1select(2)                     System Calls Manual                    select(2)
2
3
4

NAME

6       select,  pselect,  FD_CLR, FD_ISSET, FD_SET, FD_ZERO, fd_set - synchro‐
7       nous I/O multiplexing
8

LIBRARY

10       Standard C library (libc, -lc)
11

SYNOPSIS

13       #include <sys/select.h>
14
15       typedef /* ... */ fd_set;
16
17       int select(int nfds, fd_set *_Nullable restrict readfds,
18                  fd_set *_Nullable restrict writefds,
19                  fd_set *_Nullable restrict exceptfds,
20                  struct timeval *_Nullable restrict timeout);
21
22       void FD_CLR(int fd, fd_set *set);
23       int  FD_ISSET(int fd, fd_set *set);
24       void FD_SET(int fd, fd_set *set);
25       void FD_ZERO(fd_set *set);
26
27       int pselect(int nfds, fd_set *_Nullable restrict readfds,
28                  fd_set *_Nullable restrict writefds,
29                  fd_set *_Nullable restrict exceptfds,
30                  const struct timespec *_Nullable restrict timeout,
31                  const sigset_t *_Nullable restrict sigmask);
32
33   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
34
35       pselect():
36           _POSIX_C_SOURCE >= 200112L
37

DESCRIPTION

39       WARNING: select() can monitor only file descriptors  numbers  that  are
40       less  than  FD_SETSIZE (1024)—an unreasonably low limit for many modern
41       applications—and this limitation will not change.  All modern  applica‐
42       tions  should instead use poll(2) or epoll(7), which do not suffer this
43       limitation.
44
45       select() allows a program to monitor multiple file descriptors, waiting
46       until one or more of the file descriptors become "ready" for some class
47       of I/O operation (e.g., input possible).  A file descriptor is  consid‐
48       ered  ready  if it is possible to perform a corresponding I/O operation
49       (e.g., read(2), or a sufficiently small write(2)) without blocking.
50
51   fd_set
52       A structure type that can represent a set of file descriptors.  Accord‐
53       ing  to  POSIX,  the  maximum  number  of file descriptors in an fd_set
54       structure is the value of the macro FD_SETSIZE.
55
56   File descriptor sets
57       The principal arguments of select() are three "sets" of  file  descrip‐
58       tors  (declared  with  the type fd_set), which allow the caller to wait
59       for three classes of events on the specified set of  file  descriptors.
60       Each  of  the  fd_set arguments may be specified as NULL if no file de‐
61       scriptors are to be watched for the corresponding class of events.
62
63       Note well: Upon return, each of the file descriptor sets is modified in
64       place  to indicate which file descriptors are currently "ready".  Thus,
65       if using select() within a loop, the sets must be reinitialized  before
66       each call.
67
68       The contents of a file descriptor set can be manipulated using the fol‐
69       lowing macros:
70
71       FD_ZERO()
72              This macro clears (removes all file descriptors from)  set.   It
73              should  be employed as the first step in initializing a file de‐
74              scriptor set.
75
76       FD_SET()
77              This macro adds the file descriptor fd to set.   Adding  a  file
78              descriptor  that  is  already present in the set is a no-op, and
79              does not produce an error.
80
81       FD_CLR()
82              This macro removes the file descriptor fd from set.  Removing  a
83              file  descriptor  that is not present in the set is a no-op, and
84              does not produce an error.
85
86       FD_ISSET()
87              select() modifies the contents of  the  sets  according  to  the
88              rules  described  below.  After calling select(), the FD_ISSET()
89              macro can be used to test if a file descriptor is still  present
90              in  a set.  FD_ISSET() returns nonzero if the file descriptor fd
91              is present in set, and zero if it is not.
92
93   Arguments
94       The arguments of select() are as follows:
95
96       readfds
97              The file descriptors in this set are watched to see if they  are
98              ready  for reading.  A file descriptor is ready for reading if a
99              read operation will not block; in particular, a file  descriptor
100              is also ready on end-of-file.
101
102              After select() has returned, readfds will be cleared of all file
103              descriptors except for those that are ready for reading.
104
105       writefds
106              The file descriptors in this set are watched to see if they  are
107              ready  for writing.  A file descriptor is ready for writing if a
108              write operation will not block.  However, even  if  a  file  de‐
109              scriptor indicates as writable, a large write may still block.
110
111              After  select()  has  returned,  writefds will be cleared of all
112              file descriptors except for those that are ready for writing.
113
114       exceptfds
115              The file descriptors in this set are  watched  for  "exceptional
116              conditions".   For  examples of some exceptional conditions, see
