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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ATTRIBUTES | STANDARDS | EXAMPLES | NOTES | SEE ALSO | COLOPHON |
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fopencookie(3) Library Functions Manual fopencookie(3)
fopencookie - open a custom stream
Standard C library (libc, -lc)
#define _GNU_SOURCE /* See feature_test_macros(7) */
#define _FILE_OFFSET_BITS 64
#include <stdio.h>
FILE *fopencookie(void *restrict cookie, const char *restrict 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, off_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 │
└──────────────────────────────────────┴───────────────┴─────────┘
GNU.
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).
Program source
#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <unistd.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 */
};
static 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;
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;
}
static 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;
}
static int
memfile_seek(void *c, off_t *offset, int whence)
{
off_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;
}
static 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;
size_t nread;
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 (size_t 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 (long p = 0; ; p += 5) {
if (fseek(stream, p, SEEK_SET) == -1) {
perror("fseek");
exit(EXIT_FAILURE);
}
nread = fread(buf, 1, 2, stream);
if (nread == 0) {
if (ferror(stream) != 0) {
fprintf(stderr, "fread failed\n");
exit(EXIT_FAILURE);
}
printf("Reached end of file\n");
break;
}
printf("/%.*s/\n", (int) nread, buf);
}
free(mycookie.buf);
exit(EXIT_SUCCESS);
}
_FILE_OFFSET_BITS should be defined to be 64 in code that uses
non-null seek or that takes the address of fopencookie, if the
code is intended to be portable to traditional 32-bit x86 and ARM
platforms where off_t's width defaults to 32 bits.
fclose(3), fmemopen(3), fopen(3), fseek(3)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
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improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-05-17 fopencookie(3)
Pages that refer to this page: fmemopen(3), fopen(3), procio(3), stdio(3)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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