错误：在 c 中重新定义 typedef

Question

We have an assignment in school where we got a header file and we need to implement it. The header defines:

typedef struct Board* BoardP;

which to my understanding means the BoardP is a pointer to struct Board. Anyhow, my implementation is:

typedef struct Board
{
    int width;
    int height;
    char *board;
} *BoardP;

But i keep getting:

Board.c:21: error: redefinition of typedef ‘BoardP’
Board.h:4: note: previous declaration of ‘BoardP’ was here

Any ideas as to why this happens? Thanks!

EDIT: another question. As you can see my struct hold an array of characters. When I write a constructor should I initialize (malloc(sizeof(height*width)) the array first and then the struct? And how about use of free()? Should I free the array first and then the struct? Thanks

Answer 1

Drop the typedef from the definition.

struct Board
{
    int width;
    int height;
    char *board;
};

A struct is a struct; it's not like you always have to typedef it. To be honest, I only typedef structs for opaque pointers .

From style(9) :

Avoid using typedefs for structure types. Typedefs are problematic because they do not properly hide their underlying type; for example you need to know if the typedef is the structure itself or a pointer to the structure. In addition they must be declared exactly once , whereas an incomplete structure type can be mentioned as many times as necessary.

Answer 2

It happens because in your pre-written header file you have to write an implementation for, you already typedef the data type struct Board * into a "new" data type BoardP . Your explanation that the BoardP is the pointer to the data type struct Board is correct.

But, to make the code in the header work, you only have to define what's missing, which is, in this instance, only an implementation of the struct Board data type.

Your proposed solution would work if you didn't already have the typedef in the header file.

As you already have it in the header file, only the code proposed by cnicutar (the code without the typedef ) should be put in the .c file.

Answer 3

Regarding your second question...

You can malloc in either order you like, but I generally allocate the object itself before I try to allocate any sub objects, because that way you have a clear place to store the results of subobject malloc s (in the struct) rather than having to store them in otherwise useless temporary variables.

Be sure to handle your errors correctly - if the second malloc fails, you'll need to free the first one. This is actually a place that many C programmers like to use goto :

struct Board *makeBoard(...)
{
    struct Board *b = malloc(sizeof *b);
    if(b == NULL) goto end1;

    b->board = malloc(...);
    if(b->board == NULL) goto end2;

    // ...

  end2:
    free(b);
  end1:
    return NULL;
}

When you free the data, you have to free the subobjects before you free the main objects, unless you store pointers to all the subobjects in temporary variables before you free the main object. That is, the following won't work:

free(b);
free(b->board);

Because by the second statement, b is free d and won't be able to be safely dereferenced. It's cleanest just to do it in the opposite order.