friedo friedo -4 years ago 124
C Question

Get the time zone GMT offset in C

I'm using the standard

function to turn a
struct tm
into an epoch time value. The
fields are populated locally, and I need to get the epoch time as GMT.
has a
field to allow you to set the local GMT offset in seconds for just this purpose.

But I can't figure out how to get that information. Surely there must be a standard function somewhere that will return the offset? How does
do it?

Answer Source

I guess I should have done a bit more searching before asking. It turns out there's a little known timegm function which does the opposite of gmtime. It's supported on GNU and BSD which is good enough for my purposes. A more portable solution is to temporarily set the value of the TZ environment variable to "UTC" and then use mktime, then set TZ back.

But timegm works for me.

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