localtime() vs localtime_r()

David Xu davidxu at FreeBSD.org
Sun Mar 30 20:35:18 PDT 2008

According POSIX specification, it seems localtime_r() is not only
a reentrant version of localtime(), but also a performance-wise version.


[TSF] [Option Start] The localtime_r() function shall convert the time 
in seconds since the Epoch pointed to by timer into a broken-down time 
stored in the structure to which result points. The localtime_r() 
function shall also return a pointer to that same structure.

Unlike localtime(), the reentrant version is not required to set tzname.

[TSF] [Option Start] If the reentrant version does not set tzname, it 
shall not set daylight and shall not set timezone. [Option End]

Our implementation does set tzname, tomezone and daylight, it is a bit
slower than glibc because ours has to call getenv() everytime.

David Xu

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