1SLEEP(P) POSIX Programmer's Manual SLEEP(P)
2
6 sleep - suspend execution for an interval of time
7
9 #include <unistd.h>
10 unsigned sleep(unsigned seconds);
12
15 The sleep() function shall cause the calling thread to be suspended
16 from execution until either the number of realtime seconds specified by
17 the argument seconds has elapsed or a signal is delivered to the call‐
18 ing thread and its action is to invoke a signal-catching function or to
19 terminate the process. The suspension time may be longer than requested
20 due to the scheduling of other activity by the system.
21 If a SIGALRM signal is generated for the calling process during execu‐
23 tion of sleep() and if the SIGALRM signal is being ignored or blocked
24 from delivery, it is unspecified whether sleep() returns when the
25 SIGALRM signal is scheduled. If the signal is being blocked, it is also
26 unspecified whether it remains pending after sleep() returns or it is
27 discarded.
28 If a SIGALRM signal is generated for the calling process during execu‐
30 tion of sleep(), except as a result of a prior call to alarm(), and if
31 the SIGALRM signal is not being ignored or blocked from delivery, it is
32 unspecified whether that signal has any effect other than causing
33 sleep() to return.
34 If a signal-catching function interrupts sleep() and examines or
36 changes either the time a SIGALRM is scheduled to be generated, the
37 action associated with the SIGALRM signal, or whether the SIGALRM sig‐
38 nal is blocked from delivery, the results are unspecified.
39 If a signal-catching function interrupts sleep() and calls siglongjmp()
41 or longjmp() to restore an environment saved prior to the sleep() call,
42 the action associated with the SIGALRM signal and the time at which a
43 SIGALRM signal is scheduled to be generated are unspecified. It is also
44 unspecified whether the SIGALRM signal is blocked, unless the process'
45 signal mask is restored as part of the environment.
46 Interactions between sleep() and any of setitimer(), ualarm(), or
48 usleep() are unspecified.
49
51 If sleep() returns because the requested time has elapsed, the value
52 returned shall be 0. If sleep() returns due to delivery of a signal,
53 the return value shall be the "unslept" amount (the requested time
54 minus the time actually slept) in seconds.
55
57 No errors are defined.
58 The following sections are informative.
60
62 None.
63
65 None.
66
68 There are two general approaches to the implementation of the sleep()
69 function. One is to use the alarm() function to schedule a SIGALRM sig‐
70 nal and then suspend the process waiting for that signal. The other is
71 to implement an independent facility. This volume of
72 IEEE Std 1003.1-2001 permits either approach.
73 In order to comply with the requirement that no primitive shall change
75 a process attribute unless explicitly described by this volume of
76 IEEE Std 1003.1-2001, an implementation using SIGALRM must carefully
77 take into account any SIGALRM signal scheduled by previous alarm()
78 calls, the action previously established for SIGALRM, and whether
79 SIGALRM was blocked. If a SIGALRM has been scheduled before the sleep()
80 would ordinarily complete, the sleep() must be shortened to that time
81 and a SIGALRM generated (possibly simulated by direct invocation of the
82 signal-catching function) before sleep() returns. If a SIGALRM has been
83 scheduled after the sleep() would ordinarily complete, it must be
84 rescheduled for the same time before sleep() returns. The action and
85 blocking for SIGALRM must be saved and restored.
86 Historical implementations often implement the SIGALRM-based version
88 using alarm() and pause(). One such implementation is prone to infinite
89 hangups, as described in pause() . Another such implementation uses the
90 C-language setjmp() and longjmp() functions to avoid that window. That
91 implementation introduces a different problem: when the SIGALRM signal
92 interrupts a signal-catching function installed by the user to catch a
93 different signal, the longjmp() aborts that signal-catching function.
94 An implementation based on sigprocmask(), alarm(), and sigsuspend() can
95 avoid these problems.
96 Despite all reasonable care, there are several very subtle, but
98 detectable and unavoidable, differences between the two types of imple‐
99 mentations. These are the cases mentioned in this volume of
100 IEEE Std 1003.1-2001 where some other activity relating to SIGALRM
101 takes place, and the results are stated to be unspecified. All of
102 these cases are sufficiently unusual as not to be of concern to most
103 applications.
104 See also the discussion of the term realtime in alarm() .
106 Since sleep() can be implemented using alarm(), the discussion about
108 alarms occurring early under alarm() applies to sleep() as well.
109 Application writers should note that the type of the argument seconds
111 and the return value of sleep() is unsigned. That means that a Strictly
112 Conforming POSIX System Interfaces Application cannot pass a value
113 greater than the minimum guaranteed value for {UINT_MAX}, which the
114 ISO C standard sets as 65535, and any application passing a larger
115 value is restricting its portability. A different type was considered,
116 but historical implementations, including those with a 16-bit int type,
117 consistently use either unsigned or int.
118 Scheduling delays may cause the process to return from the sleep()
120 function significantly after the requested time. In such cases, the
121 return value should be set to zero, since the formula (requested time
122 minus the time actually spent) yields a negative number and sleep()
123 returns an unsigned.
124
126 None.
127
129 alarm() , getitimer() , nanosleep() , pause() , sigaction() ,
130 sigsetjmp() , ualarm() , usleep() , the Base Definitions volume of
131 IEEE Std 1003.1-2001, <unistd.h>
132
134 Portions of this text are reprinted and reproduced in electronic form
135 from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
136 -- Portable Operating System Interface (POSIX), The Open Group Base
137 Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
138 Electrical and Electronics Engineers, Inc and The Open Group. In the
139 event of any discrepancy between this version and the original IEEE and
140 The Open Group Standard, the original IEEE and The Open Group Standard
141 is the referee document. The original Standard can be obtained online
142 at http://www.opengroup.org/unix/online.html .
143IEEE/The Open Group 2003 SLEEP(P)