C Passing Complex Structures by Reference to Functions - c

Question

I replicated the guidance provided here but continue receiving the following error for each line with a function header and prototype: 'typedef "Neuron" may not be used in an elaborated type specifier'. I researched this error and didn't find much useful content. Any help would be most appreciated.
#include <stdio.h>
#include <stdlib.h>
struct neuronHead {
int x, y, z;
neuronHead *apical[25];
neuronHead *basal[25];
neuronHead *axon[25];
int time;
} neuron;
typedef struct neuron Neuron;
void setupBrain(struct Neuron ****brain); /* brain is a 3D array of structs */
void freeBrain(struct Neuron ****brain);
int main(void) {
Neuron ***brain;
setupBrain(&brain);
freeBrain(&brain); }
void setupBrain(struct Neuron ****brain) {
/* code for malloc'ing a 3D array of structures */ }
void freeBrain(struct Neuron ****brain) {
/* code for freeing the 3D array of structures */ }
I'm running on Ubuntu 14.04 and using Nvidia's NVCC compiler to run the code on GPUs, though this shouldn't be relevant to the error at hand.

Answer 1

You should replace
typedef struct neuron Neuron;
with
typedef struct neuronHead Neuron;
When writing
struct neuronHead { ... } neuron;
you introduce two elements:
a compound data type named "struct neuronHead",
and a variable named "neuron" having type "struct neuronHead".
Typedefs allow to create aliases for data types:
typedef <original_type> <alias_type>;
while in your example an <original_type> (struct neuron) is not a proper type. Proper type definitions are both:
struct neuronHead { ... } neuron; /* data type AND variable were introduced */
typedef struct neuronHead Neuron; /* data type introduced */
and:
typedef struct neuronHead { ... } Neuron; /* data type AND type alias were introduced */
Sample usage of pre-defined types:
struct neuronHead n1; /* correct */
Neuron n2; /* also correct */
struct neuron n3; /* incorrect: 'neuron' is variable name! */
Thank you.

Answer 2

In your code, there is no struct neuron you can typedef to. In your case, neuron is a global variable of type struct neuronHead.
You need to change your code to
struct neuronHead {
int x, y, z;
struct neuronHead *apical[25];
struct neuronHead *basal[25];
struct neuronHead *axon[25];
int time;
};
typedef struct neuronHead Neuron;
or,
typedef struct neuronHead Neuron;
struct neuronHead {
int x, y, z;
Neuron *apical[25];
Neuron *basal[25];
Neuron *axon[25];
int time;
};
or,
typedef struct neuronHead {
int x, y, z;
struct neuronHead *apical[25];
struct neuronHead *basal[25];
struct neuronHead *axon[25];
int time;
} Neuron;

Answer 3

Consider creating an instance of an int:
int some_int;
In C, in general, the syntax is:
<type> <variable_name>
When you have written something like this:
struct Thing {
int i;
/* Other members... */
} thing;
What this actually means is: create a variable, named thing, which is of type struct Thing, and this structure is defined as follows: { int i; /* Other members... */ }.
Then you do something like this:
typedef struct thing SomeThing;
At this point, it makes no sense, as you are trying to typedef an instance (thing).
What you probably meant was something like this:
struct Thing {
int i;
/* Other members... */
}; /* Nothing here! It's the end of type definition. */
typedef struct Thing thing; /* Typedef "struct Thing" -> "thing". */
The general syntax for typedef is:
typedef <original_type> <new_alias>
So it can be made shorter:
typedef /* Typedef the following struct... */
struct Thing {
int i;
/* Other members... */
}
thing; /* ... to such a name. */
Then you can correctly use thing as a variable type.
Also: There are no references in C, just the pointers.