man3/fopencookie.3

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.TH FOPENCOOKIE 3 2021-03-22 "Linux" "Linux Programmer's Manual"
.SH NAME
fopencookie \- opening a custom stream
.SH SYNOPSIS
.nf
.BR "#define _GNU_SOURCE" "         /* See feature_test_macros(7) */"
.B #include <stdio.h>
.PP
.BI "FILE *fopencookie(void *restrict " cookie ", const char *restrict " mode ,
.BI "                  cookie_io_functions_t " io_funcs );
.fi
.SH DESCRIPTION
The
.BR 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,
.BR fopencookie ()
is used to implement
.BR fmemopen (3),
which provides a stream interface to data that is stored in a
buffer in memory.
.PP
In order to create a custom stream the programmer must:
.IP * 3
Implement four "hook" functions that are used internally by the
standard I/O library when performing I/O on the stream.
.IP *
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
.IR "void\ *"
when passed to
.BR fopencookie ()),
but automatically supplies the cookie
as the first argument when calling the hook functions.
.IP *
Call
.BR fopencookie ()
to open a new stream and associate the cookie and hook functions
with that stream.
.PP
The
.BR fopencookie ()
function serves a purpose similar to
.BR fopen (3):
it opens a new stream and returns a pointer to a
.I FILE
object that is used to operate on that stream.
.PP
The
.I 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.
.PP
The
.I mode
argument serves the same purpose as for
.BR fopen (3).
The following modes are supported:
.IR r ,
.IR w ,
.IR a ,
.IR r+ ,
.IR w+ ,
and
.IR a+ .
See
.BR fopen (3)
for details.
.PP
The
.I 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
.PP
.in +4n
.EX
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;
.EE
.in
.PP
The four fields are as follows:
.TP
.I cookie_read_function_t *read
This function implements read operations for the stream.
When called, it receives three arguments:
.IP
    ssize_t read(void *cookie, char *buf, size_t size);
.IP
The
.I buf
and
.I size
arguments are, respectively,
a buffer into which input data can be placed and the size of that buffer.
As its function result, the
.I read
function should return the number of bytes copied into
.IR buf ,
0 on end of file, or \-1 on error.
The
.I read
function should update the stream offset appropriately.
.IP
If
.I *read
is a null pointer,
then reads from the custom stream always return end of file.
.TP
.I cookie_write_function_t *write
This function implements write operations for the stream.
When called, it receives three arguments:
.IP
    ssize_t write(void *cookie, const char *buf, size_t size);
.IP
The
.I buf
and
.I 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
.I write
function should return the number of bytes copied from
.IR buf ,
or 0 on error.
(The function must not return a negative value.)
The
.I write
function should update the stream offset appropriately.
.IP
If
.I *write
is a null pointer,
then output to the stream is discarded.
.TP
.I cookie_seek_function_t *seek
This function implements seek operations on the stream.
When called, it receives three arguments:
.IP
    int seek(void *cookie, off64_t *offset, int whence);
.IP
The
.I *offset
argument specifies the new file offset depending on which
of the following three values is supplied in
.IR whence :
.RS
.TP
.B SEEK_SET
The stream offset should be set
.I *offset
bytes from the start of the stream.
.TP
.B SEEK_CUR
.I *offset
should be added to the current stream offset.
.TP
.B SEEK_END
The stream offset should be set to the size of the stream plus
.IR *offset .
.RE
.IP
Before returning, the
.I seek
function should update
.I *offset
to indicate the new stream offset.
.IP
As its function result, the
.I seek
function should return 0 on success, and \-1 on error.
.IP
If
.I *seek
is a null pointer,
then it is not possible to perform seek operations on the stream.
.TP
.I 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:
.IP
    int close(void *cookie);
.IP
The
.I cookie
argument is the cookie that the programmer supplied when calling
.BR fopencookie ().
.IP
As its function result, the
.I close
function should return 0 on success, and
.B EOF
on error.
.IP
If
.I *close
is NULL, then no special action is performed when the stream is closed.
.SH RETURN VALUE
On success
.BR fopencookie ()
returns a pointer to the new stream.
On error, NULL is returned.
.\" .SH ERRORS
.\" It's not clear if errno ever gets set...
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.ad l
.nh
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.BR fopencookie ()
T}	Thread safety	MT-Safe
.TE
.hy
.ad
.sp 1
.SH CONFORMING TO
This function is a nonstandard GNU extension.
.SH EXAMPLES
The program below implements a custom stream whose functionality
is similar (but not identical) to that available via
.BR 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:
.PP
.in +4n
.EX
.RB "$" " ./a.out \(aqhello world\(aq"
/he/
/ w/
/d/
Reached end of file
.EE
.in
.PP
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).
.SS Program source
\&
.EX
#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;
    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 (int 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\en");
                exit(EXIT_FAILURE);
            }
            printf("Reached end of file\en");
            break;
        }

        printf("/%.*s/\en", (int) nread, buf);
    }

    exit(EXIT_SUCCESS);
}
.EE
.SH SEE ALSO
.BR fclose (3),
.BR fmemopen (3),
.BR fopen (3),
.BR fseek (3)