英文:
How to free a linked list properly
问题
第一段代码如下：
释放链表所需的任务，我使用了下面的第一段代码。但是在检查器中出现了一些错误[什么错误？]，而且建议使用第二段代码片段。但我似乎看不出两者之间的区别。为什么第一个被认为是错误的，当下一个节点的地址存储在变量tmp中时？并且在释放head变量分配的内存时，它不会丢失吗？
第一段代码：
while (head)
{
tmp = head->next;
free(head);
head = tmp;
}
第二段代码如下：
while (head)
{
tmp = head;
head = head->next;
free(tmp);
}
最后，在这两个选项中，哪一个在逻辑上被认为更准确和更高效？
英文:
Having a task that requires freeing a linked list I used this first code below. But there were some errors [what errors?] in the checker [what checker?] and instead it was suggested that I use the second code snippet. But I can't seem to see the difference. Why would the first one be considered wrong when the address of the next node is stored in the variable tmp? And wouldn't it be lost when freeing the memory allocated in the head variable?
First code:
while (head)
{
tmp = head->next;
free(head);
head = tmp;
}
Second code:
while (head)
{
tmp = head;
head = head->next;
free(tmp);
}
Finally, having these two options, which one is considered to be more accurate and efficient logic wise?
答案1
得分: 3
Given that the declarations of the variables involved are exactly the same in both versions, the two versions indeed effectively do the same thing.
There's no reason I can think of why one should be preferred over the other.
Interestingly, when compiling this with gcc, both versions produce exactly the same assembler code:
free1:
test rdi, rdi
je .L9
push rbx
mov rbx, rdi
.L3:
mov rdi, rbx
mov rbx, QWORD PTR [rbx]
call free
test rbx, rbx
jne .L3
pop rbx
ret
.L9:
ret
... and the same goes for clang, which produces the below for both versions:
free1: # @free1
test rdi, rdi
je .LBB0_4
push rbx
.LBB0_2: # =>This Inner Loop Header: Depth=1
mov rbx, qword ptr [rdi]
call free@PLT
mov rdi, rbx
test rbx, rbx
jne .LBB0_2
pop rbx
.LBB0_4:
ret
英文:
Given that the declarations of the variables involved are exactly the same in both versions, the two versions indeed effectively do the same thing.
There's no reason I can think of why one should be preferred over the other.
Interestingly, when compling this with gcc, both versions produce exactly the same assembler code:
free1:
test rdi, rdi
je .L9
push rbx
mov rbx, rdi
.L3:
mov rdi, rbx
mov rbx, QWORD PTR [rbx]
call free
test rbx, rbx
jne .L3
pop rbx
ret
.L9:
ret
... and the same goes for clang, which produces the below for both versions:
free1: # @free1
test rdi, rdi
je .LBB0_4
push rbx
.LBB0_2: # =>This Inner Loop Header: Depth=1
mov rbx, qword ptr [rdi]
call free@PLT
mov rdi, rbx
test rbx, rbx
jne .LBB0_2
pop rbx
.LBB0_4:
ret
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