c 中结构指针内的双数组

Question

Why does this double mapping array almost work, but doesn't?

My code is as follows:

#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. What is the proper way to setup a double array within a struct pointer?
2. Why does each struct pointer seem to make it´s own new array, but still they are a bit flaky?
3. What gcc compiler flags could I use to help me see this kind of error and the missing return this; in the constructor?

Answer 1

The problem is that whether you return this or not, you get undefined behavior in either case.

When you don't return this your non-void function doesn't return a value -- thus your code uses some garbage value (which might happen to be the return value from calloc ).

If you return this -- you return allocation of sizeof(CoordinateMapper) , which is just a size of a single pointer. This is less than your struct sizeof(CoordinateMapperStr) , and your other code reads/writes beyond the allocated memory. This is, again, undefined behavior.

Answer 2

@YakovGalka spotted my error. I want to add here that valgrind is indeed a static analysis tools that can detect these kinds of programming errors. By adding -Wall and -g to gcc as compiler flags and running the application with valgrind./compiled_app then these kinds of errors are easily detected.