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


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


10       /* According to POSIX.1-2001, POSIX.1-2008 */
11       #include <sys/select.h>
13       /* According to earlier standards */
14       #include <sys/time.h>
15       #include <sys/types.h>
16       #include <unistd.h>
18       int select(int nfds, fd_set *readfds, fd_set *writefds,
19                  fd_set *exceptfds, struct timeval *timeout);
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);
26       #include <sys/select.h>
28       int pselect(int nfds, fd_set *readfds, fd_set *writefds,
29                   fd_set *exceptfds, const struct timespec *timeout,
30                   const sigset_t *sigmask);
32   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
34       pselect(): _POSIX_C_SOURCE >= 200112L


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 a corre‐
41       sponding I/O operation (e.g., read(2) without  blocking,  or  a  suffi‐
42       ciently small write(2)).
44       select()  can  monitor only file descriptors numbers that are less than
45       FD_SETSIZE; poll(2) does not have this limitation.  See BUGS.
47       The operation of select() and pselect() is identical, other than  these
48       three differences:
50       (i)    select()  uses  a timeout that is a struct timeval (with seconds
51              and microseconds), while pselect() uses a struct timespec  (with
52              seconds and nanoseconds).
54       (ii)   select()  may  update  the timeout argument to indicate how much
55              time was left.  pselect() does not change this argument.
57       (iii)  select() has no  sigmask  argument,  and  behaves  as  pselect()
58              called with NULL sigmask.
60       Three  independent  sets  of  file  descriptors  are watched.  The file
61       descriptors listed in readfds will be  watched  to  see  if  characters
62       become available for reading (more precisely, to see if a read will not
63       block; in particular, a file descriptor is also ready on  end-of-file).
64       The  file  descriptors  in  writefds will be watched to see if space is
65       available for write (though a large write may still block).   The  file
66       descriptors  in  exceptfds  will be watched for exceptional conditions.
67       (For examples of some exceptional conditions,  see  the  discussion  of
68       POLLPRI in poll(2).)
70       On exit, each of the file descriptor sets is modified in place to indi‐
71       cate which file descriptors actually changed status.  (Thus,  if  using
72       select()  within  a  loop,  the  sets must be reinitialized before each
73       call.)
75       Each of the three file descriptor sets may be specified as NULL  if  no
76       file  descriptors  are  to  be  watched  for the corresponding class of
77       events.
79       Four macros are provided to manipulate the sets.   FD_ZERO()  clears  a
80       set.   FD_SET()  and  FD_CLR() respectively add and remove a given file
81       descriptor from a set.  FD_ISSET() tests to see if a file descriptor is
82       part of the set; this is useful after select() returns.
84       nfds  should  be  set to the highest-numbered file descriptor in any of
85       the three sets, plus 1.  The indicated file descriptors in each set are
86       checked, up to this limit (but see BUGS).
88       The  timeout argument specifies the interval that select() should block
89       waiting for a file descriptor to become ready.   The  call  will  block
90       until either:
92       *  a file descriptor becomes ready;
94       *  the call is interrupted by a signal handler; or
96       *  the timeout expires.
98       Note  that  the timeout interval will be rounded up to the system clock
99       granularity, and kernel scheduling delays mean that the blocking inter‐
100       val  may  overrun  by  a  small  amount.  If both fields of the timeval
101       structure are zero, then select() returns immediately.  (This is useful
102       for  polling.)   If  timeout  is  NULL (no timeout), select() can block
103       indefinitely.
105       sigmask is a pointer to a signal mask (see sigprocmask(2));  if  it  is
106       not  NULL, then pselect() first replaces the current signal mask by the
107       one pointed to by sigmask, then does the "select"  function,  and  then
108       restores the original signal mask.
110       Other than the difference in the precision of the timeout argument, the
111       following pselect() call:
113           ready = pselect(nfds, &readfds, &writefds, &exceptfds,
114                           timeout, &sigmask);
116       is equivalent to atomically executing the following calls:
118           sigset_t origmask;
120           pthread_sigmask(SIG_SETMASK, &sigmask, &origmask);
121           ready = select(nfds, &readfds, &writefds, &exceptfds, timeout);
122           pthread_sigmask(SIG_SETMASK, &origmask, NULL);
124       The reason that pselect() is needed is that if one wants  to  wait  for
125       either  a  signal  or  for  a  file descriptor to become ready, then an
126       atomic test is needed to prevent race conditions.  (Suppose the  signal
127       handler  sets  a  global  flag and returns.  Then a test of this global
128       flag followed by a call of select() could hang indefinitely if the sig‐
129       nal arrived just after the test but just before the call.  By contrast,
130       pselect() allows one to first block signals, handle  the  signals  that
131       have  come  in,  then call pselect() with the desired sigmask, avoiding
132       the race.)
134   The timeout
135       The time structures involved are defined in <sys/time.h> and look like
137           struct timeval {
138               long    tv_sec;         /* seconds */
139               long    tv_usec;        /* microseconds */
140           };
142       and
144           struct timespec {
145               long    tv_sec;         /* seconds */
146               long    tv_nsec;        /* nanoseconds */
147           };
149       (However, see below on the POSIX.1 versions.)
151       Some code calls select() with all three sets empty, nfds  zero,  and  a
152       non-NULL  timeout as a fairly portable way to sleep with subsecond pre‐
153       cision.
155       On Linux, select() modifies timeout to reflect the amount of  time  not
156       slept;  most  other  implementations  do not do this.  (POSIX.1 permits
157       either behavior.)  This causes problems  both  when  Linux  code  which
158       reads  timeout  is  ported to other operating systems, and when code is
159       ported to Linux that reuses a struct timeval for multiple select()s  in
160       a  loop  without  reinitializing  it.  Consider timeout to be undefined
161       after select() returns.


