unvis(3)

NAME

unvis, strunvis - decode a visual representation of charac

ters

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
strunvisx(char *dst, 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. Unvis is called with succes

sive characters
in c until a valid sequence is recognized, at which time the

decoded
character is available at the character pointed to by cp.

Strunvis
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 charac

ters 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 only supported flag is

VIS_HTTPSTYLE.

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 han

dled properly.
They are:

0 (zero) Another character is necessary; nothing has

been recog

nized 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 char

acter was pro

duced. 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, unvis() will decode URI

strings as
specified in RFC 1808.

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:

(void)fprintf(stderr, "bad se

quence!0);

exit(1);
}

}
if (unvis(&out, (char)0, &state, UNVIS_END) == UN

VIS_VALID)

(void) putchar(out);

SEE ALSO

vis(1), vis(3)

R. Fielding, Relative Uniform Resource Locators, RFC1808.

HISTORY

The unvis() function first appeared in 4.4BSD.

BSD December 11, 1993

BSD