I see two major linux cases:
- Ubuntu. There should be a /etc/timezone file. This file should only contain the timezone and nothing else.
- Red Hat. There should be a /etc/sysconfig/clock that contains something like: ZONE="America/Chicago"
In addition, Solaris should have an /etc/TIMEZONE file that contains a line like: TZ=US/Mountain
So based on the above, here is some straight C that I believe answers the OP's question. I have tested it on Ubuntu, CentOS (Red Hat), and Solaris (bonus).
#include <string.h>
#include <strings.h>
#include <stdio.h>
char *findDefaultTZ(char *tz, size_t tzSize);
char *getValue(char *filename, char *tag, char *value, size_t valueSize);
int main(int argc, char **argv)
{
char tz[128];
if (findDefaultTZ(tz, sizeof(tz)))
printf("Default timezone is %s.\n", tz);
else
printf("Unable to determine default timezone.\n");
return 0;
}
char *findDefaultTZ(char *tz, size_t tzSize)
{
char *ret = NULL;
/* If there is an /etc/timezone file, then we expect it to contain
* nothing except the timezone. */
FILE *fd = fopen("/etc/timezone", "r"); /* Ubuntu. */
if (fd)
{
char buffer[128];
/* There should only be one line, in this case. */
while (fgets(buffer, sizeof(buffer), fd))
{
char *lasts = buffer;
/* We don't want a line feed on the end. */
char *tag = strtok_r(lasts, " \t\n", &lasts);
/* Idiot check. */
if (tag && strlen(tag) > 0 && tag[0] != '#')
{
strncpy(tz, tag, tzSize);
ret = tz;
}
}
fclose(fd);
}
else if (getValue("/etc/sysconfig/clock", "ZONE", tz, tzSize)) /* Redhat. */
ret = tz;
else if (getValue("/etc/TIMEZONE", "TZ", tz, tzSize)) /* Solaris. */
ret = tz;
return ret;
}
/* Look for tag=someValue within filename. When found, return someValue
* in the provided value parameter up to valueSize in length. If someValue
* is enclosed in quotes, remove them. */
char *getValue(char *filename, char *tag, char *value, size_t valueSize)
{
char buffer[128], *lasts;
int foundTag = 0;
FILE *fd = fopen(filename, "r");
if (fd)
{
/* Process the file, line by line. */
while (fgets(buffer, sizeof(buffer), fd))
{
lasts = buffer;
/* Look for lines with tag=value. */
char *token = strtok_r(lasts, "=", &lasts);
/* Is this the tag we are looking for? */
if (token && !strcmp(token, tag))
{
/* Parse out the value. */
char *zone = strtok_r(lasts, " \t\n", &lasts);
/* If everything looks good, copy it to our return var. */
if (zone && strlen(zone) > 0)
{
int i = 0;
int j = 0;
char quote = 0x00;
/* Rather than just simple copy, remove quotes while we copy. */
for (i = 0; i < strlen(zone) && i < valueSize - 1; i++)
{
/* Start quote. */
if (quote == 0x00 && zone[i] == '"')
quote = zone[i];
/* End quote. */
else if (quote != 0x00 && quote == zone[i])
quote = 0x00;
/* Copy bytes. */
else
{
value[j] = zone[i];
j++;
}
}
value[j] = 0x00;
foundTag = 1;
}
break;
}
}
fclose(fd);
}
if (foundTag)
return value;
return NULL;
}
