1ALARM(3P) POSIX Programmer's Manual ALARM(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.
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13 alarm — schedule an alarm signal
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16 #include <unistd.h>
17 unsigned alarm(unsigned seconds);
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21 The alarm() function shall cause the system to generate a SIGALRM sig‐
22 nal for the process after the number of realtime seconds specified by
23 seconds have elapsed. Processor scheduling delays may prevent the
24 process from handling the signal as soon as it is generated.
25 If seconds is 0, a pending alarm request, if any, is canceled.
27 Alarm requests are not stacked; only one SIGALRM generation can be
29 scheduled in this manner. If the SIGALRM signal has not yet been gener‐
30 ated, the call shall result in rescheduling the time at which the
31 SIGALRM signal is generated.
32 Interactions between alarm() and setitimer() are unspecified.
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36 If there is a previous alarm() request with time remaining, alarm()
37 shall return a non-zero value that is the number of seconds until the
38 previous request would have generated a SIGALRM signal. Otherwise,
39 alarm() shall return 0.
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42 The alarm() function is always successful, and no return value is
43 reserved to indicate an error.
44 The following sections are informative.
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48 None.
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51 The fork() function clears pending alarms in the child process. A new
52 process image created by one of the exec functions inherits the time
53 left to an alarm signal in the image of the old process.
54 Application developers should note that the type of the argument sec‐
56 onds and the return value of alarm() is unsigned. That means that a
57 Strictly Conforming POSIX System Interfaces Application cannot pass a
58 value greater than the minimum guaranteed value for {UINT_MAX}, which
59 the ISO C standard sets as 65535, and any application passing a larger
60 value is restricting its portability. A different type was considered,
61 but historical implementations, including those with a 16-bit int type,
62 consistently use either unsigned or int.
63 Application developers should be aware of possible interactions when
65 the same process uses both the alarm() and sleep() functions.
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68 Many historical implementations (including Version 7 and System V)
69 allow an alarm to occur up to a second early. Other implementations
70 allow alarms up to half a second or one clock tick early or do not
71 allow them to occur early at all. The latter is considered most appro‐
72 priate, since it gives the most predictable behavior, especially since
73 the signal can always be delayed for an indefinite amount of time due
74 to scheduling. Applications can thus choose the seconds argument as the
75 minimum amount of time they wish to have elapse before the signal.
76 The term ``realtime'' here and elsewhere (sleep(), times()) is intended
78 to mean ``wall clock'' time as common English usage, and has nothing to
79 do with ``realtime operating systems''. It is in contrast to virtual
80 time, which could be misinterpreted if just time were used.
81 In some implementations, including 4.3 BSD, very large values of the
83 seconds argument are silently rounded down to an implementation-spe‐
84 cific maximum value. This maximum is large enough (to the order of sev‐
85 eral months) that the effect is not noticeable.
86 There were two possible choices for alarm generation in multi-threaded
88 applications: generation for the calling thread or generation for the
89 process. The first option would not have been particularly useful since
90 the alarm state is maintained on a per-process basis and the alarm that
91 is established by the last invocation of alarm() is the only one that
92 would be active.
93 Furthermore, allowing generation of an asynchronous signal for a thread
95 would have introduced an exception to the overall signal model. This
96 requires a compelling reason in order to be justified.
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99 None.
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102 alarm(), exec, fork(), getitimer(), pause(), sigaction(), sleep()
103 The Base Definitions volume of POSIX.1‐2008, <signal.h>, <unistd.h>
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107 Portions of this text are reprinted and reproduced in electronic form
108 from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
109 -- Portable Operating System Interface (POSIX), The Open Group Base
110 Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
111 cal and Electronics Engineers, Inc and The Open Group. (This is
112 POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the
113 event of any discrepancy between this version and the original IEEE and
114 The Open Group Standard, the original IEEE and The Open Group Standard
115 is the referee document. The original Standard can be obtained online
116 at http://www.unix.org/online.html .
117 Any typographical or formatting errors that appear in this page are
119 most likely to have been introduced during the conversion of the source
120 files to man page format. To report such errors, see https://www.ker‐
121 nel.org/doc/man-pages/reporting_bugs.html .
122IEEE/The Open Group 2013 ALARM(3P)