time - get time in seconds
Standard C library (libc, -lc)
#include <time.h>
long time(NULL);
Think of this function as returning a long as output and as taking only NULL as input.
#include <time.h>
time_t time(time_t *_Nullable tloc);This function gets the current date and time as seconds since January 1, 1970, 00:00:00 UTC, otherwise known as the Epoch.
time() returns the time as the number of seconds since the Epoch, 1970-01-01 00:00:00 +0000 (UTC).
If tloc is non-NULL, the return value is also stored in the
memory pointed to by tloc.
This function returns the number of seconds since January 1, 1970, 00:00:00 UTC.
On success, the value of time in seconds since the Epoch is returned.
On error, ((time_t) -1) is returned, and errno is set
to indicate the error.
#include <stdio.h>
#include <time.h>
int main(void)
{
printf("The time is now %li.\n", time(NULL));
}
The time cannot be represented as a time_t value. This can
happen if an executable with 32-bit time_t is run on a 64-bit
kernel when the time is 2038-01-19 03:14:08 UTC or later. However, when
the system time is out of time_t range in other situations, the
behavior is undefined.
tloc points outside your accessible address space (but see
BUGS).
On systems where the C library time() wrapper function invokes an implementation provided by the vdso(7) (so that there is no trap into the kernel), an invalid address may instead trigger a SIGSEGV signal.
POSIX.1 defines seconds since the Epoch using a formula that
approximates the number of seconds between a specified time and the
Epoch. This formula takes account of the facts that all years that are
evenly divisible by 4 are leap years, but years that are evenly
divisible by 100 are not leap years unless they are also evenly
divisible by 400, in which case they are leap years. This value is not
the same as the actual number of seconds between the time and the Epoch,
because of leap seconds and because system clocks are not required to be
synchronized to a standard reference. Linux systems normally follow the
POSIX requirement that this value ignore leap seconds, so that
conforming systems interpret it consistently; see POSIX.1-2018 Rationale
A.4.16.
Applications intended to run after 2038 should use ABIs with
time_t wider than 32 bits; see
time_t(3type).
On some architectures, an implementation of time() is provided in the vdso(7).
C11, POSIX.1-2008.
SVr4, 4.3BSD, C89, POSIX.1-2001.
Error returns from this system call are indistinguishable from
successful reports that the time is a few seconds before the
Epoch, so the C library wrapper function never sets errno as a
result of this call.
The tloc argument is obsolescent and should always be NULL
in new code. When tloc is NULL, the call cannot fail.