Landon Fuller

The iPhone has limited memory, and even simple applications can easily trigger a low memory warning. If you've implemented caching for performance reasons, you'll often find yourself balancing memory consumption against user experience.

Measuring the current available RAM allows one to make pre-emptive decisions about memory utilization before a low memory warning is triggered, possibly avoiding overly broad cache evictions when a memory warning is triggered.

I've seen this question come up a number of times, so here's a brief code snippet demonstrating how to determine available memory from the Mach VM statistics.

#import <mach/mach.h>
#import <mach/mach_host.h>
 
static void print_free_memory () {
    mach_port_t host_port;
    mach_msg_type_number_t host_size;
    vm_size_t pagesize;
    
    host_port = mach_host_self();
    host_size = sizeof(vm_statistics_data_t) / sizeof(integer_t);
    host_page_size(host_port, &pagesize);        
 
    vm_statistics_data_t vm_stat;
              
    if (host_statistics(host_port, HOST_VM_INFO, (host_info_t)&vm_stat, &host_size) != KERN_SUCCESS)
        NSLog(@"Failed to fetch vm statistics");
 
    /* Stats in bytes */ 
    natural_t mem_used = (vm_stat.active_count +
                          vm_stat.inactive_count +
                          vm_stat.wire_count) * pagesize;
    natural_t mem_free = vm_stat.free_count * pagesize;
    natural_t mem_total = mem_used + mem_free;
    NSLog(@"used: %u free: %u total: %u", mem_used, mem_free, mem_total);
}

For more information, see the vm_statistics man page.