man3p/localtime.3p

   1 .\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved
   2 .TH "LOCALTIME" P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
   3 .\" localtime
   4 .SH NAME
   5 localtime, localtime_r \- convert a time value to a broken-down local
   6 time
   7 .SH SYNOPSIS
   8 .LP
   9 \fB#include <time.h>
  10 .br
  11 .sp
  12 struct tm *localtime(const time_t *\fP\fItimer\fP\fB);
  13 .br
  14 \fP
  15 .LP
  16 \fBstruct tm *localtime_r(const time_t *restrict\fP \fItimer\fP\fB,
  17 .br
  18 \ \ \ \ \ \  struct tm *restrict\fP \fIresult\fP\fB); \fP
  19 \fB
  20 .br
  21 \fP
  22 .SH DESCRIPTION
  23 .LP
  24 For \fIlocaltime\fP():   The functionality described on this reference
  25 page is aligned with the ISO\ C standard. Any
  26 conflict between the requirements described here and the ISO\ C standard
  27 is unintentional. This volume of
  28 IEEE\ Std\ 1003.1-2001 defers to the ISO\ C standard.
  29 .LP
  30 The \fIlocaltime\fP() function shall convert the time in seconds since
  31 the Epoch pointed to by \fItimer\fP into a broken-down
  32 time, expressed as a local time. The function corrects for the timezone
  33 and any seasonal time adjustments.   \ Local timezone
  34 information is used as though \fIlocaltime\fP() calls \fItzset\fP().
  35 .LP
  36 The relationship between a time in seconds since the Epoch used as
  37 an argument to \fIlocaltime\fP() and the \fBtm\fP structure
  38 (defined in the \fI<time.h>\fP header) is that the result shall be
  39 as specified in the
  40 expression given in the definition of seconds since the Epoch (see
  41 the Base Definitions volume of IEEE\ Std\ 1003.1-2001,
  42 Section 4.14, Seconds Since the Epoch) corrected for timezone and
  43 any seasonal
  44 time adjustments, where the names in the structure and in the expression
  45 correspond.
  46 .LP
  47 The same relationship shall apply for \fIlocaltime_r\fP().
  48 .LP
  49 The
  50 \fIlocaltime\fP() function need not be reentrant. A function that
  51 is not required to be reentrant is not required to be
  52 thread-safe.
  53 .LP
  54 The \fIasctime\fP(), \fIctime\fP(), \fIgmtime\fP(), and \fIlocaltime\fP()
  55 functions shall return values in one of two static objects:
  56 a broken-down time structure and an array of type \fBchar\fP. Execution
  57 of any of the functions may overwrite the information
  58 returned in either of these objects by any of the other functions.
  59 .LP
  60 The \fIlocaltime_r\fP() function shall convert the time in seconds
  61 since the Epoch pointed to by \fItimer\fP into a broken-down
  62 time stored in the structure to which \fIresult\fP points. The \fIlocaltime_r\fP()
  63 function shall also return a pointer to that
  64 same structure.
  65 .LP
  66 Unlike \fIlocaltime\fP(), the reentrant version is not required to
  67 set \fItzname\fP.
  68 .SH RETURN VALUE
  69 .LP
  70 Upon successful completion, the \fIlocaltime\fP() function shall return
  71 a pointer to the broken-down time structure. If an
  72 error is detected, \fIlocaltime\fP() shall return a null pointer
  73 \ and set \fIerrno\fP to indicate the error.
  74 .LP
  75 Upon successful completion, \fIlocaltime_r\fP() shall return a pointer
  76 to the structure pointed to by the argument \fIresult\fP.
  77 .SH ERRORS
  78 .LP
  79 The \fIlocaltime\fP() function shall fail if:
  80 .TP 7
  81 .B EOVERFLOW
  82 The result cannot be represented.
  83 .sp
  84 .LP
  85 \fIThe following sections are informative.\fP
  86 .SH EXAMPLES
  87 .SS Getting the Local Date and Time
  88 .LP
  89 The following example uses the \fItime\fP() function to calculate
  90 the time elapsed, in
  91 seconds, since January 1, 1970 0:00 UTC (the Epoch), \fIlocaltime\fP()
  92 to convert that value to a broken-down time, and \fIasctime\fP() to
  93 convert the broken-down time values into a printable string.
