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.\"     @(#)unvis.3	8.2 (Berkeley) 12/11/93
.\"
.Dd March 12, 2011
.Dt UNVIS 3
.Os
.Sh NAME
.Nm unvis ,
.Nm strunvis ,
.Nm strnunvis ,
.Nm strunvisx ,
.Nm strnunvisx
.Nd decode a visual representation of characters
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In vis.h
.Ft int
.Fn unvis "char *cp" "int c" "int *astate" "int flag"
.Ft int
.Fn strunvis "char *dst" "const char *src"
.Ft int
.Fn strnunvis "char *dst" "size_t dlen" "const char *src"
.Ft int
.Fn strunvisx "char *dst" "const char *src" "int flag"
.Ft int
.Fn strnunvisx "char *dst" "size_t dlen" "const char *src" "int flag"
.Sh DESCRIPTION
The
.Fn unvis ,
.Fn strunvis
and
.Fn strunvisx
functions
are used to decode a visual representation of characters, as produced
by the
.Xr vis 3
function, back into
the original form.
.Pp
The
.Fn unvis
function is called with successive characters in
.Ar c
until a valid sequence is recognized, at which time the decoded
character is available at the character pointed to by
.Ar cp .
.Pp
The
.Fn strunvis
function decodes the characters pointed to by
.Ar src
into the buffer pointed to by
.Ar dst .
The
.Fn strunvis
function simply copies
.Ar src
to
.Ar dst ,
decoding any escape sequences along the way,
and returns the number of characters placed into
.Ar dst ,
or \-1 if an
invalid escape sequence was detected.
The size of
.Ar dst
should be equal to the size of
.Ar src
(that is, no expansion takes place during decoding).
.Pp
The
.Fn strunvisx
function does the same as the
.Fn strunvis
function,
but it allows you to add a flag that specifies the style the string
.Ar src
is encoded with.
Currently, the supported flags are:
.Dv VIS_HTTPSTYLE
and
.Dv VIS_MIMESTYLE .
.Pp
The
.Fn unvis
function implements a state machine that can be used to decode an
arbitrary stream of bytes.
All state associated with the bytes being decoded is stored outside the
.Fn unvis
function (that is, a pointer to the state is passed in), so
calls decoding different streams can be freely intermixed.
To start decoding a stream of bytes, first initialize an integer to zero.
Call
.Fn unvis
with each successive byte, along with a pointer
to this integer, and a pointer to a destination character.
The
.Fn unvis
function has several return codes that must be handled properly.
They are:
.Bl -tag -width UNVIS_VALIDPUSH
.It Li \&0 No (zero)
Another character is necessary; nothing has been recognized yet.
.It Dv UNVIS_VALID
A valid character has been recognized and is available at the location
pointed to by
.Fa cp .
.It Dv UNVIS_VALIDPUSH
A valid character has been recognized and is available at the location
pointed to by
.Fa cp ;
however, the character currently passed in should be passed in again.
.It Dv UNVIS_NOCHAR
A valid sequence was detected, but no character was produced.
This return code is necessary to indicate a logical break between characters.
.It Dv UNVIS_SYNBAD
An invalid escape sequence was detected, or the decoder is in an unknown state.
The decoder is placed into the starting state.
.El
.Pp
When all bytes in the stream have been processed, call
.Fn unvis
one more time with flag set to
.Dv UNVIS_END
to extract any remaining character (the character passed in is ignored).
.Pp
The
.Fa flag
argument is also used to specify the encoding style of the source.
If set to
.Dv VIS_HTTPSTYLE
or
.Dv VIS_HTTP1808 ,
.Fn unvis
will decode URI strings as specified in RFC 1808.
If set to
.Dv VIS_HTTP1866 ,
.Fn unvis
will decode entity references and numeric character references
as specified in RFC 1866.
If set to
.Dv VIS_MIMESTYLE ,
.Fn unvis
will decode MIME Quoted-Printable strings as specified in RFC 2045.
If set to
.Dv VIS_NOESCAPE ,
.Fn unvis
will not decode
.Ql \e
quoted characters.
.Pp
The following code fragment illustrates a proper use of
.Fn unvis .
.Bd -literal -offset indent
int state = 0;
char out;

while ((ch = getchar()) != EOF) {
again:
	switch(unvis(&out, ch, &state, 0)) {
	case 0:
	case UNVIS_NOCHAR:
		break;
	case UNVIS_VALID:
		(void)putchar(out);
		break;
	case UNVIS_VALIDPUSH:
		(void)putchar(out);
		goto again;
	case UNVIS_SYNBAD:
		errx(EXIT_FAILURE, "Bad character sequence!");
	}
}
if (unvis(&out, '\e0', &state, UNVIS_END) == UNVIS_VALID)
	(void)putchar(out);
.Ed
.Sh ERRORS
The functions
.Fn strunvis ,
.Fn strnunvis ,
.Fn strunvisx ,
and
.Fn strnunvisx
will return \-1 on error and set
.Va errno
to:
.Bl -tag -width Er
.It Bq Er EINVAL
An invalid escape sequence was detected, or the decoder is in an unknown state.
.El
.Pp
In addition the functions
.Fn strnunvis
and
.Fn strnunvisx
will can also set
.Va errno
on error to:
.Bl -tag -width Er
.It Bq Er ENOSPC
Not enough space to perform the conversion.
.El
.Sh SEE ALSO
.Xr unvis 1 ,
.Xr vis 1 ,
.Xr vis 3
.Rs
.%A R. Fielding
.%T Relative Uniform Resource Locators
.%O RFC1808
.Re
.Sh HISTORY
The
.Fn unvis
function
first appeared in
.Bx 4.4 .
The
.Fn strnunvis
and
.Fn strnunvisx
functions appeared in
.Nx 6.0
and
.Fx 9.2 .
.Sh BUGS
The names
.Dv VIS_HTTP1808
and
.Dv VIS_HTTP1866
are wrong.
Percent-encoding was defined in RFC 1738, the original RFC for URL.
RFC 1866 defines HTML 2.0, an application of SGML, from which it
inherits concepts of numeric character references and entity
references.