[RTEMS Project] #2697: Function for getting a time_t from a struct tm in UTC (like timegm on Linux and BSD)

[RTEMS trac]

#2697: Function for getting a time_t from a struct tm in UTC (like timegm on Linux
and BSD)
--------------------------------+------------------
 Reporter:  Christian Mauderer  |      Owner:
     Type:  enhancement         |     Status:  new
 Priority:  normal              |  Milestone:
Component:  Newlib              |    Version:  4.12
 Severity:  normal              |   Keywords:
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 It seems that there is no thread save possibility to convert a `struct tm`
 that holds a UTC time into a `time_t` (the inverse of the POSIX `gmtime()`
 function). `mktime` does this for the current locale only.

 Linux and BSD provide a non-POSIX-function called `timegm()` (see
 http://linux.die.net/man/3/timegm). According to the man page, the use of
 this functions should be avoided. The linked man page provides a solution
 that only uses POSIX functions. But that solution is not thread save.

 We should think about adding `timegm()` to newlib for compatibility with
 applications that use it (e.g. the civetweb web server).

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Ticket URL: <http://devel.rtems.org/ticket/2697>
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