  94 .sp
  95 .RS
  96 .nf
  97
  98 \fB#include <stdio.h>
  99 #include <time.h>
 100 .sp
 101
 102 int main(void)
 103 {
 104     time_t result;
 105 .sp
 106
 107     result = time(NULL);
 108     printf("%s%ju secs since the Epoch\\n",
 109         asctime(localtime(&result)),
 110             (uintmax_t)result);
 111     return(0);
 112 }
 113 \fP
 114 .fi
 115 .RE
 116 .LP
 117 This example writes the current time to \fIstdout\fP in a form like
 118 this:
 119 .sp
 120 .RS
 121 .nf
 122
 123 \fBWed Jun 26 10:32:15 1996
 124 835810335 secs since the Epoch
 125 \fP
 126 .fi
 127 .RE
 128 .SS Getting the Modification Time for a File
 129 .LP
 130 The following example gets the modification time for a file. The \fIlocaltime\fP()
 131 function converts the \fBtime_t\fP value of
 132 the last modification date, obtained by a previous call to \fIstat\fP(),
 133 into a \fBtm\fP
 134 structure that contains the year, month, day, and so on.
 135 .sp
 136 .RS
 137 .nf
 138
 139 \fB#include <time.h>
 140 \&...
 141 struct stat statbuf;
 142 \&...
 143 tm = localtime(&statbuf.st_mtime);
 144 \&...
 145 \fP
 146 .fi
 147 .RE
 148 .SS Timing an Event
 149 .LP
 150 The following example gets the current time, converts it to a string
 151 using \fIlocaltime\fP() and \fIasctime\fP(), and prints it to standard
 152 output using \fIfputs\fP(). It then prints the number of minutes to
 153 an event being timed.
 154 .sp
 155 .RS
 156 .nf
 157
 158 \fB#include <time.h>
 159 #include <stdio.h>
 160 \&...
 161 time_t now;
 162 int minutes_to_event;
 163 \&...
 164 time(&now);
 165 printf("The time is ");
 166 fputs(asctime(localtime(&now)), stdout);
 167 printf("There are still %d minutes to the event.\\n",
 168     minutes_to_event);
 169 \&...
 170 \fP
 171 .fi
 172 .RE
 173 .SH APPLICATION USAGE
 174 .LP
 175 The \fIlocaltime_r\fP() function is thread-safe and returns values
 176 in a user-supplied buffer instead of possibly using a static
 177 data area that may be overwritten by each call.
 178 .SH RATIONALE
 179 .LP
 180 None.
 181 .SH FUTURE DIRECTIONS
 182 .LP
 183 None.
 184 .SH SEE ALSO
 185 .LP
 186 \fIasctime\fP() , \fIclock\fP() , \fIctime\fP()
 187 , \fIdifftime\fP() , \fIgetdate\fP() , \fIgmtime\fP() , \fImktime\fP()
 188 , \fIstrftime\fP() , \fIstrptime\fP() , \fItime\fP() , \fIutime\fP()
 189 , the
 190 Base Definitions volume of IEEE\ Std\ 1003.1-2001, \fI<time.h>\fP
 191 .SH COPYRIGHT
 192 Portions of this text are reprinted and reproduced in electronic form
 193 from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
 194 -- Portable Operating System Interface (POSIX), The Open Group Base
 195 Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
 196 Electrical and Electronics Engineers, Inc and The Open Group. In the
 197 event of any discrepancy between this version and the original IEEE and
 198 The Open Group Standard, the original IEEE and The Open Group Standard
 199 is the referee document. The original Standard can be obtained online at
 200 http://www.opengroup.org/unix/online.html .