How can I use memcpy to copy data from two integers to an array of characters?

Question

I have a file that can store up to 8 bytes of data. I have a system(let's call is Sys1) which can bring me 8 bytes from the file and save it in a buffer inside the ram. I can use these 8 bytes and copy some stuff in it, and then use this system to say "OK I'm done with it, just send these data back to the file". I want to bring the 8 bytes to the buffer, copy an integer to it, which takes 4 bytes, and on the remaining 4 bytes copy another integer. I just want two integers in 8 bytes, which I believe can be done. I have managed to copy 4 bytes only, which is just 1 integer and managed to send it back to the file. So I have something like this: ``` char *data; sys1.bring_the_8_bytes(&data); //now the 8 bytes are in the buffer, // data points to the first byte of the 8 byte sequence int firstInteger = 10; int secondInteger = 20; //the two integers memcpy(data,&firstInteger,sizeof(int)); //now the values of the first integer have been copied successfully. sys1.send_the_8_bytes_back(); ```…

Accepted Answer

The address you are having problems calculating is simply `sizeof(int)` bytes past the data pointer you already have. You can just cast the `char*` to an `int*` and work with that seeing as you know you always have a valid 8-byte region to write to.

```
char *data;
sys1.bring_the_8_bytes(&data);
int *tmp = (int*)data;
tmp[0] = 10;
tmp[1] = 20;
```

Or you can just add to the `char*` (add `sizeof(int)`) and do a second `memcpy`.

How can I use memcpy to copy data from two integers to an array of characters?

Solution

Problem