UNVIS

Section: C Library Functions (3)
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BSD mandoc

NAME

unvis strunvis - decode a visual representation of characters

SYNOPSIS

Fd #include <vis.h> Ft int Fn unvis u_char *cp u_char c int *astate int flag Ft int Fn strunvis char *dst char *src

DESCRIPTION

The Fn unvis and Fn strunvis 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 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 Strunvis decodes the characters pointed to by src into the buffer pointed to by dst

The Fn 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 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 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 Fn unvis one more time with flag set to UNVIS_END to extract any remaining character (the character passed in is ignored).

The following code fragment illustrates a proper use of Fn 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 sequence!);
        exit(1);
        }
}
if (unvis(&out, (char)0, &state, UNVIS_END) == UNVIS_VALID)
        (void) putchar(out);

HISTORY

The unvis function first appeared in 4.4BSD.