test_vma.c

#include <sys/mman.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <assert.h>
#include <string.h>

void* mmap_hint()
{
#ifdef MMAP_HINT
    return rand() * 1<<16;
#else
    return 0;
#endif
}

size_t subchr_count(const char *str, char s)
{
    size_t n = 0;
    char *begin = (char *)str;
    while ((begin = strchr(begin, s)) != NULL) {
        ++begin;
        ++n;
    }
    return n;
}
size_t get_vmas()
{
    size_t n = 0;
    /// NOTE: cannot use fopen() since it creates buffer via mmap
    int fd = open("/proc/self/maps", O_RDONLY);
    assert(fd >= 0);
    char buffer[4096];
    while (read(fd, buffer, sizeof(buffer)) > 0) {
        n += subchr_count(buffer, '\n');
    }
    close(fd);
    return n;
}

int main()
{
    void *addr;

    for (size_t i = 0; i < 100; ++i) {
        addr = mmap(mmap_hint(), 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
        fprintf(stdout, "vmas: %zu\n", get_vmas());
    }

    return 0;
}

test_vma.sh

$ /tmp/test_vma | uniq -c
      1 vmas: 22
     99 vmas: 23

test_vma_hint.sh

$ gcc -g3 -o /tmp/test_vma_hint -DMMAP_HINT /tmp/test_vma.c
$ /tmp/test_vma_hint | uniq -c | head
      1 vmas: 22
      1 vmas: 24
      1 vmas: 25
      2 vmas: 26
      2 vmas: 27
      1 vmas: 28
      1 vmas: 29
      1 vmas: 30
      2 vmas: 31
      1 vmas: 32