将堆分配的指针转换为指向VLA的指针是否安全? [英] Is it safe to cast a heap allocated pointer to a pointer to a VLA?

问题描述

如果我有一个指向一些堆分配空间的指针，该空间代表一个典型的 行主要的二维数组，是否可以安全地将此指针强制转换为 VLA的等效指针，以便于进行子脚本编写?示例:

If I've got a pointer to some heap allocated space that represents a typical row-major two dimensional array, is it safe to cast this pointer to an equivalent pointer to a VLA for convenient sub-scripting? Example:

//
// Assuming 'm' was allocated and initialized something like:
//
// int *matrix = malloc(sizeof(*matrix) * rows * cols);
//
// for (int r = 0; r < rows; r++) {
//     for (int c = 0; c < cols; c++) {
//         matrix[r * cols + c] = some_value;
//     }
// }
//
// Is it safe for this function to cast 'm' to a pointer to a VLA?
//
void print_matrix(int *m, int rows, int cols) {
    int (*mp)[cols] = (int (*)[cols])m;

    for (int r = 0; r < rows; r++) {
        for (int c = 0; c < cols; c++) {
            printf(" %d", mp[r][c]);
        }
        printf("\n");
    }
}

我已经测试了上面的代码.它似乎可行，并且对我来说应该可行，但这是安全的，明确的行为吗?

I've tested the code above. It seems to work, and it makes sense to me that it should work, but is it safe, defined behavior?

万一有人想知道，这里的用例是我正在从 代表行主要2D(或3D)数组的文件/套接字/等，我想使用 VLA避免了手动计算元素索引.

In case anyone is wondering, the use case here is I'm receiving data from a file/socket/etc that represents a row-major 2D (or 3D) array and I'd like to use VLA's to avoid manually calculating indexes to the elements.

推荐答案

如果cols为0或更小，则行为未定义. C11将对VLA的支持设为可选(例如，参见此处，并标记了问题C99(在需要的地方)，如果不支持，则将宏__STDC_NO_VLA__定义为1(参见C11 6.10.8.3 p1).

The behaviour is undefined if cols is 0 or smaller. C11 made support for VLAs optional (see e.g. here, and you tagged your question C99, where they’re required), if they’re not supported, the macro __STDC_NO_VLA__ is defined to 1 (cf. C11 6.10.8.3 p1).

此外，您很安全.

感谢Ouah和Alter Mann！

Thanks to Ouah and Alter Mann!

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