man3/adjtime.3

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  25 .TH ADJTIME 3 2017-09-15 "Linux" "Linux Programmer's Manual"
  26 .SH NAME
  27 adjtime \- correct the time to synchronize the system clock
  28 .SH SYNOPSIS
  29 .nf
  30 .B #include <sys/time.h>
  31 .PP
  32 .BI "int adjtime(const struct timeval *" delta ", struct timeval *" olddelta );
  33 .fi
  34 .PP
  35 .in -4n
  36 Feature Test Macro Requirements for glibc (see
  37 .BR feature_test_macros (7)):
  38 .in
  39 .PP
  40 .BR adjtime ():
  41     Since glibc 2.19:
  42         _DEFAULT_SOURCE
  43     Glibc 2.19 and earlier:
  44         _BSD_SOURCE
  45 .SH DESCRIPTION
  46 The
  47 .BR adjtime ()
  48 function gradually adjusts the system clock (as returned by
  49 .BR gettimeofday (2)).
  50 The amount of time by which the clock is to be adjusted is specified
  51 in the structure pointed to by
  52 .IR delta .
  53 This structure has the following form:
  54 .PP
  55 .in +4n
  56 .EX
  57 struct timeval {
  58     time_t      tv_sec;     /* seconds */
  59     suseconds_t tv_usec;    /* microseconds */
  60 };
  61 .EE
  62 .in
  63 .PP
  64 If the adjustment in
  65 .I delta
  66 is positive, then the system clock is speeded up by some
  67 small percentage (i.e., by adding a small
  68 amount of time to the clock value in each second) until the adjustment
  69 has been completed.
  70 If the adjustment in
  71 .I delta
  72 is negative, then the clock is slowed down in a similar fashion.
  73 .PP
  74 If a clock adjustment from an earlier
  75 .BR adjtime ()
  76 call is already in progress
  77 at the time of a later
  78 .BR adjtime ()
  79 call, and
  80 .I delta
  81 is not NULL for the later call, then the earlier adjustment is stopped,
  82 but any already completed part of that adjustment is not undone.
  83 .PP
  84 If
  85 .I olddelta
  86 is not NULL, then the buffer that it points to is used to return
  87 the amount of time remaining from any previous adjustment that
  88 has not yet been completed.
  89 .SH RETURN VALUE
  90 On success,
  91 .BR adjtime ()
  92 returns 0.
  93 On failure, \-1 is returned, and
  94 .I errno
  95 is set to indicate the error.
  96 .SH ERRORS
  97 .TP
  98 .B EINVAL
  99 The adjustment in
 100 .I delta
 101 is outside the permitted range.
 102 .TP
 103 .B EPERM
 104 The caller does not have sufficient privilege to adjust the time.
 105 Under Linux, the
 106 .B CAP_SYS_TIME
 107 capability is required.
 108 .SH ATTRIBUTES
 109 For an explanation of the terms used in this section, see
 110 .BR attributes (7).
 111 .TS
 112 allbox;
 113 lb lb lb
 114 l l l.
 115 Interface       Attribute       Value
 116 T{
 117 .BR adjtime ()
 118 T}      Thread safety   MT-Safe
 119 .TE
 120 .SH CONFORMING TO
 121 4.3BSD, System V.
 122 .SH NOTES
 123 The adjustment that
 124 .BR adjtime ()
 125 makes to the clock is carried out in such a manner that the clock
 126 is always monotonically increasing.
 127 Using
 128 .BR adjtime ()
 129 to adjust the time prevents the problems that can be caused for certain
 130 applications (e.g.,
 131 .BR make (1))
 132 by abrupt positive or negative jumps in the system time.
 133 .PP
 134 .BR adjtime ()
 135 is intended to be used to make small adjustments to the system time.
 136 Most systems impose a limit on the adjustment that can be specified in
 137 .IR delta .
 138 In the glibc implementation,
 139 .I delta
 140 must be less than or equal to (INT_MAX / 1000000 \- 2)
 141 and greater than or equal to (INT_MIN / 1000000 + 2)
 142 (respectively 2145 and \-2145 seconds on i386).
 143 .SH BUGS
 144 A longstanding bug
 145 .\" http://sourceware.org/bugzilla/show_bug?id=2449
 146 .\" http://bugzilla.kernel.org/show_bug.cgi?id=6761
 147 meant that if
 148 .I delta
 149 was specified as NULL,
 150 no valid information about the outstanding clock adjustment was returned in
 151 .IR olddelta .
 152 (In this circumstance,
 153 .BR adjtime ()
 154 should return the outstanding clock adjustment, without changing it.)
 155 This bug is fixed
 156 .\" Thanks to the new adjtimex() ADJ_OFFSET_SS_READ flag
 157 on systems with glibc 2.8 or later and
 158 Linux kernel 2.6.26 or later.
 159 .SH SEE ALSO
 160 .BR adjtimex (2),
 161 .BR gettimeofday (2),
 162 .BR time (7)