Question

I was reading a program in C (an implementation of a server/client communication) and I saw this:

for (i = 0; i < len; i++)
 sprintf(nickmsg+i*2, "%02X", buf[i] & 0xFF);

What does this line do? I don’t understand this especially: nickmsg+i*2.
nickmsg is a char table and i is an integer. If it was just nickmsg, ok I’ll understand but there what’s the aim of this line ?

Thanks.

Answer 1

Start at the address pointed to by nickmsg and then go an additional i * 2 * CHAR_BIT / 8 bytes in memory. From there, write the hex representation of buf[i] & 0xFF, which will occupy 2 * CHAR_BIT / 8 bytes. Repeat for each i.

Assuming buf looks like

buf[0] = 20
buf[1] = 12

Then the memory pointed to by nickmsg will look like:

nickmsg
|
|
|
+ + + + +
0 2 4 6 8
140C\

Where the \ is my nomenclature for the null-terminator that sprintf writes at the end.