Scroll to navigation

UNVIS(3) Library Functions Manual UNVIS(3)


unvis, strunvis, strnunvisdecode a visual representation of characters


library “libbsd”


#include <bsd/vis.h>

unvis(char *cp, char c, int *astate, int flag);

strunvis(char *dst, char *src);

strnunvis(char *dst, char *src, size_t size);


The (), strunvis() and strnunvis() 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.

() decodes the characters pointed to by src into the buffer pointed to by dst.

() decodes the characters pointed to by src into the buffer pointed to by dst, writing a maximum of size bytes. 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). strunvis() terminates the destination string with a trailing NUL byte; strnunvis() does so if size is larger than 0.

The () 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:

Another character is necessary; nothing has been recognized yet.
A valid character has been recognized and is available at the location pointed to by cp.
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.
A valid sequence was detected, but no character was produced. This return code is necessary to indicate a logical break between characters.
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 strunvis() function returns the number of bytes written (not counting the trailing NUL byte) or -1 if an error occurred.

The strnunvis() function returns the number of bytes (not counting the trailing NUL byte) that would be needed to fully convert the input string, or -1 if an error occurred.


The following code fragment illustrates a proper use of unvis().

int state = 0;
char out;

while ((ch = getchar()) != EOF) {
	switch(unvis(&out, ch, &state, 0)) {
	case 0:
		(void) putchar(out);
		(void) putchar(out);
		goto again;
		(void)fprintf(stderr, "bad sequence!\n");
if (unvis(&out, (char)0, &state, UNVIS_END) == UNVIS_VALID)
	(void) putchar(out);


unvis(1), vis(1), vis(3)


The unvis() function first appeared in 4.4BSD.

May 31, 2007 Debian