NAME
unvis,
strunvis —
decode a visual representation of characters
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <vis.h>
int
unvis(
char
*cp,
int c,
int *astate,
int flag);
int
strunvis(
char
*dst,
const char
*src);
int
strnunvis(
char
*dst,
size_t dlen,
const char *src);
int
strunvisx(
char
*dst,
const char
*src,
int flag);
int
strnunvisx(
char
*dst,
size_t dlen,
const char *src,
int flag);
DESCRIPTION
The
unvis(),
strunvis() and
strunvisx() functions are used to decode a visual
representation of characters, as produced by the
vis(3) function, back into the
original form.
The
unvis() function is called with successive characters in
c until a valid sequence is recognized, at which time
the decoded character is available at the character pointed to by
cp.
The
strunvis() function decodes the characters pointed to by
src into the buffer pointed to by
dst. The
strunvis() function simply
copies
src to
dst, decoding any
escape sequences along the way, and returns the number of characters placed
into
dst, or -1 if an invalid escape sequence was
detected. The size of
dst should be equal to the size of
src (that is, no expansion takes place during decoding).
The
strunvisx() function does the same as the
strunvis() function, but it allows you to add a flag that
specifies the style the string
src is encoded with.
Currently, the supported flags are:
VIS_HTTPSTYLE
and
VIS_MIMESTYLE
.
The
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
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
unvis() with each
successive byte, along with a pointer to this integer, and a pointer to a
destination character. The
unvis() function has several
return codes that must be handled properly. They are:
-
-
0
(zero)
- Another character is necessary; nothing has been recognized
yet.
-
-
UNVIS_VALID
- A valid character has been recognized and is available at
the location pointed to by cp.
-
-
UNVIS_VALIDPUSH
- A valid character has been recognized and is available at
the location pointed to by cp; however, the
character currently passed in should be passed in again.
-
-
UNVIS_NOCHAR
- A valid sequence was detected, but no character was
produced. This return code is necessary to indicate a logical break
between characters.
-
-
UNVIS_SYNBAD
- An invalid escape sequence was detected, or the decoder is
in an unknown state. The decoder is placed into the starting state.
When all bytes in the stream have been processed, call
unvis()
one more time with flag set to
UNVIS_END
to extract
any remaining character (the character passed in is ignored).
The
flag argument is also used to specify the encoding
style of the source. If set to
VIS_HTTPSTYLE
or
VIS_HTTP1808
,
unvis() will decode
URI strings as specified in RFC 1808. If set to
VIS_HTTP1866
,
unvis() will decode
entity references and numeric character references as specified in RFC 1866.
If set to
VIS_MIMESTYLE
,
unvis()
will decode MIME Quoted-Printable strings as specified in RFC 2045. If set to
VIS_NOESCAPE
,
unvis() will not
decode ‘
\
’ quoted characters.
The following code fragment illustrates a proper use of
unvis().
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, '\0', &state, UNVIS_END) == UNVIS_VALID)
(void)putchar(out);
ERRORS
The functions
strunvis(),
strnunvis(),
strunvisx(), and
strnunvisx() will return
-1 on error and set
errno to:
-
-
- [
EINVAL
]
- An invalid escape sequence was detected, or the decoder is
in an unknown state.
In addition the functions
strnunvis() and
strnunvisx() will can also set
errno
on error to:
-
-
- [
ENOSPC
]
- Not enough space to perform the conversion.
SEE ALSO
unvis(1),
vis(1),
vis(3)
R. Fielding,
Relative Uniform Resource Locators,
RFC1808.
HISTORY
The
unvis() function first appeared in
4.4BSD. The
strnunvis() and
strnunvisx() functions appeared in
NetBSD
6.0.
BUGS
The names
VIS_HTTP1808
and
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.