src/tzwrapper.h

   1 ///
   2 /// \file       tzwrapper.h
   3 ///             Timezone adjustment class, wrapping the TZ environment
   4 ///             variable to make struct tm -> time_t conversions easier.
   5 ///
   6
   7 /*
   8     Copyright (C) 2010, Chris Frey <cdfrey@foursquare.net>, To God be the glory
   9     Released to the public domain.
  10     Included in Barry and Barrified the namespace July 2010
  11 */
  12
  13 #ifndef __TZWRAPPER_H__
  14
  15 #include "dll.h"
  16 #include <string>
  17 #include <time.h>
  18 #include <stdlib.h>
  19
  20 namespace Barry { namespace Sync {
  21
  22 /// Parses ISO timestamp in the format of YYYYMMDDTHHMMSS[Z]
  23 /// or YYYY-MM-DDTHH:MM:SS.uuu-HH:MM
  24 /// and places broken down time in result.
  25 /// The trailing Z is optional in the format.
  26 /// If the Z exists, utc will be set to true, otherwise false.
  27 /// If zoneminutes is not null, and if a timezone offset is
  28 /// specified, it will be filled in and *zone set to true.
  29 /// Otherwise, *zone will be set to false.
  30 /// Returns NULL on error.
  31 /// Thread-safe.
  32 BXEXPORT struct tm* iso_to_tm(const char *timestamp,
  33                                 struct tm *result,
  34                                 bool &utc,
  35                                 bool *zone = 0,
  36                                 int *zoneminutes = 0);
  37
  38 /// Turns the struct tm into an ISO timestamp in the format
  39 /// of YYYYMMDDTHHMMSS[Z].  The Z is appended if utc is true.
  40 /// This function assumes that t contains sane values, and will
  41 /// create the target string directly from its content.
  42 /// Returns the ISO timestamp, or empty string on error.
  43 /// If t contains sane values, this function should never fail.
  44 /// Thread-safe.
  45 BXEXPORT std::string tm_to_iso(const struct tm *t, bool utc);
  46
  47 /// utc_mktime() converts a struct tm that contains
  48 /// broken down time in utc to a time_t.  This function uses
  49 /// a brute-force method of conversion that does not require
  50 /// the environment variable TZ to be changed at all, and is
  51 /// therefore slightly more thread-safe in that regard.
  52 ///
  53 /// The difference between mktime() and utc_mktime() is that
  54 /// standard mktime() expects the struct tm to be in localtime,
  55 /// according to the current TZ and system setting, while utc_mktime()
  56 /// always assumes that the struct tm is in UTC, and converts it
  57 /// to time_t regardless of what TZ is currently set.
  58 ///
  59 /// The difference between utc_mktime() and TzWrapper::iso_mktime()
  60 /// is that iso_mktime() will parse straight from an ISO string,
  61 /// and if the ISO timestamp ends in a 'Z', it will behave like
  62 /// utc_mktime() except it will alter the TZ environment variable
  63 /// to do it.  If the ISO timestamp has no 'Z', then iso_mktime()
  64 /// behaves like mktime().
  65 ///
  66 BXEXPORT time_t utc_mktime(struct tm *utctime);
  67
  68 //
  69 // class TzWrapper
  70 //
  71 /// Wrapper class for the TZ environment variable.  This class allows
  72 /// setting TZ to any number of variables, and will restore the original
  73 /// setting on destruction.
  74 ///
  75 /// By default, TzWrapper does not change the environment at all, but
  76 /// only saves it.  Alternately, you can use the timezone constructor
  77 /// to save and set a new timezone on the fly.
  78 ///
  79 /// Each Set() and Unset() function returns a reference to TzWrapper,
  80 /// so that you can chain function calls like this:
  81 ///
  82 ///     time_t utc = TzWrapper("Canada/Pacific").mktime(&pacific_tm);
  83 ///
  84 /// In addition, there are two static utility functions used to
  85 /// convert ISO timestamps to struct tm* and time_t values.
  86 ///
  87 /// Note: This class is not thread-safe, since it modifies the TZ
  88 ///       environment variable without locking.  If other threads
  89 ///       use time functions, this may interfere with their behaviour.
