什么时候将内存分配给C中的局部变量 [英] When is memory allocated to local variables in C

问题描述

局部变量也称为自动变量，应该在运行时在访问函数时分配内存.

As local variables are also called automatic variables, and are supposed to be allocated memory at run time, when function is accessed.

int main(){
    int a; // declaration 
    return 0;
}

int main(){
    int a[]; // compilation error, array_size missing
    return 0;
}

int main(){
    int a[2]; // declaration, but can't work without array_size, 
              // so at compile time it is checked!
    return 0;
}

我的问题是，这仅仅是在C语言的声明中赋予array_size的规则，还是在编译时为数组分配内存(仍然是局部变量)

My question is whether it's just a rule to give array_size in declaration in C, or memory is allocated at compile time for array (still local variable)

它如何工作?

根据K& R的C编程，数组是变量. pg 161.

An array is a variable as per C programming by K&R. pg no 161.

推荐答案

声明局部变量时，它的大小在编译时是已知的，但是内存分配是在执行时发生的.

When you declare local variable, the size of it is known at a compile time, but memory allocation occurs during execution time.

因此，在您的示例中，没有大小的数组显然是编译器遇到的问题，因为它不知道要包含在汇编代码中的大小是什么.

So in your examples, array without a size is clearly a problem to compiler, as it doesn't know what is the size to include into assembler code.

如果您不知道数组的大小，则可以始终使用指针类型和malloc/free甚至alloca.前两个在堆上操作，alloca实际上使用堆栈.

If you don't know the size of an array, you can always use pointer types and malloc/free or even alloca. The first two operate on heap, and alloca actually uses stack.

值得注意的例外是静态变量.它们的存储已在编译/链接时分配，并且无法在运行时更改.

The notable exception is static variables. The storage for them is allocated at a compile/link time already and can't be changed at runtime.

示例:

int main(int argc, const char *argv[])
{
    int a; // a is a sizeof(int) allocated on stack
}

int main(int argc, const char *argv[])
{
    int a[2]; // a is a sizeof(int)*2 allocated on stack
}

int main(int argc, const char *argv[])
{
    int *a; // a is a sizeof(int*) allocated on stack (pointer)
    a = alloca(sizeof(int)*4); // a points to an area with size of 4 integers
                               // data is allocated on stack
}

int main(int argc, const char *argv[])
{
    static int a; // a is allocated in data segment, keeps the value
}

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