Valgrind prints the following message for the code below taken from http://miknight.blogspot.com/2005/11/resident-set-size-in-mac-os-x.html :

  UNKNOWN task message [id 3405, to mach_task_self(), reply 0xf03]

Looks like the wrapper for mach_msg_task (see m_syswrap/syswrap_darwin.c) doesn't handle neither id 3405 (which is task_info), nor 3406 (task_set_info).

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#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/sysctl.h>
#include <mach/task.h>
#include <mach/mach_init.h>

void getres(task_t task, unsigned int *rss, unsigned int *vs)
{
    struct task_basic_info t_info;
    mach_msg_type_number_t t_info_count = TASK_BASIC_INFO_COUNT;

    task_info(task, TASK_BASIC_INFO, (task_info_t)&t_info, &t_info_count);
    *rss = t_info.resident_size;
    *vs = t_info.virtual_size;
}

void memcheck()
{
    unsigned int rss, vs, psize;
    task_t task = MACH_PORT_NULL;

    if (task_for_pid(current_task(), getpid(), &task) != KERN_SUCCESS)
        abort();
    getres(task, &rss, &vs);
    psize = getpagesize();
    fprintf(stderr, "RSS: %u KiB, VS: %u KiB.\n", rss, vs);
}

int main() {
  memcheck();
  return 0;
}