[英]Can a single pointer be initialized in a function and if so how?
I have been given this function prototype void create_smth(smth* s)
and it is asked that this function creates, initializes and returns a smth variable.我得到了这个函数原型
void create_smth(smth* s)
并且要求这个函数创建、初始化并返回一个 smth 变量。 Is this even possible?这甚至可能吗? To my knowledge I would need a double pointer to do such a thing.
据我所知,我需要一个双指针来做这样的事情。 What I am doing in this function is
我在这个函数中做的是
s=(smth*)malloc(sizeof(smth));
This is probably the mistake.这大概是错误。
In my main I have tried在我的主要我已经尝试过
smth* smth1=NULL;
create_smth(smth1);
and和
smth smth1;
create_smth(&smth1);
but my code keeps crashing (Segmentation fault; core dump).但我的代码一直崩溃(分段错误;核心转储)。 I am starting to wonder if it is an instructors mistake but feel stupid to ask him straight up but the excercise requires what I stated above.
我开始怀疑这是否是教练的错误,但直接问他很愚蠢,但练习需要我上面所说的。 Thanks everyone.
谢谢大家。
Looks like instructor's mistake to me.对我来说看起来像是导师的错误。 You are correct that the
void create_smth(smth* s)
prototype cannot return a value, either in the traditional return
sense (it is void
), or using a double pointer.您是正确的,
void create_smth(smth* s)
原型无法返回值,无论是传统的return
意义(它是void
),还是使用双指针。 The only two ways to do this:只有两种方法可以做到这一点:
smth* creat_smth()
{
smth* mySmth = malloc(sizeof *mySmth);
// check malloc return valid pointer, initialize fields with mySmth->
return mySmth;
}
int main(void)
{
smth* smth1 = creat_smth();
...
return 0;
}
or或者
void creat_smth(smth** mySmth)
{
*mySmth = malloc(sizeof **mySmth);
// check malloc returned valid pointer, initialize fields with (*mySmth)->
}
int main(void)
{
smit* smth1;
creat_smth(&smth1);
...
return 0;
}
It's worth asking your instructor for clarification.值得向您的导师寻求澄清。 That said, it's at least somewhat common to have functions that follow the pattern
int init_smth(smth *x)
— but note the name: such functions are usually called init_•
, not create_•
(because it doesn't create the storage for this object, it just fills it).也就是说,遵循
int init_smth(smth *x)
模式的函数至少在某种程度上是常见的——但请注意名称:此类函数通常称为init_•
,而不是create_•
(因为它不会为此创建存储对象,它只是填充它)。
This makes sense when it's expected that stack-allocating the struct is the right thing to do.当预计堆栈分配结构是正确的事情时,这是有道理的。 Instead of using
malloc
, the user would pass a pointer to a locally allocated structure:用户将传递一个指向本地分配结构的指针,而不是使用
malloc
:
smth my_smth;
init_smth(&my_smth);
And inside init_smth
some moderately complex initialisation would take place (but again without malloc
: the memory for the object is already allocated).并且在
init_smth
内部会init_smth
一些中等复杂的初始化(但同样没有malloc
:对象的内存已经分配)。
However, even then it's more common for the function to return a status code indicating success (hence the return type int
in my prototype above).然而,即使如此,函数返回指示成功的状态代码也更为常见(因此在我的原型中返回类型为
int
)。
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