C Passing Complex Structures by Reference to Functions

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.

One good style to write those structs is: ``typedef struct Neuron_tag { ... } Neuron_t;`` — BitTickler, Apr 30 '15 at 06:10
Neuron is not `typedef`, not a `struct` tag. Do not put `struct` before it in the parameter list. — Ulfalizer, Apr 30 '15 at 06:10
I used to append _t to my types. Don't do it, it's specifically reserved for POSIX — phil, Apr 30 '15 at 06:45

Sourav Ghosh · Answer 1 · 2015-04-30T06:25:15.283

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;

score 0 · Answer 2 · answered Apr 30 '15 at 06:31

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.

score 0 · Accepted Answer · answered Apr 30 '15 at 06:33

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.

Thanks for untangling my spaghetti struct code. Problem solved. — Pete Janda, May 01 '15 at 03:49

C Passing Complex Structures by Reference to Functions

3 Answers3