Get Unix timestamp with C

Question

How do I get a uint unix timestamp in C    I ve googled a bit and it seems that most methods are looking for more convoluted ways to represent time  Can t I just get it as a uint

User · Answer

The most common advice is wrong  you can t just rely on time    That s used for relative timing  ISO C   doesn t specify that 1970-01-01T00 00Z is time t 0   What s worse is that you can t easily figure it out  either  Sure  you can find the calendar date of time t 0  with gmtime  but what are you going to do if that s 2000-01-01T00 00Z   How many seconds were there between 1970-01-01T00 00Z and 2000-01-01T00 00Z  It s certainly no multiple of 60  due to leap seconds

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As this is the first result on google and there s no C  20 answer yet  here s how to use std  chrono to do this    include  lt chrono gt          using namespace std  chrono  int64 t timestamp   duration cast lt milliseconds gt  system clock  now   time since epoch    count      In versions of C   before 20  system clock s epoch being Unix epoch is a de-facto convention  but it s not standardized  If you re not on C  20  use at your own risk

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include lt iostream gt   include lt ctime gt   int main         std  time t t   std  time 0       t is an integer type     std  cout  lt  lt  t  lt  lt    seconds since 01-Jan-1970 n       return 0

User · Answer

C  20 introduced a guarantee that time since epoch is relative to the UNIX epoch  and cppreference com gives an example that I ve distilled to the relevant code  and changed to  units of seconds rather than hours   include  lt iostream gt   include  lt chrono gt    int main         const auto p1   std  chrono  system clock  now          std  cout  lt  lt   quot seconds since epoch   quot                 lt  lt  std  chrono  duration cast lt std  chrono  seconds gt                      p1 time since epoch    count    lt  lt    n       Using C  17 or earlier  time   is the simplest function - seconds since Epoch  which for Linux and UNIX at least would be the UNIX epoch   Linux manpage here  The cppreference page linked above gives this example   include  lt ctime gt   include  lt iostream gt    int main         std  time t result   std  time nullptr       std  cout  lt  lt  std  asctime std  localtime  amp result                  lt  lt  result  lt  lt   quot  seconds since the Epoch n quot

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I created a global define with more information    include  lt iostream gt   include  lt ctime gt   include  lt iomanip gt    define   FILENAME      builtin strrchr   FILE             builtin strrchr   FILE           1     FILE          only show filename and not it s path  less clutter   define INFO std  cout  lt  lt  std  put time std  localtime  amp time now     y- m- d  OH  OM  OS    lt  lt     INFO     lt  lt    FILENAME    lt  lt       lt  lt    FUNCTION    lt  lt       lt  lt    LINE    lt  lt      gt  gt     define ERROR std  cout  lt  lt  std  put time std  localtime  amp time now     y- m- d  OH  OM  OS    lt  lt     ERROR     lt  lt    FILENAME    lt  lt       lt  lt    FUNCTION    lt  lt       lt  lt    LINE    lt  lt      gt  gt     static std  time t time now   std  time nullptr     Use it like this   INFO  lt  lt   Hello world   lt  lt  std  endl  ERROR  lt  lt   Goodbye world   lt  lt  std  endl    Sample output   16-06-23 21 33 19  INFO  main cpp main 6   gt  gt  Hello world 16-06-23 21 33 19  ERROR  main cpp main 7   gt  gt  Goodbye world   Put these lines in your header file   I find this very useful for debugging  etc

User · Answer

include  lt iostream gt   include  lt sys time h gt   using namespace std   int main        unsigned long int sec  time NULL     cout lt  lt sec lt  lt endl

User · Answer

Windows uses a different epoch and time units  see  Convert Windows Filetime to second in Unix Linux  What std  time   returns on Windows is  as yet  unknown to me   -

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