1SELECT(2)                  Linux Programmer's Manual                 SELECT(2)
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NAME

6       select,  pselect,  FD_CLR,  FD_ISSET, FD_SET, FD_ZERO - synchronous I/O
7       multiplexing
8

SYNOPSIS

10       /* According to POSIX.1-2001 */
11       #include <sys/select.h>
12
13       /* According to earlier standards */
14       #include <sys/time.h>
15       #include <sys/types.h>
16       #include <unistd.h>
17
18       int select(int nfds, fd_set *readfds, fd_set *writefds,
19                  fd_set *exceptfds, struct timeval *timeout);
20
21       void FD_CLR(int fd, fd_set *set);
22       int  FD_ISSET(int fd, fd_set *set);
23       void FD_SET(int fd, fd_set *set);
24       void FD_ZERO(fd_set *set);
25
26       #include <sys/select.h>
27
28       int pselect(int nfds, fd_set *readfds, fd_set *writefds,
29                   fd_set *exceptfds, const struct timespec *timeout,
30                   const sigset_t *sigmask);
31
32   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
33
34       pselect(): _POSIX_C_SOURCE >= 200112L || _XOPEN_SOURCE >= 600
35

DESCRIPTION

37       select() and  pselect()  allow  a  program  to  monitor  multiple  file
38       descriptors,  waiting  until one or more of the file descriptors become
39       "ready" for some class of I/O operation (e.g., input possible).  A file
40       descriptor  is considered ready if it is possible to perform the corre‐
41       sponding I/O operation (e.g., read(2)) without blocking.
42
43       The operation of select() and pselect() is identical, other than  these
44       three differences:
45
46       (i)    select()  uses  a timeout that is a struct timeval (with seconds
47              and microseconds), while pselect() uses a struct timespec  (with
48              seconds and nanoseconds).
49
50       (ii)   select()  may  update  the timeout argument to indicate how much
51              time was left.  pselect() does not change this argument.
52
53       (iii)  select() has no  sigmask  argument,  and  behaves  as  pselect()
54              called with NULL sigmask.
55
56       Three  independent  sets of file descriptors are watched.  Those listed
57       in readfds will be watched to see if characters  become  available  for
58       reading  (more  precisely, to see if a read will not block; in particu‐
59       lar, a file descriptor is also ready on end-of-file), those in writefds
60       will  be  watched  to  see  if  a  write  will  not block, and those in
61       exceptfds will be watched for exceptions.  On exit, the sets are  modi‐
62       fied  in place to indicate which file descriptors actually changed sta‐
63       tus.  Each of the three file descriptor sets may be specified  as  NULL
64       if no file descriptors are to be watched for the corresponding class of
65       events.
66
67       Four macros are provided to manipulate the sets.   FD_ZERO()  clears  a
68       set.   FD_SET()  and  FD_CLR() respectively add and remove a given file
69       descriptor from a set.  FD_ISSET() tests to see if a file descriptor is
70       part of the set; this is useful after select() returns.
71
72       nfds  is the highest-numbered file descriptor in any of the three sets,
73       plus 1.
74
75       The timeout argument  specifies  the  minimum  interval  that  select()
76       should  block  waiting  for  a  file descriptor to become ready.  (This
77       interval will be rounded up to the system clock granularity, and kernel
78       scheduling  delays  mean  that  the  blocking interval may overrun by a
79       small amount.)  If both fields of the timeval structure are zero,  then
80       select()  returns immediately.  (This is useful for polling.)  If time‐
81       out is NULL (no timeout), select() can block indefinitely.
82
83       sigmask is a pointer to a signal mask (see sigprocmask(2));  if  it  is
84       not  NULL, then pselect() first replaces the current signal mask by the
85       one pointed to by sigmask, then does the "select"  function,  and  then
86       restores the original signal mask.
87
88       Other than the difference in the precision of the timeout argument, the
89       following pselect() call:
90
91           ready = pselect(nfds, &readfds, &writefds, &exceptfds,
92                           timeout, &sigmask);
93
94       is equivalent to atomically executing the following calls:
95
96           sigset_t origmask;
97
98           pthread_sigmask(SIG_SETMASK, &sigmask, &origmask);
99           ready = select(nfds, &readfds, &writefds, &exceptfds, timeout);
100           pthread_sigmask(SIG_SETMASK, &origmask, NULL);
101
102       The reason that pselect() is needed is that if one wants  to  wait  for
103       either  a  signal  or  for  a  file descriptor to become ready, then an
104       atomic test is needed to prevent race conditions.  (Suppose the  signal
105       handler  sets  a  global  flag and returns.  Then a test of this global
106       flag followed by a call of select() could hang indefinitely if the sig‐
107       nal arrived just after the test but just before the call.  By contrast,
108       pselect() allows one to first block signals, handle  the  signals  that
109       have  come  in,  then call pselect() with the desired sigmask, avoiding
110       the race.)
111
112   The timeout
113       The time structures involved are defined in <sys/time.h> and look like
114
115           struct timeval {
116               long    tv_sec;         /* seconds */
117               long    tv_usec;        /* microseconds */
118           };
119
120       and
121
122           struct timespec {
123               long    tv_sec;         /* seconds */
124               long    tv_nsec;        /* nanoseconds */
125           };
126
127       (However, see below on the POSIX.1-2001 versions.)
128
129       Some code calls select() with all three sets empty, nfds  zero,  and  a
130       non-NULL  timeout as a fairly portable way to sleep with subsecond pre‐
131       cision.
132
133       On Linux, select() modifies timeout to reflect the amount of  time  not
134       slept;  most  other implementations do not do this.  (POSIX.1-2001 per‐
135       mits either behavior.)  This causes problems both when Linux code which
136       reads  timeout  is  ported to other operating systems, and when code is
137       ported to Linux that reuses a struct timeval for multiple select()s  in
138       a  loop  without  reinitializing  it.  Consider timeout to be undefined
139       after select() returns.
140