117              the discussion of POLLPRI in poll(2).
118
119              After select() has returned, exceptfds will be  cleared  of  all
120              file  descriptors except for those for which an exceptional con‐
121              dition has occurred.
122
123       nfds   This argument should be set to  the  highest-numbered  file  de‐
124              scriptor  in  any of the three sets, plus 1.  The indicated file
125              descriptors in each set are checked, up to this limit  (but  see
126              BUGS).
127
128       timeout
129              The  timeout  argument is a timeval structure (shown below) that
130              specifies the interval that select() should block waiting for  a
131              file  descriptor to become ready.  The call will block until ei‐
132              ther:
133
134              •  a file descriptor becomes ready;
135
136              •  the call is interrupted by a signal handler; or
137
138              •  the timeout expires.
139
140              Note that the timeout interval will be rounded up to the  system
141              clock  granularity,  and  kernel scheduling delays mean that the
142              blocking interval may overrun by a small amount.
143
144              If both fields of the timeval structure are zero, then  select()
145              returns immediately.  (This is useful for polling.)
146
147              If  timeout  is  specified as NULL, select() blocks indefinitely
148              waiting for a file descriptor to become ready.
149
150   pselect()
151       The pselect() system call allows an application to  safely  wait  until
152       either a file descriptor becomes ready or until a signal is caught.
153
154       The  operation of select() and pselect() is identical, other than these
155       three differences:
156
157select() uses a timeout that is a struct timeval (with  seconds  and
158          microseconds),  while pselect() uses a struct timespec (with seconds
159          and nanoseconds).
160
161select() may update the timeout argument to indicate how  much  time
162          was left.  pselect() does not change this argument.
163
164select()  has  no  sigmask argument, and behaves as pselect() called
165          with NULL sigmask.
166
167       sigmask is a pointer to a signal mask (see sigprocmask(2));  if  it  is
168       not  NULL, then pselect() first replaces the current signal mask by the
169       one pointed to by sigmask, then does the "select"  function,  and  then
170       restores  the  original  signal  mask.  (If sigmask is NULL, the signal
171       mask is not modified during the pselect() call.)
172
173       Other than the difference in the precision of the timeout argument, the
174       following pselect() call:
175
176           ready = pselect(nfds, &readfds, &writefds, &exceptfds,
177                           timeout, &sigmask);
178
179       is equivalent to atomically executing the following calls:
180
181           sigset_t origmask;
182
183           pthread_sigmask(SIG_SETMASK, &sigmask, &origmask);
184           ready = select(nfds, &readfds, &writefds, &exceptfds, timeout);
185           pthread_sigmask(SIG_SETMASK, &origmask, NULL);
186
187       The  reason  that  pselect() is needed is that if one wants to wait for
188       either a signal or for a file  descriptor  to  become  ready,  then  an
189       atomic  test is needed to prevent race conditions.  (Suppose the signal
190       handler sets a global flag and returns.  Then a  test  of  this  global
191       flag followed by a call of select() could hang indefinitely if the sig‐
192       nal arrived just after the test but just before the call.  By contrast,
193       pselect()  allows  one  to first block signals, handle the signals that
194       have come in, then call pselect() with the  desired  sigmask,  avoiding
195       the race.)
196
197   The timeout
198       The timeout argument for select() is a structure of the following type:
199
200           struct timeval {
201               time_t      tv_sec;         /* seconds */
202               suseconds_t tv_usec;        /* microseconds */
203           };
204
205       The corresponding argument for pselect() is a timespec(3) structure.
206
207       On  Linux,  select() modifies timeout to reflect the amount of time not
208       slept; most other implementations do not do this.  (POSIX.1 permits ei‐
209       ther  behavior.)  This causes problems both when Linux code which reads
210       timeout is ported to other operating systems, and when code  is  ported
211       to  Linux that reuses a struct timeval for multiple select()s in a loop
212       without reinitializing it.  Consider timeout to be undefined after  se‐
213       lect() returns.
214

RETURN VALUE

216       On  success,  select() and pselect() return the number of file descrip‐
217       tors contained in the three returned descriptor sets (that is, the  to‐
218       tal  number of bits that are set in readfds, writefds, exceptfds).  The
219       return value may be zero if the timeout expired  before  any  file  de‐
220       scriptors became ready.
221
222       On  error,  -1 is returned, and errno is set to indicate the error; the
223       file descriptor sets are unmodified, and timeout becomes undefined.
224