  90 ///
  91 class BXEXPORT TzWrapper
  92 {
  93         std::string m_orig_tz;
  94         bool m_tz_exists;
  95         bool m_dirty;
  96
  97 protected:
  98         void SaveTz()
  99         {
 100                 char *ptz = getenv("TZ");
 101                 if( ptz )
 102                         m_orig_tz = ptz;
 103                 m_tz_exists = ptz;
 104         }
 105
 106         void RestoreTz()
 107         {
 108                 if( m_dirty ) {
 109                         if( m_tz_exists )
 110                                 Set(m_orig_tz.c_str());
 111                         else
 112                                 Unset();
 113
 114                         m_dirty = false;
 115                 }
 116         }
 117
 118 public:
 119         /// Does not change TZ, only saves current setting
 120         TzWrapper()
 121                 : m_dirty(false)
 122         {
 123                 SaveTz();
 124         }
 125
 126         /// Saves current setting and sets TZ to new timezone value.
 127         /// If timezone is null, it is the same as calling Unset().
 128         explicit TzWrapper(const char *timezone)
 129                 : m_dirty(false)
 130         {
 131                 SaveTz();
 132                 Set(timezone);
 133         }
 134
 135         ~TzWrapper()
 136         {
 137                 RestoreTz();
 138         }
 139
 140         /// Set TZ to a new value.  If timezone is null, it is the
 141         /// same as calling Unset().
 142         ///
 143         /// If timezone is an empty or invalid timezone string, it
 144         /// is the same as calling SetUTC().
 145         TzWrapper& Set(const char *timezone)
 146         {
 147                 if( timezone )
 148                         setenv("TZ", timezone, 1);
 149                 else
 150                         unsetenv("TZ");
 151                 tzset();
 152                 m_dirty = true;
 153                 return *this;
 154         }
 155
 156         /// Deletes TZ from the environment, which has the same effect
 157         /// as calling SetSysLocal().  This is not a permanent
 158         /// condition, since TZ will be restored to original state
 159         /// upon destruction.
 160         TzWrapper& Unset()
 161         {
 162                 unsetenv("TZ");
 163                 tzset();
 164                 m_dirty = true;
 165                 return *this;
 166         }
 167
 168         /// Set timezone to UTC
 169         TzWrapper& SetUTC()
 170         {
 171                 return Set("");
 172         }
 173
 174         /// Set timezone via offset in minutes
 175         /// Negative minutes goes west, positive goes east
 176         /// i.e. -05:00 is EST
 177         TzWrapper& SetOffset(int zoneminutes);
 178
 179         /// Use system localtime.  This overrides any TZ value that the
 180         /// user may have set before running your program.
 181         TzWrapper& SetSysLocal()
 182         {
 183                 return Unset();
 184         }
 185
 186         /// Use the default TZ value that the user set before running
 187         /// this program.  In most cases, this will be the user's
 188         /// preferred local timezone.
 189         TzWrapper& SetDefault()
 190         {
 191                 RestoreTz();
 192                 return *this;
 193         }
 194         /// Same as SetDefault()
 195         TzWrapper& SetOrig()
 196         {
 197                 return SetDefault();
 198         }
 199
 200         //
 201         // C library wrappers, for calls like:
 202         //      time_t t = TzWrapper("Canada/Pacific").mktime(tm);
 203         //
 204         char* asctime(const struct tm *t) const { return ::asctime(t); }
 205         char* asctime_r(const struct tm *t, char *buf) const
 206                 { return ::asctime_r(t, buf); }
 207         char* ctime(const time_t *t) const { return ::ctime(t); }
 208         char* ctime_r(const time_t *t, char *buf) const
 209                 { return ::ctime_r(t, buf); }
 210         struct tm* gmtime(const time_t *t) const { return ::gmtime(t); }
 211         struct tm* gmtime_r(const time_t *t, struct tm *result) const
 212                 { return ::gmtime_r(t, result); }
 213         struct tm* localtime(const time_t *t) const { return ::localtime(t); }
 214         struct tm* localtime_r(const time_t *t, struct tm *result) const
 215                 { return ::localtime_r(t, result); }
 216         time_t mktime(struct tm *t) { return ::mktime(t); }
 217
 218         //
 219         // Additional utility functions
 220         //
 221
 222         /// Converts an ISO timestamp (YYYYMMDDTHHMMWW[Z]) into a
 223         /// unix time_t.  If the 'Z' UTC flag is not specified, then
 224         /// the timestamp will be assumed to be in the current
 225         /// default timezone.  Otherwise, SetUTC() will be used for the
 226         /// conversion.
 227         ///
 228         /// This function uses an internal TzWrapper to adjust TZ
 229         /// if necessary, which is why it is a static function
 230         /// of TzWrapper, instead of a standalone function.
 231         ///
 232         static time_t iso_mktime(const char *timestamp);
 233 };
 234
 235 }} // namespace Barry::Sync
 236
 237 #endif
 238