返回 VLA 和用法 [英] Return VLA and usage

问题描述

我有以下功能:

int* create_matrix_2(int rows, int cols)
{
    double (*A)[rows][cols] = malloc(sizeof(int[rows][cols]));

    for (int row = 0; row < rows; row++)
    {
        for (int col = 0; col < cols; col++)
        {
            *A[row][col] = row * cols + col;
        }
    }

    for (int row = 0; row < rows; row++)
    {
        for (int col = 0; col < cols; col++)
        {
            printf("%lf, " , *A[row][col]);
        }
        printf("\n");
    }
    return A;
}

我的问题是:如何从该函数返回 VLA，类型是什么，如何将其编写为函数签名，以及如何在接收返回数组的地方使用它?

My question is: How do I return the VLA from that function, what is the type and how do I write that as a function signature and how do I use it where I receive the returned array?

目前我正在尝试这样做:

At the moment I am trying to do it like this:

int (*matrix2)[height][width] = create_matrix_2(height, width);

for (int row = 0; row < height; row++)
{
    for (int col = 0; col < width; col++)
    {
        printf("%d" , (*matrix2)[row][col]);
    }
    printf("\n");
}

然后运行:gcc 2d_array_test.c -o 2d_array_test.out -std=c99 -O0

但这会导致以下问题:

2d_array_test.c: In function ‘main’:
2d_array_test.c:35:34: warning: initialization from incompatible pointer type [enabled by default]
  int (*matrix2)[height][width] = create_matrix_2(height, width);
                                  ^
2d_array_test.c: In function ‘create_matrix_2’:
2d_array_test.c:105:2: warning: return from incompatible pointer type [enabled by default]
  return A;
  ^

编辑#1:

我尝试使用alk建议的代码，但是在编译时出现了很多错误.这是一个单独的程序，其中包含您建议的带有主要功能的代码:http://pastebin.com/R6hKgvM0我收到以下错误:

I tried to use the code suggested by alk, but it gives me a lot of errors while compiling. Here is a separate programm, which contains your suggested code with a main function: http://pastebin.com/R6hKgvM0 I get the following errors:

2d_array_test_new.c: In function ‘main’:
2d_array_test_new.c:18:2: warning: passing argument 3 of ‘create_matrix’ from incompatible pointer type [enabled by default]
  if (-1 == create_matrix(height, width, &matrix))
  ^
2d_array_test_new.c:10:5: note: expected ‘int **’ but argument is of type ‘int (**)[(sizetype)(height)][(sizetype)(width)]’
 int create_matrix(size_t, size_t, int**);
     ^
2d_array_test_new.c: At top level:
2d_array_test_new.c:37:5: error: conflicting types for ‘create_matrix’
 int create_matrix(size_t rows, size_t cols, int(**a)[rows][cols])
     ^
2d_array_test_new.c:10:5: note: previous declaration of ‘create_matrix’ was here
 int create_matrix(size_t, size_t, int**);
     ^
2d_array_test_new.c: In function ‘create_matrix’:
2d_array_test_new.c:45:11: error: ‘EINVAL’ undeclared (first use in this function)
   errno = EINVAL;
           ^
2d_array_test_new.c:45:11: note: each undeclared identifier is reported only once for each function it appears in
2d_array_test_new.c:40:6: warning: variable ‘errno’ set but not used [-Wunused-but-set-variable]
  int errno;
      ^

错误似乎主要与返回类型有关.如何正确写入该数组的类型?

The errors mainly seem to be about the return type. How do I write the type of that array correctly?

推荐答案

• 参考 1^st 警告:

  warning: initialization from incompatible pointer type
  

  这里

  int (*matrix2)[height][width] = create_matrix_2(height, width);
  

  int (*matrix2)[height][width] 和 int * 根本不一样.

  参考第二个^nd警告:

  warning: return from incompatible pointer type 
  

  这是由于定义

  double (*A)[rows][cols] = malloc(sizeof(int[rows][cols]));
  

  同时从 int * create_matrix_2() 返回 A.

  int * 和 double (*A)[rows][cols] 也不一样.

  我建议你像这样改变函数:

  I propose you change the function like this:

  #include <errno.h> /* for errno and EINVAL */
  #include <stdlib.h> /* for malloc() */
  
  
  int create_matrix_2(size_t rows, size_t cols, int(**a)[rows][cols])
  {
    int result = 0;
  
    if (NULL == a)
    {
      result = -1;
      errno = EINVAL;
    }
    else
    {
      (*a) = malloc(sizeof **a);
      if (NULL == (*a))
      {
        result = -1;
      }
      else
      {
        for (size_t row = 0; row < rows; row++)
        {
          for (size_t col = 0; col < cols; col++)
          {
            (**a)[row][col] = row * cols + col;
          }
        }
  
        for (size_t row = 0; row < rows; row++)
        {
          for (size_t col = 0; col < cols; col++)
          {
            printf("%d, " , (**a)[row][col]);
          }
  
          printf("\n");
        }
      }
    }
  
    return result;
  }
  

  并这样称呼它:

  #include <stdlib.h>
  #include <stdio.h>  
  
  
  int create_matrix_2(size_t rows, size_t cols, int(**a)[rows][cols]);
  
  
  int main(void)
  {
    int result = EXIT_SUCCESS;
  
    int (*matrix2)[height][width] = NULL;
    if (-1 == create_matrix_2(height, width, &matrix2))
    {
      perror("create_matrix_2() failed");
      result = EXIT_FAILURE;
    }
    else
    {
      for (size_t row = 0; row < height; row++)
      {
        for (size_t col = 0; col < width; col++)
        {
          printf("%d, " , (*matrix2)[row][col]);
        }
  
        printf("\n");
      }
  
      free(matrix2);
    }
  
    return result;
  }
  

  这篇关于返回 VLA 和用法的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！