C: Adding integer to char pointer(address)

Question

I am getting from a function a pointer to the (empty at the start) data of a block.

This pointer is:

char* data;

And my job is to insert different types of data in this block. I know only the data type (string/int/float) and their length in bytes.

For example, for an inserting integer, I want to give to each block a form like:

|block_num|age|weight|favorite_number|

So my main question is, how can I insert the number to the block and get them like? I would like smth like data[3] to get weight each time.

I have tried to use memset and memcpy, but there was no success.

Thanks for your time!

You want to store an `int` in an address pointed by a `char*`? — user2736738, Nov 18 '17 at 11:36
If I understand correctly you want your function to return something that could be of different types (string/int/float/etc..). Working with basic pointer 'char*' and then casting is considered not type-safe. I would suggest you to define abstract base class that will hold 'a data' and define several several pure virtual methods for this class. Your function should return base-class pointer that will be used in polymorphic way. — PazO, Nov 18 '17 at 11:39
@coderredoc: Sorry, missed this. In this case I would advice to do the equivalent in C: define struct with enumerator that says 'who I am' and according to this cast the structure to other structure. — PazO, Nov 18 '17 at 11:43

user2736738 · Answer 1 · 2017-11-18T13:07:29.017

In case you want to write to an address pointed to by a char* you need to cast it to proper type (that you want) and de-reference it.

For example: (here returnedFromFunc is similar to your data).

char *returnedFromFunc = func();

you can write an int by properly casting it to int*

*((int*)(returnedFromFunc))=intValue;

To go to the next location you would simply do this:-

char *originalPtrValue = returnedfromFunc; // store it for future referece
returnedfromFunc += sizeof intVar;

Similarly you can access it also if you know the type of data you are storing.

For example if it is int

you can do this

printf("%d ", *((int*)(returnedFromFunc)));

Using the sizeof operator you can access different locations.(offseting from the initial char* value).

You need to know how much memory is there where you can write. Otherwise there is a chance that you might try to write into memory that you have no permission to.(out of the bound )

C: Adding integer to char pointer(address)

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