FreeBSD

% /compat/linux/bin/gcc multiple_threads.c -pthread -o test
% ./test 
thread_start: 3
[[0x7ffffff995b0]]
thread_start: 2
[[0x821182e30]]
thread_start: 1
[[0x84119ce30]]
thread_start: 0
[[0x867f9be30]]
00000008211d0000-00000008411d0000 rw-p 00000000 00:00 0
0000000844000000-0000000844021000 rw-p 00000000 00:00 0
0000000844021000-0000000848001000 ---p 00000000 00:00 0
0000000848001000-0000000868001000 rw-p 00000000 00:00 0
000000086c000000-000000086c021000 rw-p 00000000 00:00 0
000000086c021000-0000000870000000 ---p 6ae58000 00:00 0
00007fffdffff000-00007ffffff7f000 ---p 00000000 00:00 0
00007ffffff7f000-00007ffffffdf000 rw-p 00000000 00:00 0
00007ffffffdf000-00007ffffffff000 rw-p 00000000 00:00 0           [stack]
00007ffffffff000-0000800000000000 r-xs 00000000 00:00 0           [vdso]
  PID              START                END PRT  RES PRES REF SHD FLAG  TP PATH
 2303           0x400000           0x401000 r-x    1    2   2   1 CN--- vn /<...>/test
 2303           0x600000           0x601000 r--    1    0   1   0 CN--- vn /<...>/test
 2303           0x601000           0x623000 rw-    2    2   1   0 CN--- df 
 2303        0x800600000        0x800622000 r-x   33   34   6   3 CN--- vn /compat/linux/usr/lib64/ld-2.17.so
 2303        0x800622000        0x800623000 rw-    1    1   1   0 CN--- df 
 2303        0x800630000        0x800633000 rw-    2    2   1   0 CN--- df 
 2303        0x800821000        0x800822000 r--    1    0   1   0 CN--- vn /compat/linux/usr/lib64/ld-2.17.so
 2303        0x800822000        0x800824000 rw-    2    2   1   0 CN--- df 
 2303        0x800a00000        0x800a17000 r-x   23   24   4   2 CN--- vn /compat/linux/usr/lib64/libpthread-2.17.so
 2303        0x800a17000        0x800c16000 ---    1   24   4   2 CN--- vn /compat/linux/usr/lib64/libpthread-2.17.so
 2303        0x800c16000        0x800c17000 r--    1    0   1   0 CN--- vn /compat/linux/usr/lib64/libpthread-2.17.so
 2303        0x800c17000        0x800c18000 rw-    1    0   1   0 CN--- vn /compat/linux/usr/lib64/libpthread-2.17.so
 2303        0x800c18000        0x800c1c000 rw-    1    1   1   0 CN--- df 
 2303        0x800e00000        0x800fc3000 r-x  363  369  11   5 CN--- vn /compat/linux/usr/lib64/libc-2.17.so
 2303        0x800fc3000        0x8011c3000 ---    6  369  11   5 CN--- vn /compat/linux/usr/lib64/libc-2.17.so
 2303        0x8011c3000        0x8011c7000 r--    4    0   1   0 CN--- vn /compat/linux/usr/lib64/libc-2.17.so
 2303        0x8011c7000        0x8011c9000 rw-    2    0   1   0 CN--- vn /compat/linux/usr/lib64/libc-2.17.so
 2303        0x8011c9000        0x8011ce000 rw-    3    3   1   0 CN--- df 
 2303        0x8011ce000        0x8011cf000 ---    0    0   1   0 CN--- df 
 2303        0x8011cf000        0x8211cf000 rw-   80   80   1   0 CN--- df 
 2303        0x8211cf000        0x8211d0000 ---    0    0   1   0 CN--- df 
 2303        0x8211d0000        0x8411d0000 rw-   55   55   1   0 CN--- df 
 2303        0x844000000        0x844021000 rw-    1    1   1   0 CN--- df 
 2303        0x844021000        0x848001000 ---    0    0   1   0 CN--- df 
 2303        0x848001000        0x868001000 rw-  105  105   1   0 C---- df 
 2303        0x86c000000        0x86c021000 rw-    1    1   1   0 CN--- df 
 2303        0x86c021000        0x870000000 ---    0    0   1   0 CN--- df 
 2303     0x7fffdffff000     0x7ffffff7f000 ---    0    0   0   0 ----- -- 
 2303     0x7ffffff7f000     0x7ffffffdf000 rw-   73   73   1   0 CN-D- df 
 2303     0x7ffffffdf000     0x7ffffffff000 rw-   32   32   1   0 CN-D- df 
 2303     0x7ffffffff000     0x800000000000 r-x    1    1   4   0 ----- ph 

Linux

xubuntu@xubuntu:~$ cc multiple_threads.c -pthread -o test
xubuntu@xubuntu:~$ ./test
thread_start: 3
[[0x7ffc27e05060]]
thread_start: 2
[[0x7fb6a36e2e10]]
thread_start: 1
[[0x7fb6a311ee10]]
thread_start: 0
[[0x7fb6a2b0fe10]]
7fb6a395d000-7fb6a395e000 r--p 00000000 00:19 4481                       /usr/lib/x86_64-linux-gnu/ld-2.31.so
7fb6a395e000-7fb6a3981000 r-xp 00001000 00:19 4481                       /usr/lib/x86_64-linux-gnu/ld-2.31.so
7fb6a3981000-7fb6a3989000 r--p 00024000 00:19 4481                       /usr/lib/x86_64-linux-gnu/ld-2.31.so
7fb6a398a000-7fb6a398b000 r--p 0002c000 00:19 4481                       /usr/lib/x86_64-linux-gnu/ld-2.31.so
7fb6a398b000-7fb6a398c000 rw-p 0002d000 00:19 4481                       /usr/lib/x86_64-linux-gnu/ld-2.31.so
7fb6a398c000-7fb6a398d000 rw-p 00000000 00:00 0 
7ffc27e02000-7ffc27e6c000 rw-p 00000000 00:00 0                          [stack]
7ffc27f45000-7ffc27f48000 r--p 00000000 00:00 0                          [vvar]
7ffc27f48000-7ffc27f49000 r-xp 00000000 00:00 0                          [vdso]
ffffffffff600000-ffffffffff601000 --xp 00000000 00:00 0                  [vsyscall]
sh: 1: procstat: not found

multiple_threads.c

#define _GNU_SOURCE

#include <assert.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <pthread.h>
#include <unistd.h>

void* thread_start(void* arg) {

  long i = (long)arg;
  fprintf(stderr, "%s: %ld\n", __func__, i);

  // allocate something on stack
  int n = (1 + rand() % 4) * 100 * 1024;
  char a[n];
  memset(a, 0, n);
  fprintf(stderr, "[[%p]]\n", a);

  if (i == 0) {
    {
      char cmd[100];
#ifdef __FreeBSD__
      snprintf(cmd, sizeof(cmd), "cat /compat/linux/proc/%d/maps | tail -n 10", getpid());
#else
      snprintf(cmd, sizeof(cmd), "cat /proc/%d/maps | tail -n 10", getpid());
#endif
      system(cmd);
    }

    {
      char cmd[100];
      snprintf(cmd, sizeof(cmd), "procstat -v %d | tail -n 40", getpid());
      system(cmd);
    }
  } else {

    pthread_t t;
    int err = pthread_create(&t, NULL, &thread_start, (void*)(i - 1));
    if (err != 0) {
      fprintf(stderr, "pthread_create: %s\n", strerror(err));
    }

    sleep(1);
  }

  return 0;
}

int main() {
  thread_start((void*)3);
  return 0;
}