getmem.lua

local ffi = require 'ffi'

ffi.cdef([[
typedef long time_t;

typedef struct timeval {
 time_t tv_sec;
 time_t tv_usec;
};

struct rusage {
             struct timeval ru_utime; /* user time used */
             struct timeval ru_stime; /* system time used */
             long ru_maxrss;          /* integral max resident set size */
             long ru_ixrss;           /* integral shared text memory size */
             long ru_idrss;           /* integral unshared data size */
             long ru_isrss;           /* integral unshared stack size */
             long ru_minflt;          /* page reclaims */
             long ru_majflt;          /* page faults */
             long ru_nswap;           /* swaps */
             long ru_inblock;         /* block input operations */
             long ru_oublock;         /* block output operations */
             long ru_msgsnd;          /* messages sent */
             long ru_msgrcv;          /* messages received */
             long ru_nsignals;        /* signals received */
             long ru_nvcsw;           /* voluntary context switches */
             long ru_nivcsw;          /* involuntary context switches */
};
int getrusage(int who, struct rusage *r_usage);
]])

local time = ffi.new("struct rusage")

ffi.C.getrusage(0, time)

print(tonumber(time.ru_maxrss))