Interface in C : An example of using interface types in C

container_of.h

#ifndef container_of
#include <stddef.h>
#define container_of(ptr, type, member) ((type *)(((char *)ptr) - offsetof(type, member)))
#endif

file_logger.c

#include "file_logger.h"
#include "container_of.h"
#include <stdio.h>

static int
file_logger_log(struct ilogger *ilogger, struct ilogrecord *record)
{
    struct file_logger *this = container_of(ilogger, struct file_logger, ilogger);
    if(fprintf(this->file, "%s\n", record->format(record)) < 0)
    {
        return -1;
    }
    else
    {
        return 0;
    }
}

void
file_logger_init(struct file_logger *this, FILE *file)
{
    this->ilogger.log = file_logger_log;
    this->file = file;
}

struct file_logger
file_logger_new(FILE *file)
{
    struct file_logger new;
    file_logger_init(&new, file);
    return new;
}

file_logger.h

#ifndef FILE_LOGGER_H
#define FILE_LOGGER_H

#include "ilogger.h"
#include <stdio.h>

struct file_logger
{
    struct ilogger ilogger;
    FILE *file;
};

void
file_logger_init(struct file_logger *this, FILE *file);

struct file_logger
file_logger_new(FILE *file);

#endif

ilogger.h

#ifndef ILOGGER_H
#define ILOGGER_H

struct ilogrecord
{
    const char *(*format)(struct ilogrecord *this);
};

struct ilogger
{
    int (*log)(struct ilogger *this, struct ilogrecord *record);
};

#endif

main.c

#include "ilogger.h"
#include "string_logrecord.h"
#include "file_logger.h"

int
main(void)
{
    struct file_logger fl = file_logger_new(stdout);
    struct string_logrecord slr = string_logrecord_new("Hello, World!");
    struct ilogger *logger = &fl.ilogger;
    struct ilogrecord *logrecord = &slr.ilogrecord;
    logger->log(logger, logrecord);
    return 0;
}

Makefile

CFLAGS=-Wall -Wextra -pedantic -ansi -g

main: file_logger.o string_logrecord.o

main.o: ilogger.h file_logger.h string_logrecord.h

file_logger.o: ilogger.h file_logger.h

string_logrecord.o: ilogger.h string_logrecord.h

#https://stackoverflow.com/questions/6304794/oop-and-interfaces-in-c
#You implement interfaces using structs of function pointers.
#You can then have the interface struct embedded in your data object struct and pass the interface pointer as first parameter of every interface member function.
#In that function you then get the pointer to your container class (which is specific to your implementation) using container_of () macro.
#Search for "container_of linux kernel" for an implementation. It is a very useful macro.

string_logrecord.c

#include "string_logrecord.h"
#include "container_of.h"

static const char *
string_logrecord_format(struct ilogrecord *ilogrecord)
{
    struct string_logrecord *this = container_of(ilogrecord, struct string_logrecord, ilogrecord);
    return this->text;
}

void
string_logrecord_init(struct string_logrecord *this, const char *text)
{
    this->ilogrecord.format = string_logrecord_format;
    this->text = text;
}

struct string_logrecord
string_logrecord_new(const char *text)
{
    struct string_logrecord new;
    string_logrecord_init(&new, text);
    return new;
}

string_logrecord.h

#ifndef STRING_LOGRECORD_H
#define STRING_LOGRECORD_H

#include "ilogger.h"

struct string_logrecord
{
    struct ilogrecord ilogrecord;
    const char *text;
};

void
string_logrecord_init(struct string_logrecord *this, const char *text);

struct string_logrecord
string_logrecord_new(const char *text);

#endif