fopencookie - opening a custom stream
#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <stdio.h>
FILE *fopencookie(void *cookie, const char *mode,
cookie_io_functions_t io_funcs);
The
fopencookie() function allows the programmer to create a custom
implementation for a standard I/O stream. This implementation can store the
stream's data at a location of its own choosing; for example,
fopencookie() is used to implement
fmemopen(3), which provides a
stream interface to data that is stored in a buffer in memory.
In order to create a custom stream the programmer must:
- *
- Implement four "hook" functions that are used internally by the
standard I/O library when performing I/O on the stream.
- *
- Define a "cookie" data type, a structure that provides
bookkeeping information (e.g., where to store data) used by the
aforementioned hook functions. The standard I/O package knows nothing
about the contents of this cookie (thus it is typed as
void * when passed to fopencookie()), but
automatically supplies the cookie as the first argument when calling the
hook functions.
- *
- Call fopencookie() to open a new stream and associate the cookie
and hook functions with that stream.
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
typedef struct {
cookie_read_function_t *read;
cookie_write_function_t *write;
cookie_seek_function_t *seek;
cookie_close_function_t *close;
} cookie_io_functions_t;
The four fields are as follows:
- cookie_read_function_t *read
- This function implements read operations for the stream. When called, it
receives three arguments:
-
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.
- cookie_write_function_t *write
- This function implements write operations for the stream. When called, it
receives three arguments:
-
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 0 on error. (The function must not return
a negative value.) The write function should update the stream
offset appropriately.
- If *write is a null pointer, then output to the stream is
discarded.
- cookie_seek_function_t *seek
- This function implements seek operations on the stream. When called, it
receives three arguments:
-
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:
- SEEK_SET
- The stream offset should be set *offset bytes from the start of the
stream.
- SEEK_CUR
- *offset should be added to the current stream offset.
- SEEK_END
- The stream offset should be set to the size of the stream plus
*offset.
- Before returning, the seek function should update *offset to
indicate the new stream offset.
- 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.
- cookie_close_function_t *close
- This function closes the stream. The hook function can do things such as
freeing buffers allocated for the stream. When called, it receives one
argument:
-
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.
On success
fopencookie() returns a pointer to the new stream. On error,
NULL is returned.
For an explanation of the terms used in this section, see
attributes(7).
Interface |
Attribute |
Value |
fopencookie () |
Thread safety |
MT-Safe |
This function is a nonstandard GNU extension.
The program below implements a custom stream whose functionality is similar (but
not identical) to that available via
fmemopen(3). It implements a
stream whose data is stored in a memory buffer. The program writes its
command-line arguments to the stream, and then seeks through the stream
reading two out of every five characters and writing them to standard output.
The following shell session demonstrates the use of the program:
$ ./a.out 'hello world'
/he/
/ w/
/d/
Reached end of file
Note 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 *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);
}
fclose(3),
fmemopen(3),
fopen(3),
fseek(3)