RETURN VALUE

142       On success, select() and pselect() return the number of  file  descrip‐
143       tors  contained  in  the  three  returned descriptor sets (that is, the
144       total number of bits that are  set  in  readfds,  writefds,  exceptfds)
145       which  may  be  zero if the timeout expires before anything interesting
146       happens.  On error, -1 is returned, and errno is set appropriately; the
147       sets  and  timeout  become  undefined, so do not rely on their contents
148       after an error.
149

ERRORS

151       EBADF  An invalid file descriptor was given in one of the sets.   (Per‐
152              haps  a file descriptor that was already closed, or one on which
153              an error has occurred.)
154
155       EINTR  A signal was caught; see signal(7).
156
157       EINVAL nfds is negative  or  the  value  contained  within  timeout  is
158              invalid.
159
160       ENOMEM unable to allocate memory for internal tables.
161

VERSIONS

163       pselect()  was  added  to  Linux in kernel 2.6.16.  Prior to this, pse‐
164       lect() was emulated in glibc (but see BUGS).
165

CONFORMING TO

167       select() conforms to POSIX.1-2001 and 4.4BSD (select()  first  appeared
168       in  4.2BSD).   Generally  portable  to/from  non-BSD systems supporting
169       clones of the BSD socket layer (including System V variants).  However,
170       note  that  the  System  V  variant typically sets the timeout variable
171       before exit, but the BSD variant does not.
172
173       pselect() is defined in POSIX.1g, and in POSIX.1-2001.
174

NOTES

176       An fd_set is a fixed size buffer.  Executing FD_CLR() or FD_SET()  with
177       a value of fd that is negative or is equal to or larger than FD_SETSIZE
178       will result in undefined behavior.  Moreover, POSIX requires fd to be a
179       valid file descriptor.
180
181       Concerning  the types involved, the classical situation is that the two
182       fields of a timeval structure are typed as long (as shown  above),  and
183       the  structure  is defined in <sys/time.h>.  The POSIX.1-2001 situation
184       is
185
186           struct timeval {
187               time_t         tv_sec;     /* seconds */
188               suseconds_t    tv_usec;    /* microseconds */
189           };
190
191       where the structure is defined in <sys/select.h>  and  the  data  types
192       time_t and suseconds_t are defined in <sys/types.h>.
193
194       Concerning  prototypes,  the  classical  situation  is  that one should
195       include <time.h> for select().  The POSIX.1-2001 situation is that  one
196       should include <sys/select.h> for select() and pselect().
197
198       Libc4  and  libc5  do not have a <sys/select.h> header; under glibc 2.0
199       and later this header exists.  Under glibc 2.0 it unconditionally gives
200       the  wrong  prototype for pselect().  Under glibc 2.1 to 2.2.1 it gives
201       pselect() when _GNU_SOURCE is defined.  Since glibc 2.2.2 the  require‐
202       ments are as shown in the SYNOPSIS.
203
204   Multithreaded applications
205       If  a  file descriptor being monitored by select() is closed in another
206       thread, the result is unspecified.   On  some  UNIX  systems,  select()
207       unblocks  and  returns,  with an indication that the file descriptor is
208       ready (a subsequent I/O operation  will  likely  fail  with  an  error,
209       unless  another  the file descriptor reopened between the time select()
210       returned and the I/O operations was performed).   On  Linux  (and  some
211       other  systems),  closing  the file descriptor in another thread has no
212       effect on select().  In summary, any application that relies on a  par‐
213       ticular behavior in this scenario must be considered buggy.
214
215   Linux notes
216       The pselect() interface described in this page is implemented by glibc.
217       The underlying Linux system call is named pselect6().  This system call
218       has somewhat different behavior from the glibc wrapper function.
219
220       The  Linux  pselect6() system call modifies its timeout argument.  How‐
221       ever, the glibc wrapper function hides this behavior by using  a  local
222       variable  for  the  timeout argument that is passed to the system call.
223       Thus, the glibc pselect() function does not modify  its  timeout  argu‐
224       ment; this is the behavior required by POSIX.1-2001.
225
226       The  final  argument  of the pselect6() system call is not a sigset_t *
227       pointer, but is instead a structure of the form:
228
229           struct {
230               const sigset_t *ss;     /* Pointer to signal set */
231               size_t          ss_len; /* Size (in bytes) of object pointed
232                                          to by 'ss' */
233           };
234
235       This allows the system call to obtain both a pointer to the signal  set
236       and  its size, while allowing for the fact that most architectures sup‐
237       port a maximum of 6 arguments to a system call.
238

