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.Dd May 9, 2016
.Dt FUNOPEN 3
.Os
.Sh NAME
.Nm funopen ,
.Nm fropen ,
.Nm fwopen
.Nd open a stream
.Sh LIBRARY
.Lb libc
.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 "void *cookie" "int (*readfn)(void *, char *, int)"
.Ft FILE *
.Fn fwopen "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 can be given as an appropriately-typed
.Dv NULL
pointer.
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 single exception that they are passed the
.Fa cookie
argument specified to
.Fn funopen
in place of the traditional file descriptor 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
.In 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 fopencookie 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 .
.Pp
The
.Fn funopen
interface erroneously assumes that
.Vt fpos_t
is an integral type; see
.Xr fseek 3
for a discussion of this issue.