mmap_alloc.cpp

//My improved version of code from this blog post:
//http://voices.canonical.com/jussi.pakkanen/2013/09/14/serialising-any-c-data-structure-to-disk-with-20-lines-of-code/

#include <map>
#include <string>

#include <sys/mman.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

void *mmap_start = (void*)0x7f15b0910000;
size_t offset = 0;
void *mapping;

void * operator new(std::size_t num)
{
    printf("Trying to allocate %ld bytes.\n", num);
    long returnvalue = (long)mmap_start + offset;
    offset += (num + 8) - (num % 8);
    return (void*)returnvalue;
}
void operator delete(void * p) throw() {}

void read() {
    auto *map = reinterpret_cast<std::map<std::string,
         std::string> *>(mmap_start);
    std::string key("key");
    printf("Sizeof map: %ld.\n", (long)map->size());
    printf("Value of 'key': %s\n", (*map)[key].c_str());
}

void write() {
    auto map = new std::map<std::string, std::string>();
    std::string key("key");
    std::string value("value2");
    (*map)[key] = value;
    printf("Sizeof map: %ld.\n", (long)map->size());
    printf("Value of 'key': %s\n", (*map)[key].c_str());
}

int main(int argc, char **argv) {
    int fd = open("backingstore.dat", O_RDWR);
    if(fd < 1) {
        printf("Open failed.\n");
        return 1;
    }
    mapping = mmap(mmap_start, 10*1024*1024,
            PROT_READ | PROT_WRITE, MAP_SHARED | MAP_FIXED, fd, 0);
    if(mapping == MAP_FAILED) {
        printf("MMap failed.\n");
        return 1;
    }
    write();
    read();
    return 0;
}