164       On success, select() and pselect() return the number of  file  descrip‐
165       tors  contained  in  the  three  returned descriptor sets (that is, the
166       total number of bits that are  set  in  readfds,  writefds,  exceptfds)
167       which  may  be  zero if the timeout expires before anything interesting
168       happens.  On error, -1 is returned, and errno is set  to  indicate  the
169       error;  the  file  descriptor  sets are unmodified, and timeout becomes
170       undefined.


173       EBADF  An invalid file descriptor was given in one of the sets.   (Per‐
174              haps  a file descriptor that was already closed, or one on which
175              an error has occurred.)  However, see BUGS.
177       EINTR  A signal was caught; see signal(7).
179       EINVAL nfds is negative or exceeds  the  RLIMIT_NOFILE  resource  limit
180              (see getrlimit(2)).
182       EINVAL The value contained within timeout is invalid.
184       ENOMEM Unable to allocate memory for internal tables.


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


191       select() conforms to POSIX.1-2001, POSIX.1-2008, and  4.4BSD  (select()
192       first  appeared in 4.2BSD).  Generally portable to/from non-BSD systems
193       supporting clones of the BSD socket  layer  (including  System V  vari‐
194       ants).   However,  note  that  the  System V variant typically sets the
195       timeout variable before exit, but the BSD variant does not.
197       pselect() is defined in POSIX.1g, and in POSIX.1-2001 and POSIX.1-2008.


200       An fd_set is a fixed size buffer.  Executing FD_CLR() or FD_SET()  with
201       a value of fd that is negative or is equal to or larger than FD_SETSIZE
202       will result in undefined behavior.  Moreover, POSIX requires fd to be a
203       valid file descriptor.
205       On  some other UNIX systems, select() can fail with the error EAGAIN if
206       the system fails to allocate  kernel-internal  resources,  rather  than
207       ENOMEM  as Linux does.  POSIX specifies this error for poll(2), but not
208       for select().  Portable programs may wish to check for EAGAIN and loop,
209       just as with EINTR.
211       On  systems  that  lack  pselect(), reliable (and more portable) signal
212       trapping can be achieved using the self-pipe trick.  In this technique,
213       a  signal  handler writes a byte to a pipe whose other end is monitored
214       by select() in the main program.   (To  avoid  possibly  blocking  when
215       writing  to  a pipe that may be full or reading from a pipe that may be
216       empty, nonblocking I/O is used when reading from  and  writing  to  the
217       pipe.)
219       Concerning  the types involved, the classical situation is that the two
220       fields of a timeval structure are typed as long (as shown  above),  and
221       the structure is defined in <sys/time.h>.  The POSIX.1 situation is
223           struct timeval {
224               time_t         tv_sec;     /* seconds */
225               suseconds_t    tv_usec;    /* microseconds */
226           };
228       where  the  structure  is  defined in <sys/select.h> and the data types
229       time_t and suseconds_t are defined in <sys/types.h>.
231       Concerning prototypes, the  classical  situation  is  that  one  should
232       include  <time.h>  for  select().   The  POSIX.1  situation is that one
233       should include <sys/select.h> for select() and pselect().
235       Under glibc 2.0, <sys/select.h> gives  the  wrong  prototype  for  pse‐
236       lect().   Under glibc 2.1 to 2.2.1, it gives pselect() when _GNU_SOURCE
237       is defined.  Since glibc 2.2.2, the requirements are as  shown  in  the
238       SYNOPSIS.
240   Correspondence between select() and poll() notifications
241       Within the Linux kernel source, we find the following definitions which
242       show the correspondence between the readable, writable, and exceptional
243       condition  notifications  of  select() and the event notifications pro‐
244       vided by poll(2) (and epoll(7)):
247                               POLLERR)
248                              /* Ready for reading */
250                              /* Ready for writing */
251           #define POLLEX_SET (POLLPRI)
252                              /* Exceptional condition */
254   Multithreaded applications
255       If a file descriptor being monitored by select() is closed  in  another
256       thread,  the  result  is  unspecified.   On some UNIX systems, select()
257       unblocks and returns, with an indication that the  file  descriptor  is
258       ready  (a  subsequent  I/O  operation  will  likely fail with an error,
259       unless another the file descriptor reopened between the  time  select()
260       returned  and  the  I/O  operations was performed).  On Linux (and some
261       other systems), closing the file descriptor in another  thread  has  no
262       effect  on select().  In summary, any application that relies on a par‐
263       ticular behavior in this scenario must be considered buggy.
265   C library/kernel differences
266       The Linux kernel allows file descriptor sets of arbitrary size,  deter‐
267       mining  the  length  of  the sets to be checked from the value of nfds.
268       However, in the glibc implementation, the fd_set type is fixed in size.
269       See also BUGS.
271       The pselect() interface described in this page is implemented by glibc.
272       The underlying Linux system call is named pselect6().  This system call
273       has somewhat different behavior from the glibc wrapper function.
275       The  Linux  pselect6() system call modifies its timeout argument.  How‐
276       ever, the glibc wrapper function hides this behavior by using  a  local
277       variable  for  the  timeout argument that is passed to the system call.
278       Thus, the glibc pselect() function does not modify  its  timeout  argu‐
279       ment; this is the behavior required by POSIX.1-2001.
281       The  final  argument  of the pselect6() system call is not a sigset_t *
282       pointer, but is instead a structure of the form:
284           struct {
285               const kernel_sigset_t *ss;   /* Pointer to signal set */
286               size_t ss_len;               /* Size (in bytes) of object
287                                               pointed to by 'ss' */
288           };
290       This allows the system call to obtain both a pointer to the signal  set
291       and  its size, while allowing for the fact that most architectures sup‐
292       port a maximum of 6 arguments to a system call.  See sigprocmask(2) for
293       a  discussion  of  the difference between the kernel and libc notion of
294       the signal set.


