Stack smashing while casting and copying from one buffer to another - c

Question

I have a problem writing 8 alphanumeric symbols into my dest_buffer buffer.
I want to convert a char from buff to hexadecimal, then store the value in
different cases of my dest_buffer buffer.
For example:
buff[0] = 58 should give me dest_buffer[0] = '3' and dest_buffer[1] = 'a'
and so on to reach 8 chars.
src is a char[8*sizeof(int)+1] and buff is the same.
Unfortunatly, I only have this output:
2dbb771
*** stack smashing detected ***: ./a.out terminated
[1] 9843 abort (core dumped) ./a.out
This is the loop I use to do the job
for (i = 0; i < 4*sizeof(int); ++i)
{
snprintf (
&dest_buff[i*sizeof(int)*2],
sizeof(int)*2,
"%x",
*(int*)&buff[i * sizeof(int)]
) ;
}
I know I'm missing something but I don't know what, neither where, could
any of you help me?
Thanks
EDIT 1:
Here is another part of my code to be more precise:
int i, rndf ;
char buff[4*sizeof(int)+1];
rndf = open("/dev/urandom", O_RDONLY) ;
if( read (rndf, buff, 4*sizeof(int)) < 0)
{
fprintf (stderr, "%s\n", "An error occured while reading urandom") ;
exit (EXIT_FAILURE) ;
}
close(rndf) ;
after this come the loop

Answer 1

You have stack corruption because char buff[4*sizeof(int)+1]; is your variable.
Let's assume sizeof(int) is 4 bytes. Then variable size is 17 bytes.
By doing this: &dest_buff[i*sizeof(int)*2] you are at some time accessing (when i = 3) address offset 3 * 4 * 2 = 24 which is out of range of your variable.
Conclusion: you have undefined behaviour. Increase variable length or correct you pointer usage.