将结构指针数组传递给c中的函数

Question

I am trying to pass a array of pointers to structs to a function. then assign the address of that array to a pointer and use that pointer to index the elements in the array.

below I created three instances of a struct and assign the struct values then I create a array of pointers and have the elements in the array point to these structs.

I then pass the array of structs to a functions that prints its the value of the structs in two different ways...

The first way is I index the array that I pass to the function and print out the values in the individual structs.... this works fine!

The second way is I assign the array to a pointer and I index this pointer to print out the values in the array... this doesn't work correctly

struct name_struct_pointer {
    char *name1;
    char *name2;
};
int main(void) {


struct name_struct_pointer name_struct_pointer_inst_1 = {"name one","name two"};
struct name_struct_pointer name_struct_pointer_inst_2 = {"name three","name four"};
struct name_struct_pointer name_struct_pointer_inst_3 = {"name five","name six"};

 struct name_struct_pointer * name_struct_pointer_array[3] = {
        &name_struct_pointer_inst_1,
        &name_struct_pointer_inst_2,
        &name_struct_pointer_inst_3
};

        print_array_of_name_struct(name_struct_pointer_array,3);
        return EXIT_SUCCESS;
}

void print_array_of_name_struct(struct name_struct_pointer * name_struct_pointer_array[],int size){

    puts("print data indexed by function arg");
        for(int i = 0; i < size;i++){
            printf("print_name_struct : %s : %s \r\n",name_struct_pointer_array[i]->name1,name_struct_pointer_array[i]->name2);
        }

    struct name_struct_pointer * local_name_struct_pointer_array = name_struct_pointer_array;
    puts("print data indexed by local pointer");
    for(int i = 0; i < size;i++){
        printf("print_name_struct : %s : %s \r\n",local_name_struct_pointer_array[i].name1,local_name_struct_pointer_array[i].name2);
    }
}

I expected the output of the pointer indexing method to give the same results, but that didn't happened. I got a mangled incorrect version of my data.

print data indexed by function arg
print_name_struct : name one : name two
print_name_struct : name three : name four
print_name_struct : name five : name six
print data indexed by local pointer
print_name_struct : dP@ : vP@
print_name_struct : ïP@ : name five
print_name_struct : name six : name three

I suspect there is something going on when assign the array to the pointer. it was my understanding that if I assign the array to a pointer I could use pointer arithmetic to index the data in the array. What am I not understanding?

Answer 1

WAY 1: The *local_name_struct_pointer_array should be declared as a double pointer **local_name_struct_pointer_array , because you are assigning incompatible pointer size.

Then, you should also change how you access the members of the struct from pointer[i].member to pointer[i]->member .

WAY 2: Other way should be by keeping how you wrote the code, and only change the pointer assignment as this: *local_name_struct_pointer_array = name_struct_pointer_array[0] , so you are getting the first element of the pointer array, and then you loop over the elements.

Answer 2

one of the example to why one should not neglect the warning, especially involving the pointers. The code gives 'incompatible pointer type' warning.

Solution is declare as double pointer

Changes required

struct name_struct_pointer ** local_name_struct_pointer_array =name_struct_pointer_array;

and

printf("print_name_struct : %s : %s \r\n",local_name_struct_pointer_array[i]->name1,local_name_struct_pointer_array[i]->name2);