Every mapping created by mmap() has exactly two independent attributes. Think of them as two separate on/off switches:

These two dimensions combine to produce 4 types of mappings (covered in the next section).

The combination of file/anonymous × private/shared gives four distinct mapping types, each used for a different purpose:

Two processes can end up pointing to the same physical RAM pages in two ways:

Whether these processes actually see each other’s writes depends on MAP_PRIVATE vs MAP_SHARED.

With MAP_PRIVATE, modifications are not visible to other processes. The kernel achieves this through the copy-on-write mechanism:

This is exactly the same mechanism used by fork() for the process’s heap and stack pages.

With MAP_SHARED, all processes that map the same region share the same physical pages. There is no copy-on-write. A write by one process is immediately visible to all others sharing that mapping.

For file-backed shared mappings, writes are also propagated back to the underlying file on disk (though potentially with a delay — the kernel’s page cache handles this). This is the basis of memory-mapped I/O.

Linux exposes every mapping of a process in /proc/PID/maps. Each line shows: address range, permissions, offset, device, inode, and pathname (if file-backed).

/* Read and print your own process's memory map */
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int main(void)
{
    char path[64];
    char line[256];
    FILE *fp;

    /* /proc/self/maps is shorthand for /proc/<our-PID>/maps */
    snprintf(path, sizeof(path), "/proc/self/maps");

    fp = fopen(path, "r");
    if (fp == NULL) {
        perror("fopen /proc/self/maps");
        exit(EXIT_FAILURE);
    }

    printf("%-40s %-6s %-8s %s\n",
           "Address Range", "Perms", "Offset", "Pathname");
    printf("%s\n", "-----------------------------------------------------------");

    while (fgets(line, sizeof(line), fp) != NULL) {
        printf("%s", line);
    }

    fclose(fp);
    return 0;
}

Sample output fields:

7f8a3c000000-7f8a3c021000 r--p 00000000 08:01 12345   /lib/x86_64-linux-gnu/libc.so.6
7f8a3c021000-7f8a3c176000 r-xp 00021000 08:01 12345   /lib/x86_64-linux-gnu/libc.so.6
7fff3b400000-7fff3b421000 rw-p 00000000 00:00 0       [stack]

Notice: r-xp = read + execute + private (code), rw-p = read + write + private (stack/heap), r--s would be read + shared.