What is time NULL in C

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I learning about some basic C functions and have encountered time NULL  in some manuals   What exactly does this mean

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The time function returns the current time  as a time t value  in seconds since some point  on Unix systems  since midnight UTC January 1  1970   and it takes one argument  a time t pointer in which the time is stored   Passing NULL as the argument causes time to return the time as a normal return value but not store it anywhere else

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You can pass in a pointer to a time t object that time will fill up with the current time  and the return value is the same one that you pointed to   If you pass in NULL  it just ignores it and merely returns a new time t object that represents the current time   Nb time  amp timer   is equivalent to timer   time NULL

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int main  void             print time in seconds from 1 Jan 1970 using c        float n   time NULL          printf    2f n    n                   this prints 1481986944 00  at this moment

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The call to time NULL  returns the current calendar time  seconds since Jan 1  1970    See this reference for details   Ordinarily  if you pass in a pointer to a time t variable  that pointer variable will point to the current time

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You can pass in a pointer to a time t object that time will fill up with the current time  and the return value is the same one that you pointed to   If you pass in NULL  it just ignores it and merely returns a new time t object that represents the current time

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Answer copied from a duplicate  now-deleted question    time   is a very  very old function   It goes back to a day when the C language didn t even have type long   Once upon a time  the only way to get something like a 32-bit type was to use an array of two ints -- and that was when ints were 16 bits   So you called  int now 2   time now     and it filled the 32-bit time into now 0  and now 1   16 bits at a time    This explains why the other time-related functions  such as localtime and ctime  tend to accept their time arguments via pointers  too    Later on  dmr finished adding long to the compiler  so you could start saying  long now  time  amp now     Later still  someone realized it d be useful if time   went ahead and returned the value  rather than just filling it in via a pointer   But -- backwards compatibility is a wonderful thing -- for the benefit of all the code that was still doing time  amp now   the time   function had to keep supporting the pointer argument   Which is why -- and this is why backwards compatibility is not always such a wonderful thing -- if you re using the return value  you still have to pass NULL as a pointer   long now   time NULL      Later still  of course  we started using time t instead of plain long for times  so that  for example  it can be changed to a 64-bit type  dodging the y2 038k problem     P S   I m not actually sure the change from int  2  to long  and the change to add the return value  happened at different times  they might have happened at the same time   But note that when the time was represented as an array  it had to be filled in via a pointer  it couldn t be returned as a value  because of course C functions can t return arrays

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You have to refer to the documentation for ctime  time is a function that takes one parameter of type time t    a pointer to a time t object  and assigns to it the current time  Instead of passing this pointer  you can also pass NULL and then use the returned time t value instead

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Time   It returns the time elapsed in seconds since the epoch 1 Jan 1970

[c] What is time(NULL) in C?

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