[英]Malloc a pointer to a pointer to a structure array by reference
The code below compiles, but immediately crashes for reasons obvious to others, but not to me. 下面的代码编译,但立即崩溃的原因显而易见,但不是我。 I can't seem to get it right, can anyone tell me how to fix this.
我似乎无法做到正确,任何人都可以告诉我如何解决这个问题。
*array_ref[2] = array[0];
*array_ref[3] = array[1];
It crashes on that part everytime. 它每次都在那个部分崩溃。
typedef struct test {
char *name;
char *last_name;
} person;
int setName(person ** array, person ***array_ref) {
*array = malloc (5 * sizeof(person));
*array_ref= malloc(5 * sizeof(person*));
array[0]->name = strdup("Bob");
array[1]->name = strdup("Joseph");
array[0]->last_name = strdup("Robert");
array[1]->last_name = strdup("Clark");
*array_ref[2] = array[0];
*array_ref[3] = array[1];
return 1;
}
int main()
{
person *array;
person **array_r;
setName(&array,&array_r);
printf("First name is %s %s\n", array[0].name, array[0].last_name);
printf("Second name is %s %s\n", array_r[3]->name, array_r[3]->last_name);
while(1) {}
return 0;
}
Operator []
has higher precedence than unary operator*
. Operator
[]
优先级高于一元运算operator*
。 Hence, this: 因此,这:
*array_ref[2] = array[0];
*array_ref[3] = array[1];
actually means: 实际意味着:
*(array_ref[2]) = array[0];
*(array_ref[3]) = array[1];
Types are correct here, which is why it compiles. 类型在这里是正确的,这就是它编译的原因。 But from your code it's clear that your intent actually was:
但是从您的代码中可以清楚地看出您的意图实际上是:
(*array_ref)[2] = array[0];
(*array_ref)[3] = array[1];
So just use parentheses. 所以只需使用括号。
您为array_ref指针分配了空间,但没有为它们指向的内容分配空间。
Try changing the following in setName() 尝试在setName()中更改以下内容
*array_ref[2] = array[0];
*array_ref[3] = array[1];
to 至
*(*array_ref+2) = array[0];
*(*array_ref+3) = array[1];
This works. 这有效。
array[1]->name
is your problem. array[1]->name
是你的问题。 This should be (*array)[1].name
. 这应该是
(*array)[1].name
。 Notice how the two aren't equivalent. 注意两者是不相等的。 All the similar uses have the same problem, except for
[0]
, which accidentally does the right thing. 所有类似的用途都有同样的问题,除了
[0]
,这意外地做了正确的事情。
Remember that array
, the function parameter, isn't your array, it's a pointer to your array. 请记住,
array
(函数参数)不是您的数组,它是指向数组的指针。
In functions like this I prefer code like: 在这样的函数中,我更喜欢以下代码:
int setName(person ** out_array, person ***out_array_ref) {
person* array = malloc(5 * sizeof(person));
person** array_ref = malloc(5 * sizeof(person*));
array[0].name = strdup("Bob");
array[1].name = strdup("Joseph");
array[0].last_name = strdup("Robert");
array[1].last_name = strdup("Clark");
// I'm guessing this was your intent for array_ref, here:
array_ref[2] = &array[0];
array_ref[3] = &array[1];
*out_array = out_array;
*out_array_ref = array_ref;
return 1;
}
Note that this catches both array[1]->name
as noted by Roger Pate, and *array_ref[2] = array[0]
as (almost) noted by Pavel - whose solution (*array_ref)[2] = array[0] assigns from an unallocated person*
array[1]
- both of which are hard to notice with the extra dereference. 请注意,这会捕获Roger Pate所指出的
array[1]->name
,以及*array_ref[2] = array[0]
注意到的*array_ref[2] = array[0]
- 其解决方案(* array_ref)[2] = array [0 ]从一个未分配的person*
array[1]
分配 - 这两个都难以注意到额外的解引用。
Of course, I mostly do this because I use C++, and this increases exception safety ;). 当然,我主要这样做是因为我使用C ++,这增加了异常安全性;)。
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