带有指针的功能：成员引用基本类型（…）不是结构或联合

Question

I've the following error:

"error: member reference base type 'start' (aka 'struct no_start *') is not a structure or union".

So, I have many structures like:

typedef struct no_start * start;

struct no_start{
   prog * node_program;
};

And functions like this:

start * insert_start(prog * program){

   start * data = (start *) malloc(sizeof(start));

   data->node_program = program;

   return data;

}

I have a file functions.c where simple functions like this are at, a file structs.h where the structs are and the last functions.h, where i declare my functions of first file.

I dont understand why I'm having this errors. For each function I get as many errors as assigns.

Answer 1

You do not need to type caste the return of malloc. Doing so will create a pointer, to a pointer. Where as a non casted malloc call will return a pointer to the memory allocated for your structure

Answer 2

I wouldn't do

typedef struct no_start * start;

ie typedef something as a pointer to a structure without doing

• a typedef of the struct itself
• naming the pointer typedef something like start_ptr

otherwise people get confused. As you did yourself.

 start * data = (start *) malloc(sizeof(start));

assumes that start is a struct - it isnt. You mean

 start  data = malloc(sizeof(struct no_start));

better is

typedef struct no_start{
   prog * node_program;
} start;

typedef start * start_ptr;

     start_ptr  data = malloc(sizeof(start));

Answer 3

Expand the typedef and you'll see what went wrong:

struct no_start ** data = (struct no_start **) malloc(sizeof(struct no_start*)); 
data->node_program = program; // Nope; *data is a pointer

You could use

start data = malloc(sizeof(*data));
data->node_program = program;

But it's usually better to avoid "pointer-typedefs", except possibly if they're used for opaque types (ie where the struct definition is hidden).

If you dislike typing struct everywhere (which is unnecessary in C++), you can typedef the struct:

typedef struct no_start no_start;

no_start* insert_start(prog* program){
   no_start* data = malloc(sizeof(*data));
   data->node_program = program;
   return data;
}

Of course, in C++ you should use new , not malloc .

Answer 4

My mistake was to make a pointer of the struct.

I had typedef struct no_start * start;

Instead I needed typedef struct no_start start;