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 Swangren · Accepted Answer · 2012-04-12 16:10:02Z

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, 2012 at 16:10

answered Apr 11, 2012 at 21:50

Ed Swangren

125k24 gold badges189 silver badges271 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

Stuart Golodetz Over a year ago

Oh what the heck +1 :) I like your answer better than mine.

Mike DeSimone Over a year ago

tmp[0] = 10; tmp[1] = 20; might be a clearer way to write the last two lines.

François Andrieux Over a year ago

This is undefined behavior, it's a strict alias violation.

Ed Swangren Over a year ago

@FrançoisAndrieux: You are correct, I should probably fix this

Stuart Golodetz · Accepted Answer · 2012-04-11 21:50:09Z

3

Are you after something like this?

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

answered Apr 11, 2012 at 21:50

Stuart Golodetz

20.7k4 gold badges55 silver badges81 bronze badges

Comments

Ashish Kumar Shah · Accepted Answer · 2012-04-12 13:43:14Z

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, 2012 at 13:43

Ashish Kumar Shah

5321 gold badge9 silver badges27 bronze badges

1 Comment

Ed Swangren Over a year ago

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.

Collectives™ on Stack Overflow

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

3 Answers 3

4 Comments

Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

4 Comments

Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related