/O ?

You should probably use NSFileManager to manage files (listing,  
moving, deleting etc).

For low level I/O you would use NSFileHandle or NSStream.
For instance, to read the variable length file you could do the job  
in two lines ...

NSFileHandle	*handle = [NSFileHandle fileHandleForReadingAtpath:  
@"myfile"];
NSData                  *contents = [handle readDataToEndOfFile];

That being said, you might do it in one line with:

NSData  *contents = [NSData dataWithContentsOfFile: @"myfile"];

If you want to read the file a bit at a time ...

NSInputStream	*istream = [NSInputStream inputStreamWithFileAtPath:  
@"myfile"];
double                  number;

while ([istream read: (uint8_t*)&number maxLength: sizeof(number)] ==  
sizeof(number))
  {
     // do something with each number here.
  }

For portability, you would want the numbers to be stored in a  
standard byte ordering, so you would use NSSwapBigDoubleToHost() to  
convert from network byte ordering to host byte ordering.

Of course, you might not want to do this sort of low-level stuff, but  
rather work with objects serialised to file.

I suggest looking at property-list objects (NSString, NSNumber,  
NSArray, NSDictionary, NSData, NSDate) as combinations of these  
objects can be written to or read from file as a single operation  
(eg. write an array of numbers by using the -writeToFile:atomically:  
method and read it back using [NSArray arrayWithContentsOfFile:  
@"afile"]) as these are great for a huge variety of applications.

To deal with complex groupings of arbitrary objects I'd recommend  
using NSKeyedArchiver or NSKeyedUnarchiver

For the equivalent of printf/scanf you would normally be appending to  
a mutable string (using appendFormat:) or scanning a string (using  
the NSScanner class) and using the -writeToFile:atomically: and  
stringWithContentsOfFile: methods to perform the file I/O (or  
converting the string to an NSData object and using NSFileHandle or  
NSStream if you want to send it over a network connection).




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