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FOPENCOOKIE(3) Linux Programmer's Manual FOPENCOOKIE(3)
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
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:
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 -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.
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.
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 *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);
}
fclose(3), fmemopen(3), fopen(3), fseek(3)
This page is part of release 3.32 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
Linux 2008-12-05 FOPENCOOKIE(3)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface