struct timeval {
time_t  tv_sec;  /* seconds */


suseconds_t tv_usec; /* microseconds */


};




  and gives the number of seconds and microseconds since the Epoch (see


  time(2) ).

 The 


  tz

 argument is a 


  timezone  :




  

struct timezone {
int tz_minuteswest; /* minutes W of Greenwich */


int tz_dsttime;     /* type of dst correction */


};


 

  


 The use of the timezone struct is obsolete; the


  tz_dsttime

 field has never been used under Linux - it has not


been and will not be supported by libc or glibc.


Each and every occurrence of this field in the kernel source


(other than the declaration) is a bug. Thus, the following


is purely of historic interest.





The field


  tz_dsttime

 contains a symbolic constant (values are given below)


that indicates in which part of the year Daylight Saving Time


is in force. (Note: its value is constant throughout the year -


it does not indicate that DST is in force, it just selects an


algorithm.)


The daylight saving time algorithms defined are as follows : 


 

  


  DST_NONE /* not on dst */



 

 DST_USA /* USA style dst */



 

 DST_AUST /* Australian style dst */



 

 DST_WET /* Western European dst */



 

 DST_MET /* Middle European dst */



 

 DST_EET /* Eastern European dst */



 

 DST_CAN /* Canada */



 

 DST_GB /* Great Britain and Eire */



 

 DST_RUM /* Rumania */



 

 DST_TUR /* Turkey */



 

 DST_AUSTALT /* Australian style with shift in 1986 */


 

  


 Of course it turned out that the period in which


Daylight Saving Time is in force cannot be given


by a simple algorithm, one per country; indeed,


this period is determined by unpredictable political


decisions. So this method of representing time zones


has been abandoned. Under Linux, in a call to


  settimeofday the


  tz_dsttime

 field should be zero.


 

  


 Under Linux there is some peculiar `warp clock' semantics associated


to the


  settimeofday system call if on the very first call (after booting)


that has a non-NULL


  tz

 argument, the


  tv

 argument is NULL and the


  tz_minuteswest

 field is nonzero. In such a case it is assumed that the CMOS clock


is on local time, and that it has to be incremented by this amount


to get UTC system time.


No doubt it is a bad idea to use this feature.


 

  


 The following macros are defined to operate on a struct timeval :



 


#define	timerisset(tvp)\
((tvp)->tv_sec || (tvp)->tv_usec)


#define timercmp(tvp, uvp, cmp)\


((tvp)->tv_sec cmp (uvp)->tv_sec ||\


(tvp)->tv_sec == (uvp)->tv_sec &&\


(tvp)->tv_usec cmp (uvp)->tv_usec)


#define timerclear(tvp)\


((tvp)->tv_sec = (tvp)->tv_usec = 0)


 

  


 If either


  tv

 or 


  tz

 is null, the corresponding structure is not set or returned.


 

  


 Only the super user may use


  settimeofday .

 

EPERM

settimeofday is called by someone other than the superuser.

EINVAL

Timezone (or something else) is invalid.

EFAULT

One of
tv
or
tz
pointed outside your accessible address space.