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I would like to have an array of any type, which means that I can store anything in that array, such as structs, char, int, etc.. etc. How can I do this.. so far this is what I have in mind:
void** newArray = (void**) malloc(newBox->size * newBox->numslots);
where size is the size of each array slot and numslots is the number of slots...
The size of each array slot would be the size of a pointer to void, so it should be:
However, I am not sure how are you going to keep track of each type in order to cast the pointers to void appropriately.Code:void **newArray = malloc(sizeof(*newArray) * newBox->numslots);
hmm..what do you mean by that?Quote:
Originally Posted by laserlight
Recall that you cannot dereference a pointer to void.Quote:
Originally Posted by -EquinoX-
If I want to copy a struct to index i at the array above, can I do this.. as I tried and it didn't work..
memcpy((void*) &(newArray[i]), DiskStruct, size);
You have a dynamically allocated array of pointers to void, so I do not think you can go around copying a struct to it, or any of its elements. At best you can have a pointer to void point to the given struct.Quote:
Originally Posted by -EquinoX-
so you're saying I should use an array of pointers? is that the way to go?
if this is true then say the pointer to the given struct is changed then this will be changed as well.. I don't want this to happen.. so should I copy the pointer to the array? this is getting confusing...
or maybe is there a way so that I can copy the actual data, i.e a struct to an array... if there is how can I do that??
What do you ACTUALLY want to store, and once it is stored, how do you know what it is?
There are several ways to store generic data dynamically, but it's generally more complicated than simply malloc() a block of memory and stufff the data in there. You need to know WHAT is stored in some way - be that the order of elements, a tag of some sort, a second list, or some other way.
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Mats
I don't know, but I presumed that that was your intention.Quote:
Originally Posted by -EquinoX-
You could declare the pointers as pointing to const void.Quote:
Originally Posted by -EquinoX-
An array can only contain objects of the same type, so you cannot have an array of objects of different types, nor can you have an array of pointers to different types.Quote:
Originally Posted by -EquinoX-
At a more macro level, what exactly are you trying to do? That is, why do you want "an array of any type"?
why I want to have an array of any type is because I want to implement a mailbox system, where users can send/receive from it.. the users basically can send anything, struct, char, int.. so I am thinking this is a way to go... any ideas how people usually implement mailboxes? if I can't use an array?
And the user-code that looks in the mailbox will know what is in the mailbox how?
I have worked with a message-based OS previously, and it used separate memory allocations with a special tag to identify the content of the packet. So you would have a generic pointer to a message object, and then put that in the message-queue of the receiving process.
I think you could do something similar, but without knowing how you expect the overall concept to work, it's hard to say.
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Mats
Are those mboxes going to contain MIME attachments?
yes, this is the message for the inter process communication of the OS that I am talking about.. if you just have a pointer to that message then that would mean the message could be changed anytime during execution.... we don't want that to happen right, we want whatever the user sends and someone else receives to be the same.. if we're passing a pointer then between the period of sending and receiving the message content could be changed or lost
>>Are those mboxes going to contain MIME attachments?
no it won't, the biggest thing it's support to work is for a struct.. it should work for chars, ints, as well...
well back to my first post.. someone pointed out that an array can't have different size.. that I agree.. so therefore say I have an array of void* with a fixed size slot each.... then the size of the message, whatever the type of the message is, would be less than the size of each slots (so say the size of struct is 40 bytes and size of each slot is 100 bytes0... but now the issue is how can I declare an array of void* of each slot size is SLOT_SIZE and copy the contents that I want to store to a particular slot in the array?
Perhaps you should create an array of char[MESSAGE_MAX_SIZE] instead.
and why is that? can you show me how to copy a struct to that char[MESSAGE_MAX_SIZE] array?
You would be able to use memcpy() for that, but you have to be wary of structs that contain pointers.
well if I use an array of char then if I want to copy back from that array to recv, then how is that possible? I don't know where at what index it's stored at
Your idea was to allocate blocks of equal length, then use a block to store an object, possibly with some space wastage (and of course you cannot store objects larger than the block size).Quote:
Originally Posted by -EquinoX-
yes that's true that there might be some space wastage, but I don't care about that.. if it's an array of char then each block will be 8 bytes... am I seeing this right? so if I want to store a struct that is 40 bytes, then I can't... as size of message is greater than size of each block in the array... so I can't really use a char...Quote:
Originally Posted by laserlight
please correct me if I am seeing this the wrong way
in other words my code wants to behave like this:
find an empty slot in the array
if that empty slot size is greater than the size of message to be copied
then put the message to the mailbox
therefore say that index 0 is empty, then I can put the message to that index and then I can get it back from index 0 by doing a memcpy back
No, each block will be as large as you define it to be (you are probably thinking of 8 bit chars). You can set it as blocks of 40 bytes, if you wanted. Using an array of arrays of char might be a little easier. Still, I am not sure how to keep track of the types, as mentioned earlier, but matsp has hinted that there are ways to do so.Quote:
Originally Posted by -EquinoX-
how do you make so that each block will be 40 bytes? maybe some code example would help, as I still don't see it
I have tried to make since of this thread. What I gather is that you may be needing to set up a two-demensional array allocation, this can be set up to be dynamic as well possibly to allocate enough space for a string or ints.
I may be off base, but if not this should get you going in allocating enough space for your e-mails(numBlocks) and what they may contain(blockSize).Code:
int **my2Demail-array(int numBlocks, int blockSize)
{ int i;
int **t = malloc(numBlocks * sizeof (int *));
for (i = 0; i < blocks; i++)
t[i] = malloc(blockSize * sizeof(int));
return t;
}
Modifying this two work with structures may be a bit messy and then linking them may be preferred.
will something like this work?
I don't know why in the previous post before this you declared that as an intCode:void** newArray = (void**) malloc(sizeof(*newArray) * newBox->numslots);
for (i = 0; i < newBox->numslots; i++)
newArray[i] = malloc(sizeof(newBox->size));
There you go. An array of any type. Naturally you'll want to figure out what type you meant to use, but I'll leave that as an exercise to the reader.Code:union anytype {
char c;
char *pc;
int i;
int *pi;
...
struct someStructHere ss;
struct someStructHere *pss;
};
...
union anytype arrayof[ LOL ];
Quzah.