Basic memory management question

Hello,

I have a big memory leak in a method which simply reads from a text
file and adds each line to an NSMutableArray as an NSString, and then
return the array to the caller.

I'm positive that the memory used by the array of strings is not freed
unless Activity Monitor and ObjectAlloc both lie. ObjectAlloc does not
show any decrease in the number of allocated (and living!) CFStrings,
whatever I try to get the memory freed.

I'm new to Obj-C and from what I read in the Apple documentation,
arrays retain added objects and release removed objects. So I would
expect all lines to be released when the array gets out of scope.
Which it should do in the calling function, as I used autorelease on
the array. I tried without autorelease and an explicit release message
in the calling function, I tried removeAllObjects... Nothing works.
Any clue?

Cheers!


- (NSMutableArray*) readFileLines :(NSString*)path
{
NSError* err;
NSMutableArray* arr = [[NSMutableArray alloc] init];
NSString* contents = [NSString stringWithContentsOfFile :path
encoding:NSUTF8StringEncoding error:&err ]; // error unhandled

if( contents )
{
NSString* line;
NSScanner* scanner = [NSScanner scannerWithString :contents];
NSCharacterSet* lb = [NSCharacterSet
characterSetWithCharactersInString :@"\x0a\x0d"];

while( [scanner scanUpToCharactersFromSet :lb intoString:&line] )
[arr addObject :line];
}

return [arr autorelease];
}

On Jan 14, 2:50 pm, sebastien.waill....DeleteThis@gmail.com wrote:
> I have a big memory leak in a method which simply reads from a text
> file and adds each line to an NSMutableArray as an NSString, and then
> return the array to the caller.
>
> I'm positive that the memory used by the array of strings is not freed
> unless Activity Monitor and ObjectAlloc both lie. ObjectAlloc does not
> show any decrease in the number of allocated (and living!) CFStrings,
> whatever I try to get the memory freed.
>
> I'm new to Obj-C and from what I read in the Apple documentation,
> arrays retain added objects and release removed objects. So I would
> expect all lines to be released when the array gets out of scope.
> Which it should do in the calling function, as I used autorelease on
> the array. I tried without autorelease and an explicit release message
> in the calling function, I tried removeAllObjects... Nothing works.
> Any clue?
>
> Cheers!
>
> - (NSMutableArray*) readFileLines :(NSString*)path
> {
> NSError* err;
> NSMutableArray* arr = [[NSMutableArray alloc] init];
> NSString* contents = [NSString stringWithContentsOfFile :path
> encoding:NSUTF8StringEncoding error:&err ]; // error unhandled
>
> if( contents )
> {
> NSString* line;
> NSScanner* scanner = [NSScanner scannerWithString :contents];
> NSCharacterSet* lb = [NSCharacterSet
> characterSetWithCharactersInString :@"\x0a\x0d"];
>
> while( [scanner scanUpToCharactersFromSet :lb intoString:&line] )
> [arr addObject :line];
> }
>
> return [arr autorelease];
>
> }

Okay, so I rigged up a testbed application that got a path from
NSOpenPanel, passed it to your method, and retained the result. It
could also release the string array on command. It used bindings to
text fields to display the path and the count of the line array. I ran
it through a cycle of open, then release. I did this under
Instruments, with the ObjectAlloc and Leaks instruments running.

The application allocated 32348 CFStrings throughout its history. The
peak in allocations came when the open panel opened (adding ~19000
strings). Between loading and purging the line array, about 300
strings were allocated and released, which isn't crazy for a 60-line
file. None of those strings survived after the release.

The large number of strings allocated for the open panel stuck around.
This isn't crazy either; the shared NSOpenPanel sticks around after
its first use.

The Leaks instrument found one 16-byte block leaked by the time the
application quit. One. And none during the time the line array lived.

Your problem (if any) isn't with the method you posted (unless my
technique is much amiss). How did you determine that your method is
the source of a leak?

Remember that Foundation and AppKit allocate many, many strings for
their own purposes. This would also explain the rise in memory showin
in Activity Monitor. Also, Activity Monitor memory usage statistics
tend to rise monotonically. Malloc memory is allocated by partitioning
out large chunks of memory obtained from the OS. Those large chunks do
not get recycled; memory released within those chunks does get
recycled.

> So I would
> expect all lines to be released when the array gets out of scope.
> Which it should do in the calling function, as I used autorelease on
> the array.

Not quite. Unless you set up an NSAutoreleasePool of your own, and
release it yourself, the current pool won't be released until the next
pass through the event loop.


-- F