ERRORS

226       EBADF  An invalid file descriptor was given in one of the sets.   (Per‐
227              haps  a file descriptor that was already closed, or one on which
228              an error has occurred.)  However, see BUGS.
229
230       EINTR  A signal was caught; see signal(7).
231
232       EINVAL nfds is negative or exceeds  the  RLIMIT_NOFILE  resource  limit
233              (see getrlimit(2)).
234
235       EINVAL The value contained within timeout is invalid.
236
237       ENOMEM Unable to allocate memory for internal tables.
238

VERSIONS

240       On  some other UNIX systems, select() can fail with the error EAGAIN if
241       the system fails to allocate  kernel-internal  resources,  rather  than
242       ENOMEM  as Linux does.  POSIX specifies this error for poll(2), but not
243       for select().  Portable programs may wish to check for EAGAIN and loop,
244       just as with EINTR.
245

STANDARDS

247       POSIX.1-2008.
248

HISTORY

250       select()
251              POSIX.1-2001, 4.4BSD (first appeared in 4.2BSD).
252
253              Generally  portable to/from non-BSD systems supporting clones of
254              the BSD socket layer (including  System V  variants).   However,
255              note  that the System V variant typically sets the timeout vari‐
256              able before returning, but the BSD variant does not.
257
258       pselect()
259              Linux 2.6.16.  POSIX.1g, POSIX.1-2001.
260
261              Prior to this, it was emulated in glibc (but see BUGS).
262
263       fd_set POSIX.1-2001.
264

NOTES

266       The following header also provides the fd_set type: <sys/time.h>.
267
268       An fd_set is a fixed size buffer.  Executing FD_CLR() or FD_SET()  with
269       a value of fd that is negative or is equal to or larger than FD_SETSIZE
270       will result in undefined behavior.  Moreover, POSIX requires fd to be a
271       valid file descriptor.
272
273       The  operation  of select() and pselect() is not affected by the O_NON‐
274       BLOCK flag.
275
276   The self-pipe trick
277       On systems that lack pselect(), reliable  (and  more  portable)  signal
278       trapping can be achieved using the self-pipe trick.  In this technique,
279       a signal handler writes a byte to a pipe whose other end  is  monitored
280       by  select()  in  the  main  program.  (To avoid possibly blocking when
281       writing to a pipe that may be full or reading from a pipe that  may  be
282       empty,  nonblocking  I/O  is  used when reading from and writing to the
283       pipe.)
284
285   Emulating usleep(3)
286       Before the advent of usleep(3), some code employed a call  to  select()
287       with  all  three  sets  empty,  nfds  zero, and a non-NULL timeout as a
288       fairly portable way to sleep with subsecond precision.
289
290   Correspondence between select() and poll() notifications
291       Within the Linux kernel source, we find the following definitions which
292       show the correspondence between the readable, writable, and exceptional
293       condition notifications of select() and the  event  notifications  pro‐
294       vided by poll(2) and epoll(7):
295
296           #define POLLIN_SET  (EPOLLRDNORM | EPOLLRDBAND | EPOLLIN |
297                                EPOLLHUP | EPOLLERR)
298                              /* Ready for reading */
299           #define POLLOUT_SET (EPOLLWRBAND | EPOLLWRNORM | EPOLLOUT |
300                                EPOLLERR)
301                              /* Ready for writing */
302           #define POLLEX_SET  (EPOLLPRI)
303                              /* Exceptional condition */
304
305   Multithreaded applications
306       If  a  file descriptor being monitored by select() is closed in another
307       thread, the result is unspecified.  On some UNIX systems, select()  un‐
308       blocks  and  returns,  with  an  indication that the file descriptor is
309       ready (a subsequent I/O operation will likely fail with an  error,  un‐
310       less  another  process reopens the file descriptor between the time se‐
311       lect() returned and the I/O operation is  performed).   On  Linux  (and
312       some  other systems), closing the file descriptor in another thread has
313       no effect on select().  In summary, any application that  relies  on  a
314       particular behavior in this scenario must be considered buggy.
315
316   C library/kernel differences
317       The  Linux kernel allows file descriptor sets of arbitrary size, deter‐
318       mining the length of the sets to be checked from  the  value  of  nfds.
319       However, in the glibc implementation, the fd_set type is fixed in size.
320       See also BUGS.
321
322       The pselect() interface described in this page is implemented by glibc.
323       The underlying Linux system call is named pselect6().  This system call
324       has somewhat different behavior from the glibc wrapper function.
325
326       The Linux pselect6() system call modifies its timeout  argument.   How‐
327       ever,  the  glibc wrapper function hides this behavior by using a local
328       variable for the timeout argument that is passed to  the  system  call.
329       Thus,  the  glibc  pselect() function does not modify its timeout argu‐
330       ment; this is the behavior required by POSIX.1-2001.
331
332       The final argument of the pselect6() system call is  not  a  sigset_t *
333       pointer, but is instead a structure of the form:
334
335           struct {
336               const kernel_sigset_t *ss;   /* Pointer to signal set */
337               size_t ss_len;               /* Size (in bytes) of object
338                                               pointed to by 'ss' */
339           };
340
341       This  allows the system call to obtain both a pointer to the signal set
342       and its size, while allowing for the fact that most architectures  sup‐
343       port a maximum of 6 arguments to a system call.  See sigprocmask(2) for
344       a discussion of the difference between the kernel and  libc  notion  of
345       the signal set.
346
347   Historical glibc details
348       glibc  2.0 provided an incorrect version of pselect() that did not take
349       a sigmask argument.
350
351       From glibc 2.1 to glibc 2.2.1, one must define _GNU_SOURCE in order  to
352       obtain the declaration of pselect() from <sys/select.h>.
353

