プログラムのソース¶

#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 *stream;
    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;
    stream = fopencookie(&mycookie,"w+", memfile_func);
    if (stream == NULL) {
        perror("fopencookie");
        exit(EXIT_FAILURE);
    }
    /* Write command-line arguments to our file */
    for (j = 1; j < argc; j++)
        if (fputs(argv[j], stream) == EOF) {
            perror("fputs");
            exit(EXIT_FAILURE);
        }
    /* Read two bytes out of every five, until EOF */
    for (p = 0; ; p += 5) {
        if (fseek(stream, p, SEEK_SET) == -1) {
            perror("fseek");
            exit(EXIT_FAILURE);
        }
        nread = fread(buf, 1, 2, stream);
        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);
}