mmap memory fractions into unified memory address

mmap_shm.c

/* 
POSIX: 
gcc -std=gnu99 -o mmap_shm mmap_shm.c -lrt
 */
#include <stdio.h>
#include <stdint.h>
#include <assert.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <string.h>

int
main(void)
{
    size_t size = 0;
    void *addr[5] = {0};
    size_t naddr = sizeof(addr) / sizeof(addr[0]);
    printf("naddr = %zu\n", naddr);
    long pagesz = sysconf(_SC_PAGESIZE);
    for(int i=0 ; i<naddr; i++) {
        // addr[i] = malloc(pagesz);
        addr[i] = mmap(NULL, pagesz, PROT_READ|PROT_WRITE, MAP_SHARED|MAP_ANONYMOUS, -1, 0);
        printf("addr[%d]=%p\n", i, addr[i]);
        size += pagesz;
    }

    char path[128];
    // snprintf(path, sizeof(path), "/tmp/mmap_pid_%lu",(long)getpid());
    // int fd = open(path, O_RDWR|O_CREAT, 0666);
    snprintf(path, sizeof(path), "/%s(%lu,%p)",
             __FUNCTION__, (long)getpid(), addr);
    int fd = shm_open(path, O_RDWR | O_CREAT | O_EXCL, 0600);
    if (fd == -1)
        return -1;
    ftruncate(fd, size);
    shm_unlink(path);
    size_t offset = 0;
    void *p = mmap(addr[0], pagesz, PROT_READ | PROT_WRITE, MAP_SHARED|MAP_FIXED, fd, offset);
    assert(addr[0] != MAP_FAILED);
    offset += pagesz;
    void *p1 = mmap(addr[2], pagesz, PROT_READ | PROT_WRITE, MAP_SHARED|MAP_FIXED, fd, offset);
    assert(addr[2] != MAP_FAILED);
    offset += pagesz;
    void *p2 = mmap(addr[4], pagesz, PROT_READ | PROT_WRITE, MAP_SHARED|MAP_FIXED, fd, offset);
    assert(addr[4] != MAP_FAILED);
    offset += pagesz;

    printf("%p, %p, %p\n", p, p1, p2);
    printf("size = 0x%lx\n", offset);
    // -----------------------------------------------------------//
    char *t = (char*)mmap(0, offset, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
    printf("t= %p\n", t);
    // memset(t, ~0, offset);
    memset(t, 0, offset);
    *(char*)addr[0] = 'A';
    
    printf("addr[0][%d]=%x\n", 0, *(char*)(addr[0]));
    printf("p[%d]=%x\n", 0, *(char*)p);
    printf("t[%d]=%x\n", 0, t[0]);
    munmap(addr[0], pagesz);
    munmap(addr[2], pagesz);
    munmap(addr[4], pagesz);
    munmap(t, offset);
    for(int i=0 ; i<naddr; i++) {
        munmap(addr[i], pagesz);
    }

    getchar();
    close(fd);
    // unlink(path);
    return 0;
}