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();

now this works fine! However I'm not sure how to copy the first integer and then the second immediately.

I should actually know the address of the first byte that is exactly after the last byte that is taken by the first integer, and then use this address as a destination address in the memcpy call, is my thinking correct? if yes,

how can I achieve this?? How can I find out this address that I want?

thanks in advance

Ed S. · Accepted Answer · 2012-04-12T16:10:02.240

8

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.

edited Apr 12 '12 at 16:10

answered Apr 11 '12 at 21:50

Ed S.

122,712
22
185
265

Oh what the heck +1 :) I like your answer better than mine. – Stuart Golodetz Apr 11 '12 at 21:52
`tmp[0] = 10; tmp[1] = 20;` might be a clearer way to write the last two lines. – Mike DeSimone Apr 12 '12 at 13:49
This is undefined behavior, it's a strict alias violation. – François Andrieux Sep 28 '18 at 19:24
@FrançoisAndrieux: You are correct, I should probably fix this – Ed S. Oct 05 '18 at 16:45

score 3 · Answer 2 · answered Apr 11 '12 at 21:50

3

Are you after something like this?

memcpy(data + sizeof(int), &secondInteger, sizeof(int));

answered Apr 11 '12 at 21:50

Stuart Golodetz

20,238
4
51
80

score 0 · Answer 3 · answered Apr 12 '12 at 13:43

0

If you don't mind an array and want to do it in one call, I suggest you to copy both the integers into an int array and then write to file.

answered Apr 12 '12 at 13:43

Ashish Kumar Shah

512
1
9
26

If you didn't notice the memory that was returned via `bring_the_8_bytes` (the `data` pointer) is written to and then committed via a call to `send_the_8_bytes_back()`, so allocating new memory wouldn't work. – Ed S. Apr 12 '12 at 16:13

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

3 Answers3