How to include array of struct defined in another header in C?

Question

I have the below files:

swaps.c:

#include "swaps.h"
#include "./poolutils.h"

double someFunction(struct Pool pools[]){
//do some stuff
}

int main(){
struct Pool pool1 = {"A","C",3.0,9000.0,0.0};
struct Pool pool2 = {"B","D",20.0,20000,0.0};
struct Pool pools[N];
pools[0] = pool1;
pools[1] = pool2;
double a = someFunction(pools);
}

swaps.h has signature of someFunction
poolutils.c has some functions
poolutils.h:

#ifndef _POOLUTILS_H
#define _POOLUTILS_H

struct Pool {
    char *token1;
    char *token2;
    double reserve1;
    double reserve2;
    double fee;
};
//signatures of poolutils.c functions 
#endif

When compiling (gcc -c swaps.c poolutils.c), I am getting the below error:

In file included from swaps.c:1:
swaps.h:4:44: error: array type has incomplete element type ‘struct Pool’
    4 | double someFunction(struct Pool pools[]);

Now, I do include the header where the struct Pool is defined, so swaps.c should know about it but I understand that somehow swaps.h does not, how can I make it aware of the outside definitions ?

(gcc version 10.2.1 20210110 (Debian 10.2.1-6))

do not place data instances in header files. Because those data elements will be generated in each .c file that includes that header file. It will fail during the linking and/or running steps due to duplicate definitions of the data, — user3629249
– user3629249, Commented May 19, 2022 at 15:07

AKX · Accepted Answer · 2022-05-18 10:43:08Z

3

Simply add

#include "./poolutils.h"

to your swaps.h too, if swaps.h needs the Pool struct definition.

Since you've dutifully put include guards in poolutils.h, you can use the header multiple times in a single compilation unit.

Alternately, swap the order of your includes; the effect is the same here.

#include "./poolutils.h"
#include "swaps.h"

answered May 18, 2022 at 10:43

AKX

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3 Comments

moonrekter Over a year ago

Which option is the cleanest/recommended?

AKX Over a year ago

Depending on the size of your program, I'd probably split type/structure definitions into a separate header file you can include wherever. Beyond that, the first option; anything that depends on you having to know the correct order for your includes is bound to be brittle.

tstanisl Over a year ago

Alternatively, if all functions/types don't use struct Pool but rather a pointer to it like struct Pool* but not struct Pool [] then you could just place a forward declaration: struct Pool;

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