297       POSIX allows an implementation to define an upper limit, advertised via
298       the  constant  FD_SETSIZE, on the range of file descriptors that can be
299       specified in a file descriptor set.  The Linux kernel imposes no  fixed
300       limit,  but  the  glibc  implementation makes fd_set a fixed-size type,
301       with FD_SETSIZE defined  as  1024,  and  the  FD_*()  macros  operating
302       according  to  that  limit.   To  monitor file descriptors greater than
303       1023, use poll(2) instead.
305       According to POSIX, select() should check all specified  file  descrip‐
306       tors  in  the three file descriptor sets, up to the limit nfds-1.  How‐
307       ever, the current implementation ignores any file descriptor  in  these
308       sets  that  is greater than the maximum file descriptor number that the
309       process currently has open.  According to POSIX, any such file descrip‐
310       tor  that  is  specified  in one of the sets should result in the error
311       EBADF.
313       Glibc 2.0 provided a version of pselect() that did not take  a  sigmask
314       argument.
316       Starting  with  version  2.1,  glibc provided an emulation of pselect()
317       that was implemented using sigprocmask(2) and select().  This implemen‐
318       tation  remained  vulnerable  to the very race condition that pselect()
319       was designed to prevent.  Modern versions of glibc use the  (race-free)
320       pselect() system call on kernels where it is provided.
322       Under Linux, select() may report a socket file descriptor as "ready for
323       reading", while nevertheless a subsequent read blocks.  This could  for
324       example  happen  when  data  has arrived but upon examination has wrong
325       checksum and is discarded.  There may be other circumstances in which a
326       file  descriptor is spuriously reported as ready.  Thus it may be safer
327       to use O_NONBLOCK on sockets that should not block.
329       On Linux, select() also modifies timeout if the call is interrupted  by
330       a signal handler (i.e., the EINTR error return).  This is not permitted
331       by POSIX.1.  The Linux pselect() system call has the same behavior, but
332       the glibc wrapper hides this behavior by internally copying the timeout
333       to a local variable and passing that variable to the system call.


336       #include <stdio.h>
337       #include <stdlib.h>
338       #include <sys/time.h>
339       #include <sys/types.h>
340       #include <unistd.h>
342       int
343       main(void)
344       {
345           fd_set rfds;
346           struct timeval tv;
347           int retval;
349           /* Watch stdin (fd 0) to see when it has input. */
351           FD_ZERO(&rfds);
352           FD_SET(0, &rfds);
354           /* Wait up to five seconds. */
356           tv.tv_sec = 5;
357           tv.tv_usec = 0;
359           retval = select(1, &rfds, NULL, NULL, &tv);
360           /* Don't rely on the value of tv now! */
362           if (retval == -1)
363               perror("select()");
364           else if (retval)
365               printf("Data is available now.\n");
366               /* FD_ISSET(0, &rfds) will be true. */
367           else
368               printf("No data within five seconds.\n");
370           exit(EXIT_SUCCESS);
371       }


374       accept(2), connect(2), poll(2), read(2),  recv(2),  restart_syscall(2),
375       send(2), sigprocmask(2), write(2), epoll(7), time(7)
377       For a tutorial with discussion and examples, see select_tut(2).


380       This  page  is  part of release 4.15 of the Linux man-pages project.  A
381       description of the project, information about reporting bugs,  and  the
382       latest     version     of     this    page,    can    be    found    at
383       https://www.kernel.org/doc/man-pages/.
387Linux                             2017-09-15                         SELECT(2)