man3/timegm.3

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  25 .TH TIMEGM 3 2016-12-12 "GNU" "Linux Programmer's Manual"
  26 .SH NAME
  27 timegm, timelocal \- inverses of gmtime and localtime
  28 .SH SYNOPSIS
  29 .nf
  30 .B #include <time.h>
  31 .PP
  32 .BI "time_t timelocal(struct tm *" tm );
  33 .PP
  34 .BI "time_t timegm(struct tm *" tm );
  35 .PP
  36 .fi
  37 .in -4n
  38 Feature Test Macro Requirements for glibc (see
  39 .BR feature_test_macros (7)):
  40 .in
  41 .PP
  42 .BR timelocal (),
  43 .BR timegm ():
  44     Since glibc 2.19:
  45         _DEFAULT_SOURCE
  46     Glibc 2.19 and earlier:
  47         _BSD_SOURCE || _SVID_SOURCE
  48 .SH DESCRIPTION
  49 The functions
  50 .BR timelocal ()
  51 and
  52 .BR timegm ()
  53 are the inverses of
  54 .BR localtime (3)
  55 and
  56 .BR gmtime (3).
  57 Both functions take a broken-down time and convert it to calendar time
  58 (seconds since the Epoch, 1970-01-01 00:00:00 +0000, UTC).
  59 The difference between the two functions is that
  60 .BR timelocal ()
  61 takes the local timezone into account when doing the conversion, while
  62 .BR timegm ()
  63 takes the input value to be Coordinated Universal Time (UTC).
  64 .SH RETURN VALUE
  65 On success,
  66 these functions return the calendar time (seconds since the Epoch),
  67 expressed as a value of type
  68 .IR time_t .
  69 On error, they return the value
  70 .I (time_t)\ \-1
  71 and set
  72 .I errno
  73 to indicate the cause of the error.
  74 .SH ERRORS
  75 .TP
  76 .B EOVERFLOW
  77 The result cannot be represented.
  78 .SH ATTRIBUTES
  79 For an explanation of the terms used in this section, see
  80 .BR attributes (7).
  81 .TS
  82 allbox;
  83 lbw21 lb lb
  84 l l l.
  85 Interface       Attribute       Value
  86 T{
  87 .BR timelocal (),
  88 .BR timegm ()
  89 T}      Thread safety   MT-Safe env locale
  90 .TE
  91 .SH CONFORMING TO
  92 These functions are nonstandard GNU extensions
  93 that are also present on the BSDs.
  94 Avoid their use.
  95 .SH NOTES
  96 The
  97 .BR timelocal ()
  98 function is equivalent to the POSIX standard function
  99 .BR mktime (3).
 100 There is no reason to ever use it.
 101 .SH SEE ALSO
 102 .BR gmtime (3),
 103 .BR localtime (3),
 104 .BR mktime (3),
 105 .BR tzset (3)