Is standard C mktime thread safe on linux?

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http://stackoverflow.com – The man page of mktime didn't mention thread safety of mktime, but it did mention this which make it look like thread unsafe: Calling mktime() also sets the external variable tzname with information about the current time zone. I know on Linux mktime calls tzset to set tzname, which is a char*[]: extern char *tzname[2]; and tzset will read environment variable TZ and file /etc/localtime. So unless mktime uses a mutex to protect all these operations, I can't see how it can be thread safe. (HowTos)