mirror of https://github.com/mkerrisk/man-pages
153 lines
4.4 KiB
Plaintext
153 lines
4.4 KiB
Plaintext
.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved
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.TH "TIME" P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
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.\" time
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.SH NAME
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time \- get time
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.SH SYNOPSIS
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.LP
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\fB#include <time.h>
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.br
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.sp
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time_t time(time_t *\fP\fItloc\fP\fB);
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.br
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\fP
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.SH DESCRIPTION
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.LP
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The \fItime\fP() function shall return the value of time \ in seconds
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since the Epoch.
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.LP
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The \fItloc\fP argument points to an area where the return value is
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also stored. If \fItloc\fP is a null pointer, no value is
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stored.
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.SH RETURN VALUE
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.LP
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Upon successful completion, \fItime\fP() shall return the value of
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time. Otherwise, (\fBtime_t\fP)-1 shall be returned.
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.SH ERRORS
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.LP
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No errors are defined.
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.LP
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\fIThe following sections are informative.\fP
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.SH EXAMPLES
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.SS Getting the Current Time
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.LP
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The following example uses the \fItime\fP() function to calculate
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the time elapsed, in seconds, since the Epoch, \fIlocaltime\fP() to
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convert that value to a broken-down time, and \fIasctime\fP() to convert
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the broken-down time values into a printable string.
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.sp
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.RS
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.nf
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\fB#include <stdio.h>
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#include <time.h>
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.sp
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int main(void)
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{
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time_t result;
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.sp
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result = time(NULL);
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printf("%s%ju secs since the Epoch\\n",
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asctime(localtime(&result)),
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(uintmax_t)result);
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return(0);
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}
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\fP
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.fi
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.RE
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.LP
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This example writes the current time to \fIstdout\fP in a form like
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this:
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.sp
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.RS
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.nf
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\fBWed Jun 26 10:32:15 1996
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835810335 secs since the Epoch
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\fP
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.fi
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.RE
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.SS Timing an Event
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.LP
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The following example gets the current time, prints it out in the
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user's format, and prints the number of minutes to an event
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being timed.
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.sp
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.RS
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.nf
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\fB#include <time.h>
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#include <stdio.h>
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\&...
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time_t now;
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int minutes_to_event;
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\&...
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time(&now);
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minutes_to_event = ...;
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printf("The time is ");
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puts(asctime(localtime(&now)));
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printf("There are %d minutes to the event.\\n",
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minutes_to_event);
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\&...
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\fP
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.fi
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.RE
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.SH APPLICATION USAGE
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.LP
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None.
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.SH RATIONALE
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.LP
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The \fItime\fP() function returns a value in seconds (type \fBtime_t\fP)
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while \fItimes\fP() returns a set of values in clock ticks (type \fBclock_t\fP).
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Some historical
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implementations, such as 4.3 BSD, have mechanisms capable of returning
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more precise times (see below). A generalized timing scheme
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to unify these various timing mechanisms has been proposed but not
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adopted.
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.LP
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Implementations in which \fBtime_t\fP is a 32-bit signed integer (many
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historical implementations) fail in the year 2038.
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IEEE\ Std\ 1003.1-2001 does not address this problem. However, the
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use of the \fBtime_t\fP type is mandated in order to
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ease the eventual fix.
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.LP
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The use of the \fI<time.h>\fP header instead of \fI<sys/types.h>\fP
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allows compatibility with the ISO\ C standard.
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.LP
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Many historical implementations (including Version 7) and the 1984
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/usr/group standard use \fBlong\fP instead of \fBtime_t\fP.
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This volume of IEEE\ Std\ 1003.1-2001 uses the latter type in order
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to agree with the ISO\ C standard.
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.LP
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4.3 BSD includes \fItime\fP() only as an alternate function to the
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more flexible \fIgettimeofday\fP() function.
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.SH FUTURE DIRECTIONS
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.LP
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In a future version of this volume of IEEE\ Std\ 1003.1-2001, \fBtime_t\fP
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is likely to be required to be capable of
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representing times far in the future. Whether this will be mandated
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as a 64-bit type or a requirement that a specific date in the
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future be representable (for example, 10000 AD) is not yet determined.
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Systems purchased after the approval of this volume of
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IEEE\ Std\ 1003.1-2001 should be evaluated to determine whether their
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lifetime will extend past 2038.
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.SH SEE ALSO
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.LP
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\fIasctime\fP() , \fIclock\fP() , \fIctime\fP()
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, \fIdifftime\fP() , \fIgettimeofday\fP() , \fIgmtime\fP() , \fIlocaltime\fP()
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, \fImktime\fP() ,
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\fIstrftime\fP() , \fIstrptime\fP() , \fIutime\fP() , the Base Definitions
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volume of IEEE\ Std\ 1003.1-2001, \fI<time.h>\fP
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.SH COPYRIGHT
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Portions of this text are reprinted and reproduced in electronic form
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from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
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-- Portable Operating System Interface (POSIX), The Open Group Base
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Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
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Electrical and Electronics Engineers, Inc and The Open Group. In the
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event of any discrepancy between this version and the original IEEE and
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The Open Group Standard, the original IEEE and The Open Group Standard
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is the referee document. The original Standard can be obtained online at
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http://www.opengroup.org/unix/online.html .
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