__NSCFString appendString: crash for NSMutableString in Cocoa - cocoa

Question

I have been facing this issue and I admit that I lack of some fundamental concepts of memory managements. I've not been able to solve this and trust me, I've been trying so many things out.
In my app, there are 4 threads (can be up to 12 threads in future) which will read from RS232 ports. In each thread (threadRS232Read), I'll append the RS232 characters into respective NSMutableString using appendString. the NSMutableString will grow incredibly large with the appendString until a full test is completed. And periodically inside a test, clearRdBuffStr is called to clear the string. The app always crashes in appendString. If I get lucky, I can run few tests but normally, it crashes on the first run. Below are the code snippets and the crash log.
AppController.h
...
`#interface AppController : NSObject {
...
NSMutableString *buffStr1, *buffStr2, *buffStr3, *buffStr4;
...}
AppController.m
...
//in -(id)init
buffStr1 = [[NSMutableString alloc] initWithString:#""];
buffStr2 = [[NSMutableString alloc] initWithString:#""];
buffStr3 = [[NSMutableString alloc] initWithString:#""];
buffStr4 = [[NSMutableString alloc] initWithString:#""];
...
// in -(void)dealloc
[buffStr1 release];
[buffStr2 release];
[buffStr3 release];
[buffStr4 release];`
In another file RS232RW.m, a thread will be used to update buffStr1 to 4
RS232RW.m
- (void)threadRS232Read:(id)argument {
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
....
//read from RS232 port buffer and etc...
NSString *buffStr = [NSString stringWithUTF8String:buff];
switch (portNo) {
case 0:
if (buffStr != nil)
{
[buffStr1 appendString:buffStr];
}
break;
case 1:
if (buffStr != nil)
{
[buffStr2 appendString:buffStr];
}
break;
case 2:
if (buffStr != nil)
{
[buffStr3 appendString:buffStr];
}
break;
case 3:
if (buffStr != nil)
{
[buffStr4 appendString:buffStr];
}
break;
case 4:
if (buffStr != nil)
{
[buffStr5 appendString:buffStr];
}
break;
....
// clearRdBuffStr will be called by the other part of the program to clear this buffer.
-(void) clearRdBuffStr:(int) portNo {
switch (portNo) {
case 0:
[buffStr1 setString:#""];
break;
case 1:
[buffStr2 setString:#""];
break;
case 2:
[buffStr3 setString:#""];
break;
case 3:
[buffStr4 setString:#""];
break;
....
The app always crashes at 1 of the appendString above.
The crash log is as below:
....
Crashed Thread: 3
Exception Type: EXC_CRASH (SIGABRT)
Exception Codes: 0x0000000000000000, 0x0000000000000000
Application Specific Information:
objc[584]: garbage collection is OFF
*** error for object 0x6d02b600: double free
....
Thread 3 Crashed:
0 libsystem_kernel.dylib 0x905819c6 __pthread_kill + 10
1 libsystem_c.dylib 0x926ecf78 pthread_kill + 106
2 libsystem_c.dylib 0x926ddbdd abort + 167
3 libsystem_c.dylib 0x92701508 szone_error + 333
4 libsystem_c.dylib 0x92702dd1 free_small_botch + 102
5 com.apple.CoreFoundation 0x97ee51e8 __CFAllocatorSystemDeallocate + 24
6 com.apple.CoreFoundation 0x97ee51ba CFAllocatorDeallocate + 266
7 com.apple.CoreFoundation 0x97ee50a2 __CFStrDeallocateMutableContents + 178
8 com.apple.CoreFoundation 0x97ee422b __CFStringChangeSizeMultiple + 3147
9 com.apple.CoreFoundation 0x97f86010 __CFStringCheckAndReplace + 496
10 com.apple.CoreFoundation 0x97f95dad -[__NSCFString appendString:] + 45
11 com.TopTestDFU 0x00109f6d -[AppController(RS232RW)
threadRS232Read:] + 752
12 com.apple.Foundation 0x92aabf7d -[NSThread main] + 45
13 com.apple.Foundation 0x92aabf2d __NSThread__main__ + 1582
14 libsystem_c.dylib 0x926eaed9 _pthread_start + 335
15 libsystem_c.dylib 0x926ee6de thread_start + 34

Answer 1

The Cocoa mutable objects such as NSMutableString are not thread safe. It's up to you to arrange synchronisation using some form of lock.
Check out NSLock for one option.
You might try making the 4 NSMutableStrings into atomic properties rather than ivars. It might work in this case, although atomic doesn't actually guarantee thread safety.
[edit]
... actually, if in your case, you are sure that each thread is using a different port number, then it's probably not the NSMutableStrings as such that are the problem. It's probably NSString *buffStr = [NSString stringWithUTF8String:buff]; is being called by one thread before another has finished appending buffStr. So you need a lock around the that as well as the append.