BUGS

355       POSIX allows an implementation to define an upper limit, advertised via
356       the constant FD_SETSIZE, on the range of file descriptors that  can  be
357       specified  in a file descriptor set.  The Linux kernel imposes no fixed
358       limit, but the glibc implementation makes  fd_set  a  fixed-size  type,
359       with  FD_SETSIZE  defined  as 1024, and the FD_*() macros operating ac‐
360       cording to that limit.  To monitor file descriptors greater than  1023,
361       use poll(2) or epoll(7) instead.
362
363       The implementation of the fd_set arguments as value-result arguments is
364       a design error that is avoided in poll(2) and epoll(7).
365
366       According to POSIX, select() should check all specified  file  descrip‐
367       tors  in  the three file descriptor sets, up to the limit nfds-1.  How‐
368       ever, the current implementation ignores any file descriptor  in  these
369       sets  that  is greater than the maximum file descriptor number that the
370       process currently has open.  According to POSIX, any such file descrip‐
371       tor  that  is  specified  in one of the sets should result in the error
372       EBADF.
373
374       Starting with glibc 2.1, glibc provided an emulation of pselect()  that
375       was implemented using sigprocmask(2) and select().  This implementation
376       remained vulnerable to the very race condition that pselect()  was  de‐
377       signed  to  prevent.  Modern versions of glibc use the (race-free) pse‐
378       lect() system call on kernels where it is provided.
379
380       On Linux, select() may report a socket file descriptor  as  "ready  for
381       reading",  while nevertheless a subsequent read blocks.  This could for
382       example happen when data has arrived but upon examination has the wrong
383       checksum and is discarded.  There may be other circumstances in which a
384       file descriptor is spuriously reported as ready.  Thus it may be  safer
385       to use O_NONBLOCK on sockets that should not block.
386
387       On  Linux, select() also modifies timeout if the call is interrupted by
388       a signal handler (i.e., the EINTR error return).  This is not permitted
389       by POSIX.1.  The Linux pselect() system call has the same behavior, but
390       the glibc wrapper hides this behavior by internally copying the timeout
391       to a local variable and passing that variable to the system call.
392

EXAMPLES

394       #include <stdio.h>
395       #include <stdlib.h>
396       #include <sys/select.h>
397
398       int
399       main(void)
400       {
401           int             retval;
402           fd_set          rfds;
403           struct timeval  tv;
404
405           /* Watch stdin (fd 0) to see when it has input. */
406
407           FD_ZERO(&rfds);
408           FD_SET(0, &rfds);
409
410           /* Wait up to five seconds. */
411
412           tv.tv_sec = 5;
413           tv.tv_usec = 0;
414
415           retval = select(1, &rfds, NULL, NULL, &tv);
416           /* Don't rely on the value of tv now! */
417
418           if (retval == -1)
419               perror("select()");
420           else if (retval)
421               printf("Data is available now.\n");
422               /* FD_ISSET(0, &rfds) will be true. */
423           else
424               printf("No data within five seconds.\n");
425
426           exit(EXIT_SUCCESS);
427       }
428

SEE ALSO

430       accept(2),  connect(2),  poll(2), read(2), recv(2), restart_syscall(2),
431       send(2), sigprocmask(2), write(2), timespec(3), epoll(7), time(7)
432
433       For a tutorial with discussion and examples, see select_tut(2).
434
435
436
437Linux man-pages 6.05              2023-05-03                         select(2)
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