Concatenate with memcpy

Question

I'm trying to add two strings together using memcpy. The first memcpy does contain the data, I require. The second one does not however add on. Any idea why?

if (strlen(g->db_cmd) < MAX_DB_CMDS )
{
      memcpy(&g->db_cmd[strlen(g->db_cmd)],l->db.param_value.val,strlen(l->db.param_value.val));
      memcpy(&g->db_cmd[strlen(g->db_cmd)],l->del_const,strlen(l->del_const));
      g->cmd_ctr++;
}

Answer 1

size_t len = strlen(l->db.param_value.val);

memcpy(g->db_cmd, l->db.param_value.val, len);
memcpy(g->db_cmd + len, l->del_const, strlen(l->del_cost)+1);

This gains you the following:

• Less redundant calls to strlen. Each of those must traverse the string, so it's a good idea to minimize these calls.
• The 2nd memcpy needs to actually append, not replace. So the first argument has to differ from the previous call.
• Note the +1 in the 3rd arg of the 2nd memcpy. That is for the NUL terminator.

I'm not sure your if statement makes sense either. Perhaps a more sane thing to do would be to make sure that g->db_cmd has enough space for what you are about to copy. You would do that via either sizeof (if db_cmd is an array of characters) or by tracking how big your heap allocations are (if db_cmd was acquired via malloc). So perhaps it would make most sense as:

size_t param_value_len = strlen(l->db.param_value.val),
       del_const_len = strlen(l->del_const);

// Assumption is that db_cmd is a char array and hence sizeof(db_cmd) makes sense.
// If db_cmd is a heap allocation, replace the sizeof() with how many bytes you
// asked malloc for.
//
if (param_value_len + del_const_len < sizeof(g->db_cmd))
{
   memcpy(g->db_cmd, l->db.param_value.val, param_value_len);
   memcpy(g->db_cmd + param_value_len, l->del_const, del_const_len + 1);
}
else
{
   // TODO: your buffer is not big enough.  handle that.
}