tidy code for asynchronous IO is a good thread to start in.

Portable solutions don't really exist. It also differs for socket streams and files, on all platforms.

libevent is a good abstraction.

Writing ODBC is not for the faint hearted.

Take below as example, notice async mostly used in multi-thread,

// FILE NAME: a.c
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <pthread.h>

typedef void (*pcb)(int a);
typedef struct parameter
{
    int a;
    pcb callback;
} parameter;

void *callback_thread(void *p1)
{
    //do something
    parameter *p = (parameter *)p1;
    while (1)
    {
        printf("GetCallBack print! \n");
        sleep(3); //delay 3s
        p->callback(p->a);
    }
}

extern int SetCallBackFun(int a, pcb callback)
{
    printf("SetCallBackFun print! \n");
    parameter *p = malloc(sizeof(parameter));
    p->a = 10;
    p->callback = callback;

    pthread_t thing1;
    pthread_create(&thing1, NULL, callback_thread, (void *)p);
    pthread_join(thing1, NULL);
}

// FILE NAME: b.c
#include "boo.c"
#include <stdio.h>

void fCallBack(int a)
{
    //do something
    printf("a = %d\n",a);
    printf("fCallBack print! \n");
}


int main(void)
{
    SetCallBackFun(4,fCallBack);

    return 0;
}

Output is below,

SetCallBackFun print!
GetCallBack print!
a = 10
fCallBack print!
GetCallBack print!
a = 10
fCallBack print!
GetCallBack print!
a = 10
fCallBack print!
GetCallBack print!
a = 10
fCallBack print!
GetCallBack print!
a = 10
fCallBack print!
GetCallBack print!

...

In terms of calling function, there are three types: sync, back, and async.

The tricky thing is the last two are highly correlated, why is that?

Perhaps a graph would make it clear, i.e.