fopencookie(3) Library Functions Manual fopencookie(3)
NAME
fopencookie - open a custom stream
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#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);
DESCRIPTION
The fopencookie() function allows the programmer to create a custom im-
plementation 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 follow-
ing modes are supported: r, w, a, r+, w+, and a+. See fopen(3) for de-
tails.
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 dis-
carded.
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 in-
dicate 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.
RETURN VALUE
On success fopencookie() returns a pointer to the new stream. On er-
ror, NULL is returned.
ATTRIBUTES
For an explanation of the terms used in this section, see attrib-
utes(7).
┌───────────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├───────────────────────────────────────────┼───────────────┼─────────┤
│ fopencookie() │ Thread safety │ MT-Safe │
└───────────────────────────────────────────┴───────────────┴─────────┘
STANDARDS
GNU.
EXAMPLES
The program below implements a custom stream whose functionality is
similar (but not identical) to that available via fmemopen(3). It im-
plements 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 */
};
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;
}
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, 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;
}
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);
}
NOTES
_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.
SEE ALSO
fclose(3), fmemopen(3), fopen(3), fseek(3)
Linux man-pages 6.7 2023-12-29 fopencookie(3)
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