adjtime(3)

1adjtime(3)                 Library Functions Manual                 adjtime(3)
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NAME

6       adjtime - correct the time to synchronize the system clock
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LIBRARY

9       Standard C library (libc, -lc)
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SYNOPSIS

12       #include <sys/time.h>
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14       int adjtime(const struct timeval *delta, struct timeval *olddelta);
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16   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
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18       adjtime():
19           Since glibc 2.19:
20               _DEFAULT_SOURCE
21           glibc 2.19 and earlier:
22               _BSD_SOURCE
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DESCRIPTION

25       The  adjtime() function gradually adjusts the system clock (as returned
26       by gettimeofday(2)).  The amount of time by which the clock  is  to  be
27       adjusted  is  specified  in  the  structure  pointed to by delta.  This
28       structure has the following form:
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30           struct timeval {
31               time_t      tv_sec;     /* seconds */
32               suseconds_t tv_usec;    /* microseconds */
33           };
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35       If the adjustment in delta  is  positive,  then  the  system  clock  is
36       speeded  up by some small percentage (i.e., by adding a small amount of
37       time to the clock value in each second) until the adjustment  has  been
38       completed.   If  the adjustment in delta is negative, then the clock is
39       slowed down in a similar fashion.
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41       If a clock adjustment from an earlier  adjtime()  call  is  already  in
42       progress  at  the time of a later adjtime() call, and delta is not NULL
43       for the later call, then the earlier adjustment is stopped, but any al‐
44       ready completed part of that adjustment is not undone.
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46       If  olddelta  is not NULL, then the buffer that it points to is used to
47       return the amount of time remaining from any previous  adjustment  that
48       has not yet been completed.
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RETURN VALUE

51       On success, adjtime() returns 0.  On failure, -1 is returned, and errno
52       is set to indicate the error.
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ERRORS

55       EINVAL The adjustment in delta is outside the permitted range.
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57       EPERM  The caller does not have  sufficient  privilege  to  adjust  the
58              time.  Under Linux, the CAP_SYS_TIME capability is required.
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ATTRIBUTES

61       For  an  explanation  of  the  terms  used  in  this  section,  see at‐
62       tributes(7).
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64       ┌────────────────────────────────────────────┬───────────────┬─────────┐
65       │Interface                                   │ Attribute     │ Value   │
66       ├────────────────────────────────────────────┼───────────────┼─────────┤
67       │adjtime()                                   │ Thread safety │ MT-Safe │
68       └────────────────────────────────────────────┴───────────────┴─────────┘
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STANDARDS

71       None.
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HISTORY

74       4.3BSD, System V.
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NOTES

77       The adjustment that adjtime() makes to the clock is carried out in such
78       a manner that the clock is always monotonically increasing.  Using adj‐
79       time() to adjust the time prevents the problems that can be caused  for
80       certain  applications  (e.g.,  make(1))  by abrupt positive or negative
81       jumps in the system time.
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83       adjtime() is intended to be used to make small adjustments to the  sys‐
84       tem  time.   Most  systems impose a limit on the adjustment that can be
85       specified in delta.  In the glibc implementation, delta  must  be  less
86       than  or  equal to (INT_MAX / 1000000 - 2) and greater than or equal to
87       (INT_MIN / 1000000 + 2) (respectively 2145 and -2145 seconds on i386).
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BUGS

90       A longstanding bug meant that if delta was specified as NULL, no  valid
91       information about the outstanding clock adjustment was returned in old‐
92       delta.  (In this circumstance, adjtime() should return the  outstanding
93       clock  adjustment,  without changing it.)  This bug is fixed on systems
94       with glibc 2.8 or later and Linux kernel 2.6.26 or later.
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