BUGS

240       Glibc 2.0 provided a version of pselect() that did not take  a  sigmask
241       argument.
242
243       Starting  with  version  2.1,  glibc provided an emulation of pselect()
244       that was implemented using sigprocmask(2) and select().  This implemen‐
245       tation  remained  vulnerable  to the very race condition that pselect()
246       was designed to prevent.  Modern versions of glibc use the  (race-free)
247       pselect() system call on kernels where it is provided.
248
249       On  systems  that  lack  pselect(), reliable (and more portable) signal
250       trapping can be achieved using the self-pipe trick.  In this technique,
251       a  signal  handler writes a byte to a pipe whose other end is monitored
252       by select() in the main program.   (To  avoid  possibly  blocking  when
253       writing  to  a pipe that may be full or reading from a pipe that may be
254       empty, nonblocking I/O is used when reading from  and  writing  to  the
255       pipe.)
256
257       Under Linux, select() may report a socket file descriptor as "ready for
258       reading", while nevertheless a subsequent read blocks.  This could  for
259       example  happen  when  data  has arrived but upon examination has wrong
260       checksum and is discarded.  There may be other circumstances in which a
261       file  descriptor is spuriously reported as ready.  Thus it may be safer
262       to use O_NONBLOCK on sockets that should not block.
263
264       On Linux, select() also modifies timeout if the call is interrupted  by
265       a signal handler (i.e., the EINTR error return).  This is not permitted
266       by POSIX.1-2001.  The Linux pselect() system call has the  same  behav‐
267       ior,  but  the  glibc wrapper hides this behavior by internally copying
268       the timeout to a local variable and passing that variable to the system
269       call.
270

EXAMPLE

272       #include <stdio.h>
273       #include <stdlib.h>
274       #include <sys/time.h>
275       #include <sys/types.h>
276       #include <unistd.h>
277
278       int
279       main(void)
280       {
281           fd_set rfds;
282           struct timeval tv;
283           int retval;
284
285           /* Watch stdin (fd 0) to see when it has input. */
286           FD_ZERO(&rfds);
287           FD_SET(0, &rfds);
288
289           /* Wait up to five seconds. */
290           tv.tv_sec = 5;
291           tv.tv_usec = 0;
292
293           retval = select(1, &rfds, NULL, NULL, &tv);
294           /* Don't rely on the value of tv now! */
295
296           if (retval == -1)
297               perror("select()");
298           else if (retval)
299               printf("Data is available now.\n");
300               /* FD_ISSET(0, &rfds) will be true. */
301           else
302               printf("No data within five seconds.\n");
303
304           exit(EXIT_SUCCESS);
305       }
306

SEE ALSO

308       accept(2),  connect(2),  poll(2),  read(2),  recv(2), send(2), sigproc‐
309       mask(2), write(2), epoll(7), time(7)
310
311       For a tutorial with discussion and examples, see select_tut(2).
312

COLOPHON

314       This page is part of release 3.53 of the Linux  man-pages  project.   A
315       description  of  the project, and information about reporting bugs, can
316       be found at http://www.kernel.org/doc/man-pages/.
317
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320Linux                             2012-08-17                         SELECT(2)
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