path: root/man-pages-posix-2013/man3p/localtime.3p

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.TH LOCALTIME "3P" 2013 "IEEE/The Open Group" "POSIX Programmer's Manual"
.SH PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.

.SH NAME
localtime,
localtime_r
\(em convert a time value to a broken-down local time
.SH SYNOPSIS
.LP
.nf
#include <time.h>
.P
struct tm *localtime(const time_t *\fItimer\fP);
struct tm *localtime_r(const time_t *restrict \fItimer\fP,
    struct tm *restrict \fIresult\fP);
.fi
.SH DESCRIPTION
For
\fIlocaltime\fR():
The functionality described on this reference page is aligned with the
ISO\ C standard. Any conflict between the requirements described here and the
ISO\ C standard is unintentional. This volume of POSIX.1\(hy2008 defers to the ISO\ C standard.
.P
The
\fIlocaltime\fR()
function shall convert the time in seconds since the Epoch pointed
to by
.IR timer
into a broken-down time, expressed as a local time. The function
corrects for the timezone and any seasonal time adjustments.
Local timezone information is used as though
\fIlocaltime\fR()
calls
\fItzset\fR().
.P
The relationship between a time in seconds since the Epoch used as an
argument to
\fIlocaltime\fR()
and the
.BR tm
structure (defined in the
.IR <time.h> 
header) is that the result shall be as specified in the expression
given in the definition of seconds since the Epoch (see the Base Definitions volume of POSIX.1\(hy2008,
.IR "Section 4.15" ", " "Seconds Since the Epoch")
corrected for timezone and any seasonal time adjustments, where the
names in the structure and in the expression correspond.
.P
The same relationship shall apply for
\fIlocaltime_r\fR().
.P
The
\fIlocaltime\fR()
function need not be thread-safe.
.P
The
\fIasctime\fR(),
\fIctime\fR(),
\fIgmtime\fR(),
and
\fIlocaltime\fR()
functions shall return values in one of two static objects: a
broken-down time structure and an array of type
.BR char .
Execution of any of the functions may overwrite the information
returned in either of these objects by any of the other functions.
.P
The
\fIlocaltime_r\fR()
function shall convert the time in seconds since the Epoch pointed
to by
.IR timer
into a broken-down time stored in the structure to which
.IR result
points. The
\fIlocaltime_r\fR()
function shall also return a pointer to that same structure.
.P
Unlike
\fIlocaltime\fR(),
the
\fIlocaltime_r\fR()
function is not required to set
.IR tzname .
If
\fIlocaltime_r\fR()
does not set
.IR tzname ,
it shall not set
.IR daylight
and shall not set
.IR timezone .
.SH "RETURN VALUE"
Upon successful completion, the
\fIlocaltime\fR()
function shall return a pointer to the broken-down time structure.
If an error is detected,
\fIlocaltime\fR()
shall return a null pointer
and set
.IR errno
to indicate the error.
.P
Upon successful completion,
\fIlocaltime_r\fR()
shall return a pointer to the structure pointed to by the argument
.IR result .
If an error is detected,
\fIlocaltime_r\fR()
shall return a null pointer and set
.IR errno
to indicate the error.
.SH ERRORS
The
\fIlocaltime\fR()
and
\fIlocaltime_r\fR()
functions shall fail if:
.TP
.BR EOVERFLOW
The result cannot be represented.
.LP
.IR "The following sections are informative."
.SH EXAMPLES
.SS "Getting the Local Date and Time"
.P
The following example uses the
\fItime\fR()
function to calculate the time elapsed, in seconds, since January 1,
1970 0:00 UTC (the Epoch),
\fIlocaltime\fR()
to convert that value to a broken-down time, and
\fIasctime\fR()
to convert the broken-down time values into a printable string.
.sp
.RS 4
.nf
\fB
#include <stdio.h>
#include <time.h>
.P
int main(void)
{
    time_t result;
.P
    result = time(NULL);
    printf("%s%ju secs since the Epoch\en",
        asctime(localtime(&result)),
            (uintmax_t)result);
    return(0);
}
.fi \fR
.P
.RE
.P
This example writes the current time to
.IR stdout
in a form like this:
.sp
.RS 4
.nf
\fB
Wed Jun 26 10:32:15 1996
835810335 secs since the Epoch
.fi \fR
.P
.RE
.SS "Getting the Modification Time for a File"
.P
The following example prints the last data modification timestamp
in the local timezone for a given file.
.sp
.RS 4
.nf
\fB
#include <stdio.h>
#include <time.h>
#include <sys/stat.h>
.P
int
print_file_time(const char *pathname)
{
    struct stat statbuf;
    struct tm *tm;
    char timestr[BUFSIZ];
.P
    if(stat(pathname, &statbuf) =\|= \(mi1)
        return \(mi1;
    if((tm = localtime(&statbuf.st_mtime)) =\|= NULL)
        return \(mi1;
    if(strftime(timestr, sizeof(timestr), "%Y-%m-%d %H:%M:%S", tm) =\|= 0)
        return \(mi1;
    printf("%s: %s.%09ld\en", pathname, timestr, statbuf.st_mtim.tv_nsec);
    return 0;
}
.fi \fR
.P
.RE
.SS "Timing an Event"
.P
The following example gets the current time, converts it to a string
using
\fIlocaltime\fR()
and
\fIasctime\fR(),
and prints it to standard output using
\fIfputs\fR().
It then prints the number of minutes to an event being timed.
.sp
.RS 4
.nf
\fB
#include <time.h>
#include <stdio.h>
\&...
time_t now;
int minutes_to_event;
\&...
time(&now);
printf("The time is ");
fputs(asctime(localtime(&now)), stdout);
printf("There are still %d minutes to the event.\en",
    minutes_to_event);
\&...
.fi \fR
.P
.RE
.SH "APPLICATION USAGE"
The
\fIlocaltime_r\fR()
function is thread-safe and returns values in a user-supplied buffer
instead of possibly using a static data area that may be overwritten by
each call.
.SH RATIONALE
None.
.SH "FUTURE DIRECTIONS"
None.
.SH "SEE ALSO"
.IR "\fIasctime\fR\^(\|)",
.IR "\fIclock\fR\^(\|)",
.IR "\fIctime\fR\^(\|)",
.IR "\fIdifftime\fR\^(\|)",
.IR "\fIgetdate\fR\^(\|)",
.IR "\fIgmtime\fR\^(\|)",
.IR "\fImktime\fR\^(\|)",
.IR "\fIstrftime\fR\^(\|)",
.IR "\fIstrptime\fR\^(\|)",
.IR "\fItime\fR\^(\|)",
.IR "\fItzset\fR\^(\|)",
.IR "\fIutime\fR\^(\|)"
.P
The Base Definitions volume of POSIX.1\(hy2008,
.IR "Section 4.15" ", " "Seconds Since the Epoch",
.IR "\fB<time.h>\fP"
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, Copyright (C) 2013 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
(This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.unix.org/online.html .

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