Are multiple MAP_PRIVATE mappings of the same file, in the same process, still private?

data.txt

0123456789ABCDEF

multimap.c

/**
 * Answering the question:
 * "Are multiple MAP_PRIVATE mappings of the same file, in the same process,
 * still private?"
 * https://stackoverflow.com/q/66925360/119527
 */
#include <errno.h>
#include <error.h>
#include <fcntl.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <sys/mman.h>
#include <sys/stat.h>

static void *map_file(int fd, size_t size)
{
    void *m;

    m = mmap(NULL, size, PROT_READ|PROT_WRITE, MAP_PRIVATE, fd, 0);
    if (m == MAP_FAILED)
        error(3, errno, "Failed to mmap fd %d\n", fd);

    return m;
}

int main(int argc, char **argv)
{
    const int nprint = 16;
    static const char replace[] = "MODIFIED DATA!!!";

    const char *path;
    int fd;
    struct stat statbuf;
    size_t sz;
    void *m[2];
    int i;

    if (argc < 2)
        error(1, 0, "Usage: multimap PATH");
    path = argv[1];

    fd = open(path, O_RDONLY);
    if (fd < 0)
        error(2, errno, "Failed to open %s\n", path);

    if (fstat(fd, &statbuf) < 0)
        error(3, errno, "Failed to stat fd %d\n", fd);
    sz = statbuf.st_size;

    if (sz < nprint)
        error(4, 0, "Data too small");

    for (i = 0; i < 2; i++) {
        m[i] = map_file(fd, sz);
        printf("Initial m[%d] data: \"%.*s\"\n", i, nprint, (char*)m[i]);
    }

    memcpy(m[0], replace, sizeof(replace));

    for (i = 0; i < 2; i++) {
        printf("Final m[%d] data: \"%.*s\"\n", i, nprint, (char*)m[i]);
        munmap(m[i], sz);
    }
    close(fd);

    return 0;
}

result.md

Linux output

$ uname -rv
4.19.0-16-amd64 #1 SMP Debian 4.19.181-1 (2021-03-19)

$ gcc -o multimap -Wall -Werror multimap.c

$ ./multimap data.txt
Initial m[0] data: "0123456789ABCDEF"
Initial m[1] data: "0123456789ABCDEF"
Final m[0] data: "MODIFIED DATA!!!"
Final m[1] data: "0123456789ABCDEF"