1TIME(3P) POSIX Programmer's Manual TIME(3P)
2
6 This manual page is part of the POSIX Programmer's Manual. The Linux
7 implementation of this interface may differ (consult the corresponding
8 Linux manual page for details of Linux behavior), or the interface may
9 not be implemented on Linux.
10
12 time — get time
13
15 #include <time.h>
16 time_t time(time_t *tloc);
18
20 The functionality described on this reference page is aligned with the
21 ISO C standard. Any conflict between the requirements described here
22 and the ISO C standard is unintentional. This volume of POSIX.1‐2017
23 defers to the ISO C standard.
24 The time() function shall return the value of time in seconds since the
26 Epoch.
27 The tloc argument points to an area where the return value is also
29 stored. If tloc is a null pointer, no value is stored.
30
32 Upon successful completion, time() shall return the value of time. Oth‐
33 erwise, (time_t)-1 shall be returned.
34
36 The time() function may fail if:
37 EOVERFLOW
39 The number of seconds since the Epoch will not fit in an object
40 of type time_t.
41 The following sections are informative.
43
45 Getting the Current Time
46 The following example uses the time() function to calculate the time
47 elapsed, in seconds, since the Epoch, localtime() to convert that value
48 to a broken-down time, and asctime() to convert the broken-down time
49 values into a printable string.
50 #include <stdio.h>
53 #include <time.h>
54 int main(void)
56 {
57 time_t result;
58 result = time(NULL);
60 printf("%s%ju secs since the Epoch\n",
61 asctime(localtime(&result)),
62 (uintmax_t)result);
63 return(0);
64 }
65 This example writes the current time to stdout in a form like this:
67 Wed Jun 26 10:32:15 1996
70 835810335 secs since the Epoch
71 Timing an Event
73 The following example gets the current time, prints it out in the
74 user's format, and prints the number of minutes to an event being
75 timed.
76 #include <time.h>
79 #include <stdio.h>
80 ...
81 time_t now;
82 int minutes_to_event;
83 ...
84 time(&now);
85 minutes_to_event = ...;
86 printf("The time is ");
87 puts(asctime(localtime(&now)));
88 printf("There are %d minutes to the event.\n",
89 minutes_to_event);
90 ...
91
93 None.
94
96 The time() function returns a value in seconds while clock_gettime()
97 and gettimeofday() return a struct timespec (seconds and nanoseconds)
98 and struct timeval (seconds and microseconds), respectively, and are
99 therefore capable of returning more precise times. The times() function
100 is also capable of more precision than time() as it returns a value in
101 clock ticks, although it returns the elapsed time since an arbitrary
102 point such as system boot time, not since the epoch.
103 Implementations in which time_t is a 32-bit signed integer (many his‐
105 torical implementations) fail in the year 2038. POSIX.1‐2008 does not
106 address this problem. However, the use of the time_t type is mandated
107 in order to ease the eventual fix.
108 On some systems the time() function is implemented using a system call
110 that does not return an error condition in addition to the return
111 value. On these systems it is impossible to differentiate between valid
112 and invalid return values and hence overflow conditions cannot be reli‐
113 ably detected.
114 The use of the <time.h> header instead of <sys/types.h> allows compati‐
116 bility with the ISO C standard.
117 Many historical implementations (including Version 7) and the 1984
119 /usr/group standard use long instead of time_t. This volume of
120 POSIX.1‐2017 uses the latter type in order to agree with the ISO C
121 standard.
122
124 In a future version of this volume of POSIX.1‐2017, time_t is likely to
125 be required to be capable of representing times far in the future.
126 Whether this will be mandated as a 64-bit type or a requirement that a
127 specific date in the future be representable (for example, 10000 AD) is
128 not yet determined. Systems purchased after the approval of this volume
129 of POSIX.1‐2017 should be evaluated to determine whether their lifetime
130 will extend past 2038.
131
133 asctime(), clock(), clock_getres(), ctime(), difftime(), futimens(),
134 gettimeofday(), gmtime(), localtime(), mktime(), strftime(), strp‐
135 time(), times(), utime()
136 The Base Definitions volume of POSIX.1‐2017, <time.h>
138
140 Portions of this text are reprinted and reproduced in electronic form
141 from IEEE Std 1003.1-2017, Standard for Information Technology -- Por‐
142 table Operating System Interface (POSIX), The Open Group Base Specifi‐
143 cations Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of
144 Electrical and Electronics Engineers, Inc and The Open Group. In the
145 event of any discrepancy between this version and the original IEEE and
146 The Open Group Standard, the original IEEE and The Open Group Standard
147 is the referee document. The original Standard can be obtained online
148 at http://www.opengroup.org/unix/online.html .
149 Any typographical or formatting errors that appear in this page are
151 most likely to have been introduced during the conversion of the source
152 files to man page format. To report such errors, see https://www.ker‐
153 nel.org/doc/man-pages/reporting_bugs.html .
154IEEE/The Open Group 2017 TIME(3P)