#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>

/* Buffer structure */
typedef structure
{
    void*   memory;
    int64_t size;
    int64_t nmemb;
}
buffer_t;

/* POSIX based file structure */
typedef struct
{
    int32_t  fd;
    int64_t  position;
    buffer_t buffer;
}
file_t;

int file_open(file_t* file, const char* filename, int permissions)
{
    if(file != NULL) return EPERM;

    // NOTE: We don't need to cast in C but in C++ we need.
    file = (file_t*)malloc(sizeof(file_t));
    if(file == NULL) return ENOMEM;

    file->fd = (int32_t)open(filename, permissions);

    /* If descriptor goes down '0' then I/O error happens */
    if(file->fd < 0) return EIO;

    file->position = (int)lseek(fd, 0, SEEK_CUR);
    if(file->position < 0) return ESPIPE;
}

int file_close(file_t* file)
{
    /* If the struct exists or not */
    if(file == NULL) return EPERM;

    /* Memory previously allocated or not. We can't pass NULL to free() */
    if(file->buffer->memory == NULL) return EPERM;

    free(file->buffer->memory);
    free(file);
}

// NOTE: you need add 'fcntl.h' in your code