灵活数组成员不在结构体错误末尾的原因是什么? [英] What is the cause of flexible array member not at end of struct error?

问题描述

我想知道为什么我在调用 malloc 时总是收到 error: flexible array member not at end of struct 错误.我有一个带有可变长度数组的结构，但我不断收到此错误.

I am wondering why I keep getting error: flexible array member not at end of struct error when I call malloc. I have a struct with a variable length array, and I keep getting this error.

结构是，

typedef struct {
  size_t N;
  double data[];
  int label[];
} s_col; 

对 malloc 的调用是，

and the call to malloc is,

col = malloc(sizeof(s_col) + lc * (sizeof(double) + sizeof(int)));

这是对 malloc 的正确调用吗?

Is this the correct call to malloc?

推荐答案

一个结构体中只能有一个灵活的数组成员，并且它必须始终是结构体的最后一个成员.换句话说，在这种情况下，您在调用 malloc 之前就出错了，以至于实际上无法为此结构正确调用 malloc.

You can only have one flexible array member in a struct, and it must always be the last member of the struct. In other words, in this case you've gone wrong before you call malloc, to the point that there's really no way to call malloc correctly for this struct.

要执行您似乎想要的操作(具有相同数量的 data 和 label 成员的数组)，您可以考虑以下内容:

To do what you seem to want (arrays of the same number of data and label members), you could consider something like:

struct my_pair { 
    double data;
    int label;
};

typedef struct { 
   size_t N;
   struct my_pair data_label[];
};

请注意，这有点不同:它不是一个 double 数组，后面跟着一个 int 数组，而是一个 数组double 后跟一个 int，然后是下一个 double，下一个 int，依此类推.这是否足够接近相同取决于您如何使用数据(例如，为了传递给需要连续数组的外部函数，您可能必须以不同的方式做事).

Note that this is somewhat different though: instead of an array of doubles followed by an array of ints, it gives you an array of one double followed by one int, then the next double, next int, and so on. Whether this is close enough to the same or not will depend on how you're using the data (e.g., for passing to an external function that expects a contiguous array, you'll probably have to do things differently).

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