How to solve circular dependencies in C language

Question

The system software includes two sub-modules mod_a and mod_b, which lead to circular dependencies due to the following reasons

1. mod_a and mod_b need to use the public definition of sys, so they need to include sys_com.h
2. sys_com.h provides a memory management interface. You need to know the memory node size (sizeof) of each module, so it contains mod_a.h and mod_b.h.

My personal idea is to create a new com.h, move the definitions of MOD_A_TYPE_1 and MOD_B_TYPE_1 to this file, and sys_com.h, mod_a.h and mod_b.h all include header files. We often encounter similar problems during the project development process. What are your principles for dealing with them?Thanks in advance.

// mod_a.h

#include "sys_com.h"

typedef struct
{
    int len;
} MOD_A_TYPE_1;

// mod_b.h

#include "sys_com.h"

typedef struct
{
    int size;
} MOD_B_TYPE_1;

//sys_com.h

#include "mod_a.h"
#include "mod_b.h"

#define SYS_MOD_A_TYPE_1_SIZE           sizeof(MOD_A_TYPE_1)
#define SYS_MOD_B_TYPE_1_SIZE           sizeof(MOD_B_TYPE_1) 

// pool_type
#define SYS_MEM_POOL_MOD_A_TYPE_1       1
#define SYS_MEM_POOL_MOD_B_TYPE_1       2

// @pool_type: SYS_MEM_POOL_MOD_A_TYPE_1/SYS_MEM_POOL_MOD_B_TYPE_1
void* sys_alloc_pool_mem(int pool_type)

Answer 1

sys_com.h provides a memory management interface. You need to know the memory node size (sizeof) of each module, so it contains mod_a.h and mod_b.h.

Do you? Why? It seems to me that your API has a flag to choose the struct type but it does not need to use the size directly. The way I would do this is to put the #define for the sizes directly in the C file. In fact, I question the need for the #defines at all. What advantage do they give you over directly using sizeof(type)? I think sys_com.h should look like this:

// pool_type
#define SYS_MEM_POOL_MOD_A_TYPE_1       1
#define SYS_MEM_POOL_MOD_B_TYPE_1       2

// @pool_type: SYS_MEM_POOL_MOD_A_TYPE_1/SYS_MEM_POOL_MOD_B_TYPE_1
void* sys_alloc_pool_mem(int pool_type)

and sys_com.c should do the following

#include sys_com.h
#include "mod_a.h"
#include "mod_b.h"

// If you really must have the #defines
#define SYS_MOD_A_TYPE_1_SIZE           sizeof(MOD_A_TYPE_1)
#define SYS_MOD_B_TYPE_1_SIZE           sizeof(MOD_B_TYPE_1) 

void* sys_alloc_pool_mem(int pool_type)
{
// Implementation using the #defines or better sizeof(type)
}

Also consider putting include guards¹ on all the headers to prevent circular includes e.g. in mod_a.h

#if !defined(_MOD_A_H)
#define _MOD_A_H

typedef struct
{
    int len;
} MOD_A_TYPE_1;

#endif

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¹ Many C compilers have #pragma once to do the same thing but it is not standard.