Memory Mapped File

Hi,

I'm working on an assignment in which I have to use a memory mapped file.

In another assignment I've done this before like this:

Code:

---

static unsigned long *Buff = NULL;
static HANDLE hFileMap;

    // Create File Mapping
    hFileMap = CreateFileMapping(INVALID_HANDLE_VALUE, NULL, PAGE_READWRITE, 0, maxSize + 1, "memMappedFile");
    if (!hFileMap)
    {
        perror("Unable to create file mapping,Error");
        return;
    }
   
    // Map View Of File (access memory mapped file and place it in memory)
    if ( !(Buff = MapViewOfFile(hFileMap, FILE_MAP_WRITE, 0, 0, 0) ) )
    {
        perror("Unable to create view of mapped file,Error");
       
        // Cleanup
        CloseHandle(hFileMap);
       
        return;
    }

---

I've used the code above to create a circulair buffer in C.

I'm, however, using using C++ now and the above code (when modified) fails.

I want to map an arraylist:

string neighbourList[5][7]

...to make it available for another proces.

Can someone help me out here?

I'm not sure if I can map a two dimensional array at all, but I'm receiving the following error:

error C2440: '=' : cannot convert from 'LPVOID' to 'std::string [5][7]'

But the memory held by an object is not (guaranteed to be) a contiguous chunk of memory. So the string is perhaps a structure like this:

Code:

---

class string {
  char *strdata;
  int len;
};

---

So whilst you can solve the problem of storing a "string array" in the buffer by using (void *) to make it a compatible pointer, you are not storing the string content in the file.

You probably need to rethink this strategy.

--
Mats

Don't try to host any std:: objects in shared memory - there's too many things that can and will go wrong. Stick with built-in data types or simple structures composed of built-in types.

gg

See last message...

to copy C-strings use strcpy

Besided Vart's comment, are you sure this is right:

Code:

---

MapViewOfFile(hNeighbourListMemMap, FILE_MAP_WRITE, 5, 5, 0)

// This is how MSDN describes the function:
LPVOID WINAPI MapViewOfFile(
  __in  HANDLE hFileMappingObject,    hNeighbourListMemMap  OK
  __in  DWORD dwDesiredAccess,        FILE_MAP_WRITE  OK
  __in  DWORD dwFileOffsetHigh,        5 * 4GB                  ???
  __in  DWORD dwFileOffsetLow,          + 5 bytes.       
  __in  SIZE_T dwNumberOfBytesToMap  0                        OK
);

---

--
Mats

I've got a bit of a mess right now...sorry for that!

Code:

---

        static HANDLE hFileMap;
        static unsigned long *Buff = NULL;

    // Create File Mapping (use invalid handle, don't link to real file, +1 for synchronization)
    hFileMap = CreateFileMapping(INVALID_HANDLE_VALUE, NULL, PAGE_READWRITE, 0, 100, "memMappedFile");
    if (!hFileMap)
    {
        perror("Unable to create file mapping,Error");
        return;
    }
   
    // Map View Of File (access memory mapped file and place it in memory)
    if ( !(Buff = MapViewOfFile(hFileMap, FILE_MAP_READ, 0, 0, 0) ) )
    {
        perror("Unable to create view of mapped file,Error");
       
        // Cleanup
        CloseHandle(hFileMap);
       
        return;
    }

        Buff[0] = 100;

---

Can someone tell me how the VOID thing works in C++.
I'm getting the following error:

"error C2440: '=' : cannot convert from 'LPVOID' to 'unsigned long *'"

I need to experiment a bit more with MMF after I get this working to ask a proper question. :)

MapViewOfFile returns void*, but Buff is unsigned long*.
C++ requires an explicit cast to compile this (because you have two different pointer types).

Ok, that works! Thanks!

