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