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#include <stdlib.h>
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#include <sys/types.h>
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#include <fcntl.h>
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#include <time.h>
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#include <ctype.h>
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#include <string.h>
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#include <unistd.h>
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#define TZFILE	"/etc/TZ"
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static char TZ[128];
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static char std[32] = "GMT0";
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static char dst[32];
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char *tzname[2] = {
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	std, dst
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};
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time_t tzoffset, tzdstoffset;
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int tzdst = 0;
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static int
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offset(char *env, time_t *off)
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{
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	int n, sign;
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	size_t len, retlen;
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	retlen = 0;
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	sign = 1;
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	/*
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	 * strictly, no sign is allowed in the 'time' part of the
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	 * dst start/stop rules, but who cares?
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	 */
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	if (*env == '-' || *env == '+') {
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		if (*env++ == '-')
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			sign = -1;
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		retlen++;
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	}
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	if ((len = strspn(env, ":0123456789")) == 0)
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		return 0;
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	retlen += len;
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	for (*off = 0; len && isdigit(*env); len--)		/* hours */
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		*off = *off*10 + (*env++ - '0')*60*60;
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	if (len) {
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		if (*env++ != ':')
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			return 0;
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		len--;
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	}
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	for (n = 0; len && isdigit(*env); len--)		/* minutes */
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		n = n*10 + (*env++ - '0')*60;
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	*off += n;
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	if (len) {
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		if (*env++ != ':')
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			return 0;
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		len--;
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	}
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	for (n = 0; len && isdigit(*env); len--)		/* seconds */
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		n = n*10 + (*env++ - '0');
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	*off = (*off + n)*sign;
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	return retlen;
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}
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/*
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 * TZ=stdoffset[dst[offset][,start[/time],end[/time]]]
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 */
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void
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tzset(void)
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{
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	char *env, *p, envbuf[128];
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	int fd, i;
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	size_t len, retlen;
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	time_t off;
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	/*
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	 * get the TZ environment variable and check for validity.
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	 * the implementation-defined manner for dealing with the
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	 * leading ':' format is to reject it.
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	 * if it's ok, stash a copy away for quick comparison next time.
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	 */
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	if ((env = getenv("TZ")) == 0) {
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		if ((fd = open(TZFILE, O_RDONLY)) == -1)
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			return;
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		if (read(fd, envbuf, sizeof(envbuf)-1) == -1) {
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			close(fd);
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			return;
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		}
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		close(fd);
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		for (i = 0; i < sizeof(envbuf); i++) {
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			if (envbuf[i] != '\n')
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				continue;
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			envbuf[i] = '\0';
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			break;
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		}
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		env = envbuf;
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	}
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	if (strcmp(env, TZ) == 0)
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		return;
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	if (*env == 0 || *env == ':')
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		return;
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	strncpy(TZ, env, sizeof(TZ)-1);
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	TZ[sizeof(TZ)-1] = 0;
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	/*
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	 * get the 'std' string.
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	 * before committing, check there is a valid offset.
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	 */
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	if ((len = strcspn(env, ":0123456789,-+")) == 0)
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		return;
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	if ((retlen = offset(env+len, &off)) == 0)
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		return;
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	for (p = std, i = len+retlen; i; i--)
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		*p++ = *env++;
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	*p = 0;
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	tzoffset = -off;
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	/*
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	 * get the 'dst' string (if any).
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	 */
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	if (*env == 0 || (len = strcspn(env, ":0123456789,-+")) == 0)
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		return;
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	for (p = dst; len; len--)
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		*p++ = *env++;
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	*p = 0;
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	/*
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	 * optional dst offset.
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	 * default is one hour.
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	 */
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	tzdst = 1;
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	if (retlen = offset(env+len, &off)) {
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		tzdstoffset = -off;
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		env += retlen;
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	}
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	else
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		tzdstoffset = tzoffset + 60*60;
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	/*
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	 * optional rule(s) for start/end of dst.
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	 */
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	if (*env == 0 || *env != ',' || *(env+1) == 0)
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		return;
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	env++;
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	/*
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	 * we could go on...
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	 * but why bother?
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	 */
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	USED(env);
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}