1SLEEP(3P) POSIX Programmer's Manual SLEEP(3P)
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6 This manual page is part of the POSIX Programmer's Manual. The Linux
7 implementation of this interface may differ (consult the corresponding
8 Linux manual page for details of Linux behavior), or the interface may
9 not be implemented on Linux.
10
13 sleep — suspend execution for an interval of time
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16 #include <unistd.h>
17 unsigned sleep(unsigned seconds);
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21 The sleep() function shall cause the calling thread to be suspended
22 from execution until either the number of realtime seconds specified by
23 the argument seconds has elapsed or a signal is delivered to the call‐
24 ing thread and its action is to invoke a signal-catching function or to
25 terminate the process. The suspension time may be longer than requested
26 due to the scheduling of other activity by the system.
27 If a SIGALRM signal is generated for the calling process during execu‐
29 tion of sleep() and if the SIGALRM signal is being ignored or blocked
30 from delivery, it is unspecified whether sleep() returns when the
31 SIGALRM signal is scheduled. If the signal is being blocked, it is also
32 unspecified whether it remains pending after sleep() returns or it is
33 discarded.
34 If a SIGALRM signal is generated for the calling process during execu‐
36 tion of sleep(), except as a result of a prior call to alarm(), and if
37 the SIGALRM signal is not being ignored or blocked from delivery, it is
38 unspecified whether that signal has any effect other than causing
39 sleep() to return.
40 If a signal-catching function interrupts sleep() and examines or
42 changes either the time a SIGALRM is scheduled to be generated, the
43 action associated with the SIGALRM signal, or whether the SIGALRM sig‐
44 nal is blocked from delivery, the results are unspecified.
45 If a signal-catching function interrupts sleep() and calls siglongjmp()
47 or longjmp() to restore an environment saved prior to the sleep() call,
48 the action associated with the SIGALRM signal and the time at which a
49 SIGALRM signal is scheduled to be generated are unspecified. It is
50 also unspecified whether the SIGALRM signal is blocked, unless the sig‐
51 nal mask of the process is restored as part of the environment.
52 Interactions between sleep() and setitimer() are unspecified.
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56 If sleep() returns because the requested time has elapsed, the value
57 returned shall be 0. If sleep() returns due to delivery of a signal,
58 the return value shall be the ``unslept'' amount (the requested time
59 minus the time actually slept) in seconds.
60
62 No errors are defined.
63 The following sections are informative.
65
67 None.
68
70 None.
71
73 There are two general approaches to the implementation of the sleep()
74 function. One is to use the alarm() function to schedule a SIGALRM sig‐
75 nal and then suspend the calling thread waiting for that signal. The
76 other is to implement an independent facility. This volume of
77 POSIX.1‐2008 permits either approach.
78 In order to comply with the requirement that no primitive shall change
80 a process attribute unless explicitly described by this volume of
81 POSIX.1‐2008, an implementation using SIGALRM must carefully take into
82 account any SIGALRM signal scheduled by previous alarm() calls, the
83 action previously established for SIGALRM, and whether SIGALRM was
84 blocked. If a SIGALRM has been scheduled before the sleep() would ordi‐
85 narily complete, the sleep() must be shortened to that time and a
86 SIGALRM generated (possibly simulated by direct invocation of the sig‐
87 nal-catching function) before sleep() returns. If a SIGALRM has been
88 scheduled after the sleep() would ordinarily complete, it must be
89 rescheduled for the same time before sleep() returns. The action and
90 blocking for SIGALRM must be saved and restored.
91 Historical implementations often implement the SIGALRM-based version
93 using alarm() and pause(). One such implementation is prone to infi‐
94 nite hangups, as described in pause(). Another such implementation
95 uses the C-language setjmp() and longjmp() functions to avoid that win‐
96 dow. That implementation introduces a different problem: when the
97 SIGALRM signal interrupts a signal-catching function installed by the
98 user to catch a different signal, the longjmp() aborts that signal-
99 catching function. An implementation based on sigprocmask(), alarm(),
100 and sigsuspend() can avoid these problems.
101 Despite all reasonable care, there are several very subtle, but
103 detectable and unavoidable, differences between the two types of imple‐
104 mentations. These are the cases mentioned in this volume of
105 POSIX.1‐2008 where some other activity relating to SIGALRM takes place,
106 and the results are stated to be unspecified. All of these cases are
107 sufficiently unusual as not to be of concern to most applications.
108 See also the discussion of the term realtime in alarm().
110 Since sleep() can be implemented using alarm(), the discussion about
112 alarms occurring early under alarm() applies to sleep() as well.
113 Application developers should note that the type of the argument sec‐
115 onds and the return value of sleep() is unsigned. That means that a
116 Strictly Conforming POSIX System Interfaces Application cannot pass a
117 value greater than the minimum guaranteed value for {UINT_MAX}, which
118 the ISO C standard sets as 65535, and any application passing a larger
119 value is restricting its portability. A different type was considered,
120 but historical implementations, including those with a 16-bit int type,
121 consistently use either unsigned or int.
122 Scheduling delays may cause the process to return from the sleep()
124 function significantly after the requested time. In such cases, the
125 return value should be set to zero, since the formula (requested time
126 minus the time actually spent) yields a negative number and sleep()
127 returns an unsigned.
128
130 None.
131
133 alarm(), getitimer(), nanosleep(), pause(), sigaction(), sigsetjmp()
134 The Base Definitions volume of POSIX.1‐2008, <unistd.h>
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138 Portions of this text are reprinted and reproduced in electronic form
139 from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
140 -- Portable Operating System Interface (POSIX), The Open Group Base
141 Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
142 cal and Electronics Engineers, Inc and The Open Group. (This is
143 POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the
144 event of any discrepancy between this version and the original IEEE and
145 The Open Group Standard, the original IEEE and The Open Group Standard
146 is the referee document. The original Standard can be obtained online
147 at http://www.unix.org/online.html .
148 Any typographical or formatting errors that appear in this page are
150 most likely to have been introduced during the conversion of the source
151 files to man page format. To report such errors, see https://www.ker‐
152 nel.org/doc/man-pages/reporting_bugs.html .
153IEEE/The Open Group 2013 SLEEP(3P)