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    <p>While testing and debugging your app with XCode you can use this <code>logMemUsage()</code> function to NSLog the used/free space and watch how things are going while you test your app. This function logs any change in usage > 100kb. It outputs to the debug log like this (on the simulator the free space is huge):</p> <pre><code>2011-11-02 21:55:58.928 hello[971:207] Memory used 21884.9 (+21885), free 1838366.8 kb 2011-11-02 21:55:59.936 hello[971:207] Memory used 28512.3 (+6627), free 1830809.6 kb 2011-11-02 21:56:01.936 hello[971:207] Memory used 28803.1 ( +291), free 1830129.6 kb 2011-11-02 21:56:02.936 hello[971:207] Memory used 29712.4 ( +909), free 1830142.0 kb </code></pre> <p>You decide where to call <code>logMemUsage</code> in your app. I happen to have a function that is called by a timer every second and so I put it in there. I suggest using <code>#ifdef</code> around these so this code is only included in Debug builds.</p> <pre><code>#import "mach/mach.h" vm_size_t usedMemory(void) { struct task_basic_info info; mach_msg_type_number_t size = sizeof(info); kern_return_t kerr = task_info(mach_task_self(), TASK_BASIC_INFO, (task_info_t)&amp;info, &amp;size); return (kerr == KERN_SUCCESS) ? info.resident_size : 0; // size in bytes } vm_size_t freeMemory(void) { mach_port_t host_port = mach_host_self(); mach_msg_type_number_t host_size = sizeof(vm_statistics_data_t) / sizeof(integer_t); vm_size_t pagesize; vm_statistics_data_t vm_stat; host_page_size(host_port, &amp;pagesize); (void) host_statistics(host_port, HOST_VM_INFO, (host_info_t)&amp;vm_stat, &amp;host_size); return vm_stat.free_count * pagesize; } void logMemUsage(void) { // compute memory usage and log if different by &gt;= 100k static long prevMemUsage = 0; long curMemUsage = usedMemory(); long memUsageDiff = curMemUsage - prevMemUsage; if (memUsageDiff &gt; 100000 || memUsageDiff &lt; -100000) { prevMemUsage = curMemUsage; NSLog(@"Memory used %7.1f (%+5.0f), free %7.1f kb", curMemUsage/1000.0f, memUsageDiff/1000.0f, freeMemory()/1000.0f); } } </code></pre>
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    1. COThe "freeMemory()" routine is exactly what I was looking for! Is there a reason why I shouldn't use this in product mode? I would like to know about how much memory is left, and then decide whether or not I can perform an operation that might make my app run out of memory.
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    2. COAs a follow up ... I used this on my original iPad, and I tried running a loop that just kept allocating small bits of memory. Interestingly, "free + used" kept changing (I guess not too surprising). My app started with 29MB free and 14MB used. As time went on, iOS would purge other apps from memory to make room for my app. When my app finally died, it had used up about 116MB. This is only about half of the memory on the device, but I guess the OS needs its share. After a fresh reboot, my App was able to get a bit more memory (125MB).
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    3. COLooking at how memory works in iOS, I guess it is hard to tell how much memory might be available to my App, even with the routines you provided. Even when my App first was down to 2MB of free memory remaining, iOS was still able to free up another 80MB to keep my App going. I guess I might have to resort to checking the device, and then restricting operations based on how much memory those devices have available (the iPad2 can handle much larger images than a 2G iTouch for example).
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