C-为什么这样的程序可以静态分配内存？

Question

int main(int argn, char** argv)
{
    int a;
    scanf("%d",&a);
    int b[a];

    printf("size of array = %dn", sizeof(b));

    return 1;
}

Answer 1

这个是c99中新加的, 这篇帖子有详细介绍: http://topic.csdn.net/t/20061019/10/5093368.html

Answer 2

Variable Length Arrays are not approved by C++ standard. C++ Standard mandates that the size of an array must be an compile time constant.

Variable-length automatic arrays are allowed in ISO C99, and as an extension GCC accepts them in C89 mode and in C++. (However, GCC’s implementation of variable-length arrays does not yet conform in detail to the ISO C99 standard.) These arrays are declared like any other automatic arrays, but with a length that is not a constant expression. The storage is allocated at the point of declaration and deallocated when the brace-level is exited.
另外，VLA 需要支持 sizeof 运算， 动态sizeof 也是C99的一个特有特性。
目前很多C++编译器尚不能支持动态数组特性(VC++2005不支持此特性， GCC3.2之后支持）
关于VLA,需要注意一下几点：
1. VLA的空间是函数栈中是一个automatic，在声明处分配，在作用域结束处释放。
2. VLA不能位于在静态存储区(包括全局变量和静态变量)中，也就是说不能添加static修饰或在任何函数体外定义

参考链接：
C语言变长数组之剖析
GCC变长数组VLA (allowed in ISO C99)