xref: /dragonfly/lib/libc/stdio/funopen.3 (revision a765cedf) 
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30.\"     @(#)funopen.3	8.1 (Berkeley) 6/9/93
31.\" $FreeBSD: src/lib/libc/stdio/funopen.3,v 1.16 2007/01/09 00:28:06 imp Exp $
32.\" $DragonFly: src/lib/libc/stdio/funopen.3,v 1.3 2006/05/26 19:39:37 swildner Exp $
33.\"
34.Dd February, 4 2023
35.Dt FUNOPEN 3
36.Os
37.Sh NAME
38.Nm funopen ,
39.Nm fropen ,
40.Nm fwopen
41.Nd open a stream
42.Sh LIBRARY
43.Lb libc
44.Sh SYNOPSIS
45.In stdio.h
46.Ft FILE *
47.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 *)"
48.Ft FILE *
49.Fn fropen "void *cookie" "int (*readfn)(void *, char *, int)"
50.Ft FILE *
51.Fn fwopen "void *cookie" "int (*writefn)(void *, const char *, int)"
52.Sh DESCRIPTION
53The
54.Fn funopen
55function
56associates a stream with up to four
57.Dq Tn I/O No functions .
58Either
59.Fa readfn
60or
61.Fa writefn
62must be specified;
63the others can be given as an appropriately-typed
64.Dv NULL
65pointer.
66These
67.Tn I/O
68functions will be used to read, write, seek and
69close the new stream.
70.Pp
71In general, omitting a function means that any attempt to perform the
72associated operation on the resulting stream will fail.
73If the close function is omitted, closing the stream will flush
74any buffered output and then succeed.
75.Pp
76The calling conventions of
77.Fa readfn ,
78.Fa writefn ,
79.Fa seekfn
80and
81.Fa closefn
82must match those, respectively, of
83.Xr read 2 ,
84.Xr write 2 ,
85.Xr lseek 2 ,
86and
87.Xr close 2
88with the single exception that they are passed the
89.Fa cookie
90argument specified to
91.Fn funopen
92in place of the traditional file descriptor argument.
93.Pp
94Read and write
95.Tn I/O
96functions are allowed to change the underlying buffer
97on fully buffered or line buffered streams by calling
98.Xr setvbuf 3 .
99They are also not required to completely fill or empty the buffer.
100They are not, however, allowed to change streams from unbuffered to buffered
101or to change the state of the line buffering flag.
102They must also be prepared to have read or write calls occur on buffers other
103than the one most recently specified.
104.Pp
105All user
106.Tn I/O
107functions can report an error by returning \-1.
108Additionally, all of the functions should set the external variable
109.Va errno
110appropriately if an error occurs.
111.Pp
112An error on
113.Fn closefn
114does not keep the stream open.
115.Pp
116As a convenience, the include file
117.In stdio.h
118defines the macros
119.Fn fropen
120and
121.Fn fwopen
122as calls to
123.Fn funopen
124with only a read or write function specified.
125.Sh RETURN VALUES
126Upon successful completion,
127.Fn funopen
128returns a
129.Dv FILE
130pointer.
131Otherwise,
132.Dv NULL
133is returned and the global variable
134.Va errno
135is set to indicate the error.
136.Sh ERRORS
137.Bl -tag -width Er
138.It Bq Er EINVAL
139The
140.Fn funopen
141function
142was called without either a read or write function.
143The
144.Fn funopen
145function
146may also fail and set
147.Va errno
148for any of the errors
149specified for the routine
150.Xr malloc 3 .
151.El
152.Sh SEE ALSO
153.Xr fcntl 2 ,
154.Xr open 2 ,
155.Xr fclose 3 ,
156.Xr fopen 3 ,
157.Xr fopencookie 3 ,
158.Xr fseek 3 ,
159.Xr setbuf 3
160.Sh HISTORY
161The
162.Fn funopen
163functions first appeared in
164.Bx 4.4 .
165.Sh BUGS
166The
167.Fn funopen
168function
169may not be portable to systems other than
170.Bx .
171.Pp
172The
173.Fn funopen
174interface erroneously assumes that
175.Vt fpos_t
176is an integral type; see
177.Xr fseek 3
178for a discussion of this issue.
179