Retrieve RAM info on a Mac?

Question

I need to retrieve the total amount of RAM present in a system and the total RAM currently being used, so I can calculate a percentage. This is similar to: Retrieve system infor

Answer 1

You can figure out the answer to this question by looking at the source of the top command. You can download the source from http://opensource.apple.com/. The 10.7.2 source is available as an archive here or in browsable form here. I recommend downloading the archive and opening top.xcodeproj so you can use Xcode to find definitions (command-clicking in Xcode is very useful).

The top command displays physical memory (RAM) numbers after the label "PhysMem". Searching the project for that string, we find it in the function update_physmem in globalstats.c. It computes the used and free memory numbers from the vm_stat member of struct libtop_tsamp_t.

You can command-click on "vm_stat" to find its declaration as a membor of libtop_tsamp_t in libtop.h. It is declared as type vm_statistics_data_t. Command-clicking that jumps to its definition in /usr/include/mach/vm_statistics.h.

Searching the project for "vm_stat", we find that it is filled in by function libtop_tsamp_update_vm_stats in libtop.c:

mach_msg_type_number_t count = sizeof(tsamp->vm_stat) / sizeof(natural_t);
kr = host_statistics(libtop_port, HOST_VM_INFO, (host_info_t)&tsamp->vm_stat, &count);
if (kr != KERN_SUCCESS) {
    return kr;
}

You will need to figure out how libtop_port is set if you want to call host_statistics. I'm sure you can figure that out for yourself.

Answer 2

Getting the machine's physical memory is simple with sysctl:

int mib [] = { CTL_HW, HW_MEMSIZE };
int64_t value = 0;
size_t length = sizeof(value);

if(-1 == sysctl(mib, 2, &value, &length, NULL, 0))
    // An error occurred

// Physical memory is now in value

VM stats are only slightly trickier:

mach_msg_type_number_t count = HOST_VM_INFO_COUNT;
vm_statistics_data_t vmstat;
if(KERN_SUCCESS != host_statistics(mach_host_self(), HOST_VM_INFO, (host_info_t)&vmstat, &count))
    // An error occurred

You can then use the data in vmstat to get the information you'd like:

double total = vmstat.wire_count + vmstat.active_count + vmstat.inactive_count + vmstat.free_count;
double wired = vmstat.wire_count / total;
double active = vmstat.active_count / total;
double inactive = vmstat.inactive_count / total;
double free = vmstat.free_count / total;

There is also a 64-bit version of the interface.

Answer 3

It's been 4 years but I just wanted to add some extra info on calculating total RAM.

To get the total RAM, we should also consider Pages occupied by compressor and Pages speculative in addition to Kyle Jones answer.

You can check out this post for where the problem occurs.

Answer 4

You're not supposed to read from /usr/bin/vm_stat, rather you're supposed to run it; it is a program. Look at the first four lines of output

Pages free:                  1880145.
Pages active:                  49962.
Pages inactive:                43609.
Pages wired down:             123353.

Add the numbers in the right column and multiple by the system page size (as returned by getpagesize()) and you get the total amount of physical memory in the system in bytes.

vm_stat isn't setuid on Mac OS, so I assume there is a non-privileged API somewhere to access this information and that vm_stat is using it. But I don't know what that interface is.