与memcpy串联 [英] Concatenate with memcpy

问题描述

我正在尝试使用memcpy将两个字符串添加在一起.我需要第一个memcpy包含数据.但是第二个没有添加.知道为什么吗?

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++;
}

推荐答案

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);

这将为您带来以下好处:

This gains you the following:

• 减少对strlen的冗余调用.每个字符串都必须遍历该字符串，因此最好将这些调用减至最少.
• 第二个memcpy需要实际添加，而不是替换.因此，第一个参数必须与上一个调用不同.
• 请注意第二个memcpy的第三个arg中的+1.那是针对NUL终结者的.

• 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.

我也不确定您的if陈述是否有意义.也许更明智的做法是确保g->db_cmd有足够的空间来存储您要复制的内容.您可以通过sizeof(如果db_cmd是字符数组)或通过跟踪堆分配量(如果通过malloc获取db_cmd)来做到这一点.因此，也许最有意义的是:

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.
}

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