[英]Random value in struct field
I'm trying to learn some C and I've started with a simple deck of cards program.我正在尝试学习一些 C 语言,并且我已经从一个简单的纸牌程序开始。
The card is struct containing two ints for face and suit.该卡是包含两个整数的结构体,用于面和花色。 And I'm building a deck by allocating 52 cards, and going thorough a loop.
我正在通过分配 52 张卡片并进行循环来构建一副套牌。
card *Deck = (card*)malloc(sizeof(card) * 52);
for(int i = 0; i < 4; i++){
for(int j = 1; j < 14; j++){
Deck->face = j;
Deck->suit = i;
Deck += sizeof(card);
}
}
Deck -= sizeof(card) * 52;
printDeck(Deck);
free(Deck);
I have a printCard
function that takes a single pointer and prints the values, and a print deck function that calls printCard
52 times.我有一个接受单个指针并打印值的
printCard
函数,以及一个调用printCard
52 次的打印组函数。 If i print the cards as they are being created, everything goes well, but if i print the cards after all have been created for some reason I get a segfault at the 6th card.如果我在创建卡片时打印卡片,一切都会顺利,但是如果我在卡片创建之后因为某种原因打印卡片,我会在第 6 张卡片上出现段错误。 I fired up
gdb
and noticed this output:我启动了
gdb
并注意到了这个输出:
Seven of Diamonds<br>
(null) of Diamonds<br>
Nine of Diamonds<br>
The rest of the output is normal.其余输出正常。 When checking for the value of the 6th card i got:
在检查第 6 张卡的价值时,我得到:
(gdb) p c->face
$10 = 2675
If i am skipping the 6th card the output is fine and everything works the way it should.如果我跳过第 6 张卡,输出很好,并且一切正常。 I'm new to C but i can't really see how that value got put in that field in my code.
我是 C 的新手,但我真的看不到该值是如何在我的代码中放入该字段的。
struct card{
int face;
int suit;
};
void printCard(card* c){
printf("%s of %s\n", face_str[c->face], suit_str[c->suit]);
}
Edit:编辑:
The printDeck code:打印甲板代码:
Before:前:
void printDeck(card* c){
for(int i = 0; i < 52; i++){
printCard(c);
c += sizeof(card);
}
}
After:后:
void printDeck(card* c){
for(int i = 0; i < 52; i++){
printCard(c);
c++;
}
}
printCard didn't changed:打印卡没有改变:
void printCard(card* c){
printf("%s of %s\n", face_str[c->face], suit_str[c->suit]);
}
The problem in your code is this line:您的代码中的问题是这一行:
Deck += sizeof(card);
Here, Deck
is a card *
(pointer to card
).这里,
Deck
是一张card *
(指向card
指针)。 If you want to advance of one position, pointer arithmetic does the job for you, and doing Deck + 1
already advances sizeof(card)
ahead.如果你想前进一个位置,指针算术会为你完成这项工作,而做
Deck + 1
已经提前了sizeof(card)
。 Adding or subtracting from a pointer p
always advances sizeof(*p)
.从指针
p
添加或减去总是会增加sizeof(*p)
。
The correct code would be:正确的代码是:
Deck += 1;
// or
Deck++;
The same mistake is done after the loop, here:循环后犯了同样的错误,这里:
Deck -= sizeof(card) * 52;
Which should be:应该是:
Deck -= 52;
I'm new to C but i can't really see how that value got put in that field in my code.
我是 C 的新手,但我真的看不到该值是如何在我的代码中放入该字段的。
When you wrongly modify the variable Deck
, you end up accessing memory that is beyond the size of the allocated chunk (which is sizeof(card) * 52
).当您错误地修改变量
Deck
,您最终会访问超出分配块大小(即sizeof(card) * 52
)的内存。 This is undefined behavior in C, and therefore trying to print such a value could result in random values being printed (if not worse).这是 C 中未定义的行为,因此尝试打印这样的值可能会导致打印随机值(如果不是更糟的话)。
I would suggest you to completely avoid modifying the value of Deck
, and instead index it like an array:我建议你完全避免修改
Deck
的值,而是像数组一样索引它:
card *Deck = malloc(sizeof(card) * 52);
for(int i = 0; i < 4; i++){
for(int j = 1; j < 14; j++){
Deck[i*13 + j]->face = j;
Deck[i*13 + j]->suit = i;
}
}
printDeck(Deck);
free(Deck);
It would be interesting to show us the code of the printDeck
function.向我们展示
printDeck
函数的代码会很有趣。
The problem you have in the given code is Deck += sizeof(card)
.您在给定代码中遇到的问题是
Deck += sizeof(card)
。 You assumed that this would move Deck sizeof(card)
bytes further.您假设这会进一步移动 Deck
sizeof(card)
字节。 But that is not correct.但这是不正确的。 When Deck is a pointer on
card
, incrementing Deck by one will increment its address by sizeof(card)
bytes.当 Deck 是
card
上的一个指针时,将 Deck 增加 1 会将其地址增加sizeof(card)
个字节。
So your code should be所以你的代码应该是
card *Deck = malloc(sizeof(card) * 52);
for(int i = 0; i < 4; i++){
for(int j = 1; j < 14; j++){
Deck->face = j;
Deck->suit = i;
Deck++;
}
}
Deck -= 52;
printDeck(Deck);
free(Deck);
You might have made the same error in printDeck
.您可能在
printDeck
犯了同样的错误。
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