Question

I’m getting the following error while trying to run an iOS app I’m developing:

appname(6097,0x3e835d98) malloc: *** error for object 0x20104600: pointer being freed was not allocated
*** set a breakpoint in malloc_error_break to debug

How can I find out which call to free() generated this error? If I add a breakpoint for malloc_error_break, the whole stack trace I get is a call to “malloc_error_break” which looks like this:

libsystem_c.dylib`malloc_error_break:
0x32f20fa8:  push   {r7, lr}                           < Thread 1: breakpoint 1.1
0x32f20faa:  mov    r7, sp
0x32f20fac:  nop    
0x32f20fae:  nop    
0x32f20fb0:  pop    {r7, pc}
0x32f20fb2:  nop    

If I don’t add a breakpoint for malloc_error_break, I get a similarly useless stack trace, but this time with just a __pthread_kill and an assembler instruction.

Is there anything else I can do to get a better idea about the code that generated this error?

Answer 1

I have just created a new project called “Test” with the following main method:

void* data = malloc(100);    
free(data);
free(data);

Then I open “Product->Edit Scheme”, click on the “Test.app” tab and click on “Diagnostics”. Then I select “Enable Guard Malloc”.

Now, if I run the application, I get a break on the second free.