#define _GNU_SOURCE
#include <sys/types.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>

#define INIT_BUF_SIZE 4

struct memfile_cookie {
    char   *buf;        /* Dynamically sized buffer for data */
    size_t  allocated;  /* Size of buf */
    size_t  endpos;     /* Number of characters in buf */
    off_t   offset;     /* Current file offset in buf */
};

ssize_t
memfile_write(void *c, const char *buf, size_t size)
{
    char *new_buff;
    struct memfile_cookie *cookie = c;

    /* Buffer too small? Keep doubling size until big enough */

    while (size + cookie−>offset > cookie−>allocated) {
        new_buff = realloc(cookie−>buf, cookie−>allocated * 2);
        if (new_buff == NULL) {
            return −1;
        } else {
            cookie−>allocated *= 2;
            cookie−>buf = new_buff;
        }
    }

    memcpy(cookie−>buf + cookie−>offset, buf, size);

    cookie−>offset += size;
    if (cookie−>offset > cookie−>endpos)
        cookie−>endpos = cookie−>offset;

    return size;
}

ssize_t
memfile_read(void *c, char *buf, size_t size)
{
    ssize_t xbytes;
    struct memfile_cookie *cookie = c;

    /* Fetch minimum of bytes requested and bytes available */

    xbytes = size;
    if (cookie−>offset + size > cookie−>endpos)
        xbytes = cookie−>endpos − cookie−>offset;
    if (xbytes < 0)     /* offset may be past endpos */
       xbytes = 0;

    memcpy(buf, cookie−>buf + cookie−>offset, xbytes);

    cookie−>offset += xbytes;
    return xbytes;
}

int
memfile_seek(void *c, off64_t *offset, int whence)
{
    off64_t new_offset;
    struct memfile_cookie *cookie = c;

    if (whence == SEEK_SET)
        new_offset = *offset;
    else if (whence == SEEK_END)
        new_offset = cookie−>endpos + *offset;
    else if (whence == SEEK_CUR)
        new_offset = cookie−>offset + *offset;
    else
        return −1;

    if (new_offset < 0)
        return −1;

    cookie−>offset = new_offset;
    *offset = new_offset;
    return 0;
}

int
memfile_close(void *c)
{
    struct memfile_cookie *cookie = c;

    free(cookie−>buf);
    cookie−>allocated = 0;
    cookie−>buf = NULL;

    return 0;
}

int
main(int argc, char *argv[])
{
    cookie_io_functions_t  memfile_func = {
        .read  = memfile_read,
        .write = memfile_write,
        .seek  = memfile_seek,
        .close = memfile_close
    };
    FILE *fp;
    struct memfile_cookie mycookie;
    ssize_t nread;
    long p;
    int j;
    char buf[1000];

    /* Set up the cookie before calling fopencookie() */

    mycookie.buf = malloc(INIT_BUF_SIZE);
    if (mycookie.buf == NULL) {
        perror("malloc");
        exit(EXIT_FAILURE);
    }

    mycookie.allocated = INIT_BUF_SIZE;
    mycookie.offset = 0;
    mycookie.endpos = 0;

    fp = fopencookie(&mycookie,"w+", memfile_func);
    if (fp == NULL) {
        perror("fopencookie");
        exit(EXIT_FAILURE);
    }

    /* Write command−line arguments to our file */

    for (j = 1; j < argc; j++)
        if (fputs(argv[j], fp) == EOF) {
            perror("fputs");
            exit(EXIT_FAILURE);
        }

    /* Read two bytes out of every five, until EOF */

    for (p = 0; ; p += 5) {
        if (fseek(fp, p, SEEK_SET) == −1) {
            perror("fseek");
            exit(EXIT_FAILURE);
        }
        nread = fread(buf, 1, 2, fp);
        if (nread == −1) {
            perror("fread");
            exit(EXIT_FAILURE);
        }
        if (nread == 0) {
            printf("Reached end of file\n");
            break;
        }

        printf("/%.*s/\n", nread, buf);
    }

    exit(EXIT_SUCCESS);
}