How to read from memory using a file descriptor

memory_fd.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include <sys/mman.h>
#include <sys/stat.h>        /* For mode constants */
#include <fcntl.h>           /* For O_* constants */

const char source[] = 
    "Generalized algebraic data types (GADTs) are a feature of functional "
    "programming languages which generalize ordinary algebraic data types by "
    "permitting value constructors to return special types. This article is "
    "a tutorial about GADTs in Haskell programming language as they are "
    "implemented by the Glasgow Haskell Compiler (GHC) as a language extension";

int main(void)
{
    void *addr;
    ssize_t size;
    int fd, source_len;

    source_len = strlen(source) + 1;
    fd = shm_open("/alex", O_RDWR | O_CREAT | O_TRUNC, 0600);
    printf("Got descriptor: %d\n", fd);
    if(ftruncate(fd, source_len) < 0)
    {
        perror("ftruncate");
        return 42;
    }

    if((addr = mmap(NULL, source_len, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0)) == MAP_FAILED)
    {
        perror("mmap");
        return 42;
    }
    memcpy(addr, source, source_len);
    if(munmap(addr, source_len) < 0)
    {
        perror("munmap");
        return 42;
    }

    /* now the BIG test... reading from fd */
    addr = calloc(source_len, 1);
    size = read(fd, addr, source_len);
    printf("Sizes: %d, %d\n", source_len, size);
    printf("Result: %s\n", addr);

    free(addr);
    close(fd);
    return 0;
}