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1 perry 1 SELECT
2 !!!SELECT
3 NAME
4 SYNOPSIS
5 DESCRIPTION
6 RETURN VALUE
7 ERRORS
8 NOTES
9 EXAMPLE
10 CONFORMING TO
11 SEE ALSO
12 ----
13 !!NAME
14
15
16 select, pselect, FD_CLR, FD_ISSET, FD_SET, FD_ZERO - synchronous I/O multiplexing
17 !!SYNOPSIS
18
19
20 __#include __
21 #include __
22 #include __
23
24
25 __int select(int__ ''n''__, fd_set
26 *__''readfds''__, fd_set *__''writefds''__,
27 fd_set *__''exceptfds''__, struct timeval
28 *__''timeout''__);__
29
30
31 __int pselect(int__ ''n''__, fd_set
32 *__''readfds''__, fd_set *__''writefds''__,
33 fd_set *__''exceptfds''__, const struct timespec
34 *__''timeout''__, sigset_t *__
35 ''sigmask''__);__
36
37
38 __FD_CLR(int__ ''fd''__, fd_set
39 *__''set''__);
40 FD_ISSET(int__ ''fd''__, fd_set
41 *__''set''__);
42 FD_SET(int__ ''fd''__, fd_set *__''set''__);
43 FD_ZERO(fd_set *__''set''__);__
44 !!DESCRIPTION
45
46
47 The functions __select__ and __pselect__ wait for a
48 number of file descriptors to change status.
49
50
51 Their function is identical, with three
52 differences:
53
54
55 (i)
56
57
58 The __select__ function uses a timeout that is a
59 ''struct timeval'' (with seconds and microseconds), while
60 __pselect__ uses a ''struct timespec'' (with seconds
61 and nanoseconds).
62
63
64 (ii)
65
66
67 The __select__ function may update the ''timeout''
68 parameter to indicate how much time was left. The
69 __pselect__ function does not change this
70 parameter.
71
72
73 (iii)
74
75
76 The __select__ function has no ''sigmask'' parameter,
77 and behaves as __pselect__ called with NULL
78 ''sigmask''.
79
80
81 Three independent sets of descriptors are watched. Those
82 listed in ''readfds'' will be watched to see if
83 characters become available for reading (more precisely, to
84 see if a read will not block - in particular, a file
85 descriptor is also ready on end-of-file), those in
86 ''writefds'' will be watched to see if a write will not
87 block, and those in ''exceptfds'' will be watched for
88 exceptions. On exit, the sets are modified in place to
89 indicate which descriptors actually changed
90 status.
91
92
93 Four macros are provided to manipulate the sets.
94 __FD_ZERO__ will clear a set. __FD_SET__ and
95 __FD_CLR__ add or remove a given descriptor from a set.
96 __FD_ISSET__ tests to see if a descriptor is part of the
97 set; this is useful after __select__
98 returns.
99
100
101 ''n'' is the highest-numbered descriptor in any of the
102 three sets, plus 1.
103
104
105 ''timeout'' is an upper bound on the amount of time
106 elapsed before __select__ returns. It may be zero,
107 causing __select__ to return immediately. (This is useful
108 for polling.) If ''timeout'' is NULL (no timeout),
109 __select__ can block indefinitely.
110
111
112 ''sigmask'' is a pointer to a signal mask (see
113 sigprocmask(2)); if it is not NULL, then
114 __pselect__ first replaces the current signal mask by the
115 one pointed to by ''sigmask'', then does the `select'
116 function, and then restores the original signal mask
117 again.
118
119
120 The idea of __pselect__ is that if one wants to wait for
121 an event, either a signal or something on a file descriptor,
122 an atomic test is needed to prevent race conditions.
123 (Suppose the signal handler sets a global flag and returns.
124 Then a test of this global flag followed by a call of
125 __select__() could hang indefinitely if the signal
126 arrived just after the test but just before the call. On the
127 other hand, __pselect__ allows one to first block
128 signals, handle the signals that have come in, then call
129 __pselect__() with the desired ''sigmask'', avoiding
130 the race.) Since Linux today does not have a
131 ''pselect''() system call, the current glibc2 routine
132 still contains this race.
133 !!RETURN VALUE
134
135
136 On success, __select__ and __pselect__ return the
137 number of descriptors contained in the descriptor sets,
138 which may be zero if the timeout expires before anything
139 interesting happens. On error, -1 is returned, and
140 ''errno'' is set appropriately; the sets and
141 ''timeout'' become undefined, so do not rely on their
142 contents after an error.
143 !!ERRORS
144
145
146 __EBADF__
147
148
149 An invalid file descriptor was given in one of the
150 sets.
151
152
153 __EINTR__
154
155
156 A non blocked signal was caught.
157
158
159 __EINVAL__
160
161
162 ''n'' is negative.
163
164
165 __ENOMEM__
166
167
168 __select__ was unable to allocate memory for internal
169 tables.
170 !!NOTES
171
172
173 Some code calls __select__ with all three sets empty,
174 __n__ zero, and a non-null ''timeout'' as a fairly
175 portable way to sleep with subsecond precision.
176
177
178 On Linux, ''timeout'' is modified to reflect the amount
179 of time not slept; most other implementations do not do
180 this. This causes problems both when Linux code which reads
181 ''timeout'' is ported to other operating systems, and
182 when code is ported to Linux that reuses a struct timeval
183 for multiple __select__s in a loop without reinitializing
184 it. Consider ''timeout'' to be undefined after
185 __select__ returns.
186 !!EXAMPLE
187
188
189 #include
190 !!CONFORMING TO
191
192
193 4.4BSD (the __select__ function first appeared in
194 4.2BSD). Generally portable to/from non-BSD systems
195 supporting clones of the BSD socket layer (including System
196 V variants). However, note that the System V variant
197 typically sets the timeout variable before exit, but the BSD
198 variant does not.
199
200
201 The __pselect__ function is defined in IEEE Std
202 1003.1g-2000 (POSIX.1g). It is found in glibc2.1 and later.
203 Glibc2.0 has a function with this name, that however does
204 not take a ''sigmask'' parameter.
205 !!SEE ALSO
206
207
208 accept(2), connect(2), poll(2),
209 read(2), recv(2), send(2),
210 sigprocmask(2), write(2)
211 ----
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