Memory leaks are blocks of allocated memory that the program no longer references. Leaks waste space by filling up pages of memory with inaccessible data and waste time due to extra paging activity. Leaked memory eventually forces the system to allocate additional virtual memory pages for the application, the allocation of which could have been avoided by reclaiming the leaked memory.
For apps that use malloc, memory leaks are bugs and should always be fixed. For apps that use only Objective-C objects, the compiler’s ARC feature deallocates objects for you and generally avoids the problem of memory leaks. However, apps that mix the use of Objective-C objects and C-based structures must manage the ownership of objects more directly to ensure that the objects are not leaked.
The malloc library can only reclaim the memory you tell it to reclaim. If you call malloc or any routine that allocates memory, you must balance that call with a corresponding free. A typical memory leak occurs when you forget to deallocate memory for a pointer embedded in a data structure. If you allocate memory for embedded pointers in your code, make sure you release the memory for that pointer prior to deallocating the data structure itself.
Another typical memory leak example occurs when you allocate memory, assign it to a pointer, and then assign a different value to the pointer without freeing the first block of memory. In this example, overwriting the address in the pointer erases the reference to the original block of memory, making it impossible to release.
Finding Leaks Using Instruments
The Instruments application can be used to find leaks in both OS X and iPhone applications. To find leaks, create a new document template in Instruments and add the Leaks instrument to it. The Leaks instrument provides leak-detection capabilities identical to those in the leaks command-line tool. The Leaks instrument records all allocation events that occur in your application and then periodically searches the application’s writable memory, registers, and stack for references to any active memory blocks. If it does not find a reference to a block in one of these places, it deems the block a “leak” and displays the relevant information in the Detail pane.
In the Detail pane, you can view leaked memory blocks using Table and Outline modes. In Table mode,
Instruments displays the complete list of leaked blocks, sorted by size. Selecting an entry in the table and clicking the arrow button next to the memory address shows the allocation history for the memory block at that address. Selecting an entry from this allocation history then shows the stack trace for that event in the Extended Detail pane of the document window. In Outline mode, the Leaks instrument displays leaks organized by call tree, which you can use to get information about the leaks in a particular branch of your code.
For apps that use malloc, memory leaks are bugs and should always be fixed. For apps that use only Objective-C objects, the compiler’s ARC feature deallocates objects for you and generally avoids the problem of memory leaks. However, apps that mix the use of Objective-C objects and C-based structures must manage the ownership of objects more directly to ensure that the objects are not leaked.
The malloc library can only reclaim the memory you tell it to reclaim. If you call malloc or any routine that allocates memory, you must balance that call with a corresponding free. A typical memory leak occurs when you forget to deallocate memory for a pointer embedded in a data structure. If you allocate memory for embedded pointers in your code, make sure you release the memory for that pointer prior to deallocating the data structure itself.
Another typical memory leak example occurs when you allocate memory, assign it to a pointer, and then assign a different value to the pointer without freeing the first block of memory. In this example, overwriting the address in the pointer erases the reference to the original block of memory, making it impossible to release.
Finding Leaks Using Instruments
The Instruments application can be used to find leaks in both OS X and iPhone applications. To find leaks, create a new document template in Instruments and add the Leaks instrument to it. The Leaks instrument provides leak-detection capabilities identical to those in the leaks command-line tool. The Leaks instrument records all allocation events that occur in your application and then periodically searches the application’s writable memory, registers, and stack for references to any active memory blocks. If it does not find a reference to a block in one of these places, it deems the block a “leak” and displays the relevant information in the Detail pane.
In the Detail pane, you can view leaked memory blocks using Table and Outline modes. In Table mode,
Instruments displays the complete list of leaked blocks, sorted by size. Selecting an entry in the table and clicking the arrow button next to the memory address shows the allocation history for the memory block at that address. Selecting an entry from this allocation history then shows the stack trace for that event in the Extended Detail pane of the document window. In Outline mode, the Leaks instrument displays leaks organized by call tree, which you can use to get information about the leaks in a particular branch of your code.
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