If anyone can use it, here is the code:

Code:

---

        static HANDLE hFileMap;
        static char *Buff = NULL;

    // Create File Mapping (use invalid handle, don't link to real file, +1 for synchronization)
    hFileMap = CreateFileMapping(INVALID_HANDLE_VALUE, NULL, PAGE_READWRITE, 0, 100, "memMappedFile");
    if (!hFileMap)
    {
                perror("Unable to create file mapping,Error");
                return;
    }
   
        Buff = static_cast<char*> ( MapViewOfFile(hFileMap, FILE_MAP_WRITE, 0, 0, 0) );
       
        Buff[0] = 'H'; // uncomment for client,
        Buff[1] = 'i';  // to read from buffer

        cout << "Buff0 = " << Buff[0] << ", Buff1=" << Buff[1] << ".\n";

---

The next step is create a char array...

I've got the following code so far for a char array:

Code:

---

        static HANDLE hFileMap;
        static char *Buff[2];

    // Create File Mapping (use invalid handle, don't link to real file, +1 for synchronization)
    hFileMap = CreateFileMapping(INVALID_HANDLE_VALUE, NULL, PAGE_READWRITE, 0, 100, "memMappedFile");
    if (!hFileMap)
    {
                perror("Unable to create file mapping,Error");
                return;
    }
   
        *Buff = static_cast<char*> ( MapViewOfFile(hFileMap, FILE_MAP_WRITE, 0, 0, 0) );
       
        char *test;
        test = "hello";

        Buff[0] = "hello";
        Buff[1] = "world";

        cout << "Buff0 = " << Buff[0] << ", Buff1 = " << Buff[1] << ".\n";

---

...The client does not read anything from the buffer.

If I use

Code:

---

strcpy(*Buff, test);

---

...and then read from Buff[0] it works.

But I cannot assign anything to Buff[1]...

Can anyone point me in the right direction? Or am I doing something completely wrong here?

What are you doing with multiple pointers? You need only one pointer to the memory area.
Therefore

Code:

---

static char *Buff[2];

---

Should be

Code:

---

static char* Buff;

---

And once more - do not do this:

Code:

---

Buff[0] = "hello";
Buff[1] = "world";

---

You are putting pointers in the shared memory. Pointers which will be invalid in the other process.
Therefore you must use strcpy to copy the string into the shared memory.

Ok, I've changed my code.

Now, when I use:

Code:

---

        char *test;
        test = "hello";

        strcpy(Buff, test);

---

...It assigns the string, but is there a way to read the string like this:

Buff[0] returns "hello"
Buff[1] returns "world"

When I use Buff[0] it returns the first character and so on.

Also, I can only use one string in Buff, if I use strcpy a second time, it overwrites the first string, whereas by using Buff[0] = "hello" and Buff[1] = "world" I can have multiple strings.

You really have no grasp of C-style strings.
So... here you go. Read up.
http://www.cprogramming.com/tutorial/c/lesson9.html

And this:

Code:

---

        char *test;
        test = "hello";

        strcpy(Buff, test);

---

Is the same as

Code:

---

        strcpy(Buff, "hello");

---

And a C-style string is basically an array of chars. So perhaps read a little about arrays, too.

I know that both code is the same, and I also know that a string is actually an array of characters.

What I meant was that I want to assign a pointer char array, so I can have some sort of string array to assign multiple string values to the buffer with use of the index like this:

Buff[0] = "hello"
Buff[1] = "world"

I know I can just put all strings behind each other with \0 too, but I like the above method and want to use that in my 'client' too...

Aaah, I'll guess I will have to read some more about C tough.

Don't do it. It will never work to assign like that.
strcpy and strcat are your friends. Just as long as you treat the shared memory as char*.

Quote:

---

Originally Posted by kevinv2u

I know that both code is the same, and I also know that a string is actually an array of characters.

What I meant was that I want to assign a pointer char array, so I can have some sort of string array to assign multiple string values to the buffer with use of the index like this:

Buff[0] = "hello"
Buff[1] = "world"

I know I can just put all strings behind each other with \0 too, but I like the above method and want to use that in my 'client' too...

Aaah, I'll guess I will have to read some more about C tough.

---

You could alternatively serialise the length before each string. But the key is that you ARE doing serialisation / deserialisation here.