c 中结构指针内的双数组

Question

为什么这个双映射数组几乎可以工作，但不能？

我的代码如下：

#include <stdlib.h>
#include <stdio.h>

typedef struct {
    double mapping [3][3];
} CoordinateMapperStr;
typedef CoordinateMapperStr * CoordinateMapper;

CoordinateMapper CoordinateMapper_Constructor(void)
{
    CoordinateMapper this = (CoordinateMapper) calloc (1, sizeof(CoordinateMapper));
    //return this; // <- I was missing this return, but still the rest worked the same
}

void CoordinateMapper_Initialize(CoordinateMapper this, double numb)
{
    for (int i=0; i < 3; i=i+1) {
        for (int j=0; j < 3; j=j+1) {
            this->mapping[i][j] = numb;
            printf("mapping(%d, %d) = %f\n", i, j, this->mapping[i][j]);
        }
    }
}

void CoordinateMapper_Print(CoordinateMapper this)
{
    for (int i=0; i < 3; i=i+1) {
        for (int j=0; j < 3; j=j+1) {
            printf("mapping(%d, %d) = %f\n", i, j, this->mapping[i][j]);
        }
    }
}

int main()
{
    CoordinateMapper mapper_1 = CoordinateMapper_Constructor();
    CoordinateMapper_Initialize(mapper_1, 1);
    printf("Init 1 done\n");

    CoordinateMapper_Print(mapper_1);
    printf("Print 1 done\n");

    CoordinateMapper mapper_2 = CoordinateMapper_Constructor();
    CoordinateMapper_Initialize(mapper_2, 2);
    printf("Init 2 done\n");

    CoordinateMapper_Print(mapper_1);
    printf("Second print 1 done\n");

    CoordinateMapper_Print(mapper_2);
    printf("Print 2 done\n");
}

// Here is the corresponding output
user:~/path$ gcc src/test_3.c -o test_3
user:~/path$ ./test_3
mapping(0, 0) = 1.000000
mapping(0, 1) = 1.000000
mapping(0, 2) = 1.000000
mapping(1, 0) = 1.000000
mapping(1, 1) = 1.000000
mapping(1, 2) = 1.000000
mapping(2, 0) = 1.000000
mapping(2, 1) = 1.000000
mapping(2, 2) = 1.000000
Init 1 done
mapping(0, 0) = 1.000000
mapping(0, 1) = 1.000000
mapping(0, 2) = 1.000000
mapping(1, 0) = 1.000000
mapping(1, 1) = 0.000000 // This is not correct
mapping(1, 2) = 0.000000 // This is not correct
mapping(2, 0) = 0.000000 // This is not correct
mapping(2, 1) = 1.000000
mapping(2, 2) = 1.000000
Print 1 done
mapping(0, 0) = 2.000000
mapping(0, 1) = 2.000000
mapping(0, 2) = 2.000000
mapping(1, 0) = 2.000000
mapping(1, 1) = 2.000000
mapping(1, 2) = 2.000000
mapping(2, 0) = 2.000000
mapping(2, 1) = 2.000000
mapping(2, 2) = 2.000000
Init 2 done
mapping(0, 0) = 1.000000
mapping(0, 1) = 1.000000
mapping(0, 2) = 1.000000
mapping(1, 0) = 1.000000
mapping(1, 1) = 0.000000 // This is not correct
mapping(1, 2) = 0.000000 // This is not correct
mapping(2, 0) = 0.000000 // This is not correct
mapping(2, 1) = 1.000000
mapping(2, 2) = 1.000000
Second print 1 done
mapping(0, 0) = 2.000000
mapping(0, 1) = 2.000000
mapping(0, 2) = 2.000000
mapping(1, 0) = 2.000000
mapping(1, 1) = 2.000000
mapping(1, 2) = 2.000000
mapping(2, 0) = 2.000000
mapping(2, 1) = 2.000000
mapping(2, 2) = 2.000000
Print 2 done

1. 在结构指针中设置双精度数组的正确方法是什么？
2. 为什么每个结构指针似乎都让它成为自己的新数组，但它们仍然有点不稳定？
3. 我可以使用哪些gcc编译器标志来帮助我查看此类错误，而缺少的则return this; 在构造函数中？

Answer 1

问题是无论你是否return this ，在任何一种情况下你都会得到未定义的行为。

当您不return this值时，您的非 void function 不会返回值 - 因此您的代码使用了一些垃圾值（可能恰好是calloc的返回值）。

如果你return this ——你返回sizeof(CoordinateMapper)的分配，它只是单个指针的大小。 这小于您的 struct sizeof(CoordinateMapperStr) ，并且您的其他代码在分配的 memory 之外读取/写入。 这又是未定义的行为。

Answer 2

@YakovGalka 发现了我的错误。 这里要补充一点，valgrind确实是一个static分析工具，可以检测出这类编程错误。 通过将-Wall和-g添加到gcc作为编译器标志并使用valgrind./compiled_app运行应用程序，可以轻松检测到这些类型的错误。