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.Dd $Mdocdate: June 5 2013 $
.Dt FUNOPEN 3
.Os
.Sh NAME
.Nm funopen ,
.Nm fropen ,
.Nm fwopen
.Nd open a stream
.Sh SYNOPSIS
.In stdio.h
.Ft FILE *
.Fn funopen "const void *cookie" "int (*readfn)(void *, char *, int)" \
    "int (*writefn)(void *, const char *, int)" \
    "fpos_t (*seekfn)(void *, fpos_t, int)" \
    "int (*closefn)(void *)"
.Ft FILE *
.Fn fropen "const void *cookie" "int (*readfn)(void *, char *, int)"
.Ft FILE *
.Fn fwopen "const void *cookie" "int (*writefn)(void *, const char *, int)"
.Sh DESCRIPTION
The
.Fn funopen
function associates a stream with up to four
.Dq Tn I/O No functions .
Either
.Fa readfn
or
.Fa writefn
must be specified;
the others may be given as
.Dv NULL
pointers.
These
.Tn I/O
functions will be used to read, write, seek, and close the new stream.
.Pp
In general, omitting a function means that any attempt to perform the
associated operation on the resulting stream will fail.
If the close function is omitted, closing the stream will flush
any buffered output and then succeed.
.Pp
The calling conventions of
.Fa readfn ,
.Fa writefn ,
.Fa seekfn ,
and
.Fa closefn
must match those, respectively, of
.Xr read 2 ,
.Xr write 2 ,
.Xr lseek 2 ,
and
.Xr close 2
with the exceptions that they are passed the
.Fa cookie
argument specified to
.Fn funopen
in place of the traditional file descriptor argument and that
the seek function takes an
.Li fpos_t
argument and not an
.Li off_t
argument.
.Pp
Read and write
.Tn I/O
functions are allowed to change the underlying buffer
on fully buffered or line buffered streams by calling
.Xr setvbuf 3 .
They are also not required to completely fill or empty the buffer.
They are not, however, allowed to change streams from unbuffered to buffered
or to change the state of the line buffering flag.
They must also be prepared to have read or write calls occur on buffers other
than the one most recently specified.
.Pp
All user
.Tn I/O
functions can report an error by returning \-1.
Additionally, all of the functions should set the external variable
.Va errno
appropriately if an error occurs.
.Pp
An error on
.Fn closefn
does not keep the stream open.
.Pp
As a convenience, the include file
.Aq Pa stdio.h
defines the macros
.Fn fropen
and
.Fn fwopen
as calls to
.Fn funopen
with only a read or write function specified.
.Sh RETURN VALUES
Upon successful completion,
.Fn funopen
returns a
.Dv FILE
pointer.
Otherwise,
.Dv NULL
is returned and the global variable
.Va errno
is set to indicate the error.
.Sh ERRORS
.Bl -tag -width Er
.It Bq Er EINVAL
The
.Fn funopen
function was called without either a read or write function.
The
.Fn funopen
function may also fail and set
.Va errno
for any of the errors specified for the routine
.Xr malloc 3 .
.El
.Sh SEE ALSO
.Xr fcntl 2 ,
.Xr open 2 ,
.Xr fclose 3 ,
.Xr fopen 3 ,
.Xr fseek 3 ,
.Xr setbuf 3
.Sh HISTORY
The
.Fn funopen
functions first appeared in
.Bx 4.4 .
.Sh BUGS
The
.Fn funopen
function may not be portable to systems other than
.Bx .