#define _GNU_SOURCE
#include <stdio.h>
FILE *fopencookie(void *cookie, const char *mode,
cookie_io_functions_t io_funcs);
In order to create a custom stream the programmer must:
The fopencookie() function serves a purpose similar to fopen(3): it opens a new stream and returns a pointer to a FILE object that is used to operate on that stream.
The cookie argument is a pointer to the caller's cookie structure that is to be associated with the new stream. This pointer is supplied as the first argument when the standard I/O library invokes any of the hook functions described below.
The mode argument serves the same purpose as for fopen(3). The following modes are supported: r, w, a, r+, w+, and a+. See fopen(3) for details.
The io_funcs argument is a structure that contains four fields pointing to the programmer-defined hook functions that are used to implement this stream. The structure is defined as follows
struct cookie_io_functions_t {
cookie_read_function_t *read;
cookie_write_function_t *write;
cookie_seek_function_t *seek;
cookie_close_function_t *close;
};
The four fields are as follows:
ssize_t read(void *cookie, char *buf, size_t size);
The buf and size arguments are, respectively, a buffer into which input data can be placed and the size of that buffer. As its function result, the read function should return the number of bytes copied into buf, 0 on end of file, or -1 on error. The read function should update the stream offset appropriately.
If *read is a NULL pointer, then reads from the custom stream always return end of file.
ssize_t write(void *cookie, const char *buf, size_t size);
The buf and size arguments are, respectively, a buffer of data to be output to the stream and the size of that buffer. As its function result, the write function should return the number of bytes copied from buf, or -1 on error. The write function should update the stream offset appropriately.
If *write is a NULL pointer, then output to the stream is discarded.
int seek(void *cookie, off64_t *offset, int whence);
The *offset argument specifies the new file offset depending on which of the following three values is supplied in whence:
As its function result, the seek function should return 0 on success, and -1 on error.
If *seek is a NULL pointer, then it is not possible to perform seek operations on the stream.
int close(void *cookie);
The cookie argument is the cookie that the programmer supplied when calling fopencookie().
As its function result, the close function should return 0 on success, and EOF on error.
If *close is NULL, then no special action is performed when the stream is closed.
$ ./a.out 'hello world' /he/ / w/ /d/ Reached end of fileNote that a more general version of the program below could be improved to more robustly handle various error situations (e.g., opening a stream with a cookie that already has an open stream; closing a stream that has already been closed).
#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);
}