# select

## NAME



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


## LIBRARY



           Standard C library (_libc_ , _-lc_)
    


## SYNOPSIS



           **#include <sys/select.h>**
    
           **typedef** /* ... */ **fd_set;**
    
           **int select(int** _nfds_**, fd_set *_Nullable restrict** _readfds_**,**
                      **fd_set *_Nullable restrict** _writefds_**,**
                      **fd_set *_Nullable restrict** _exceptfds_**,**
                      **struct timeval *_Nullable restrict** _timeout_**);**
    
           **void FD_CLR(int** _fd_**, fd_set ***_set_**);**
           **int  FD_ISSET(int** _fd_**, fd_set ***_set_**);**
           **void FD_SET(int** _fd_**, fd_set ***_set_**);**
           **void FD_ZERO(fd_set ***_set_**);**
    
           **int pselect(int** _nfds_**, fd_set *_Nullable restrict** _readfds_**,**
                      **fd_set *_Nullable restrict** _writefds_**,**
                      **fd_set *_Nullable restrict** _exceptfds_**,**
                      **const struct timespec *_Nullable restrict** _timeout_**,**
                      **const sigset_t *_Nullable restrict** _sigmask_**);**
    
       Feature Test Macro Requirements for glibc (see
       feature_test_macros(7)):
    
           **pselect**():
               _POSIX_C_SOURCE >= 200112L
    


## DESCRIPTION



           **WARNING** : **select**() can monitor only file descriptors numbers that
           are less than **FD_SETSIZE**(1024)—an unreasonably low limit for many
           modern applications—and this limitation will not change.  All
           modern applications should instead use poll(2) or epoll(7), which
           do not suffer this limitation.
    
           **select**() allows a program to monitor multiple file descriptors,
           waiting until one or more of the file descriptors become "ready"
           for some class of I/O operation (e.g., input possible).  A file
           descriptor is considered ready if it is possible to perform a
           corresponding I/O operation (e.g., read(2), or a sufficiently
           small write(2)) without blocking.
    
       **fd_set**
           A structure type that can represent a set of file descriptors.
           According to POSIX, the maximum number of file descriptors in an
           _fd_set_ structure is the value of the macro **FD_SETSIZE**.
    
       **File descriptor sets**
           The principal arguments of **select**() are three "sets" of file
           descriptors (declared with the type _fd_set_), which allow the
           caller to wait for three classes of events on the specified set of
           file descriptors.  Each of the _fd_set_ arguments may be specified
           as NULL if no file descriptors are to be watched for the
           corresponding class of events.
    
           **Note well** : Upon return, each of the file descriptor sets is
           modified in place to indicate which file descriptors are currently
           "ready".  Thus, if using **select**() within a loop, the sets _must be_
           _reinitialized_ before each call.
    
           The contents of a file descriptor set can be manipulated using the
           following macros:
    
           **FD_ZERO**()
                  This macro clears (removes all file descriptors from) _set_.
                  It should be employed as the first step in initializing a
                  file descriptor set.
    
           **FD_SET**()
                  This macro adds the file descriptor _fd_ to _set_.  Adding a
                  file descriptor that is already present in the set is a no-
                  op, and does not produce an error.
    
           **FD_CLR**()
                  This macro removes the file descriptor _fd_ from _set_.
                  Removing a file descriptor that is not present in the set
                  is a no-op, and does not produce an error.
    
           **FD_ISSET**()
                  **select**() modifies the contents of the sets according to the
                  rules described below.  After calling **select**(), the
                  **FD_ISSET**() macro can be used to test if a file descriptor
                  is still present in a set.  **FD_ISSET**() returns nonzero if
                  the file descriptor _fd_ is present in _set_ , and zero if it is
                  not.
    
       **Arguments**
           The arguments of **select**() are as follows:
    
           _readfds_
                  The file descriptors in this set are watched to see if they
                  are ready for reading.  A file descriptor is ready for
                  reading if a read operation will not block; in particular,
                  a file descriptor is also ready on end-of-file.
    
                  After **select**() has returned, _readfds_ will be cleared of all
                  file descriptors except for those that are ready for
                  reading.
    
           _writefds_
                  The file descriptors in this set are watched to see if they
                  are ready for writing.  A file descriptor is ready for
                  writing if a write operation will not block.  However, even
                  if a file descriptor indicates as writable, a large write
                  may still block.
    
                  After **select**() has returned, _writefds_ will be cleared of
                  all file descriptors except for those that are ready for
                  writing.
    
           _exceptfds_
                  The file descriptors in this set are watched for
                  "exceptional conditions".  For examples of some exceptional
                  conditions, see the discussion of **POLLPRI** in poll(2).
    
                  After **select**() has returned, _exceptfds_ will be cleared of
                  all file descriptors except for those for which an
                  exceptional condition has occurred.
    
           _nfds_   This argument should be set to the highest-numbered file
                  descriptor in any of the three sets, plus 1.  The indicated
                  file descriptors in each set are checked, up to this limit
                  (but see BUGS).
    
           _timeout_
                  The _timeout_ argument is a _timeval_ structure (shown below)
                  that specifies the interval that **select**() should block
                  waiting for a file descriptor to become ready.  The call
                  will block until either:
    
                  •  a file descriptor becomes ready;
    
                  •  the call is interrupted by a signal handler; or
    
                  •  the timeout expires.
    
                  Note that the _timeout_ interval will be rounded up to the
                  system clock granularity, and kernel scheduling delays mean
                  that the blocking interval may overrun by a small amount.
    
                  If both fields of the _timeval_ structure are zero, then
                  **select**() returns immediately.  (This is useful for
                  polling.)
    
                  If _timeout_ is specified as NULL, **select**() blocks
                  indefinitely waiting for a file descriptor to become ready.
    
       **pselect()**
           The **pselect**() system call allows an application to safely wait
           until either a file descriptor becomes ready or until a signal is
           caught.
    
           The operation of **select**() and **pselect**() is identical, other than
           these three differences:
    
           •  **select**() uses a timeout that is a _struct timeval_ (with seconds
              and microseconds), while **pselect**() uses a _struct timespec_ (with
              seconds and nanoseconds).
    
           •  **select**() may update the _timeout_ argument to indicate how much
              time was left.  **pselect**() does not change this argument.
    
           •  **select**() has no _sigmask_ argument, and behaves as **pselect**()
              called with NULL _sigmask_.
    
           _sigmask_ is a pointer to a signal mask (see sigprocmask(2)); if it
           is not NULL, then **pselect**() first replaces the current signal mask
           by the one pointed to by _sigmask_ , then does the "select" function,
           and then restores the original signal mask.  (If _sigmask_ is NULL,
           the signal mask is not modified during the **pselect**() call.)
    
           Other than the difference in the precision of the _timeout_
           argument, the following **pselect**() call:
    
               ready = pselect(nfds, &readfds, &writefds, &exceptfds,
                               timeout, &sigmask);
    
           is equivalent to _atomically_ executing the following calls:
    
               sigset_t origmask;
    
               pthread_sigmask(SIG_SETMASK, &sigmask, &origmask);
               ready = select(nfds, &readfds, &writefds, &exceptfds, timeout);
               pthread_sigmask(SIG_SETMASK, &origmask, NULL);
    
           The reason that **pselect**() is needed is that if one wants to wait
           for either a signal or for a file descriptor to become ready, then
           an atomic test is needed to prevent race conditions.  (Suppose the
           signal handler sets a global flag and returns.  Then a test of
           this global flag followed by a call of **select**() could hang
           indefinitely if the signal arrived just after the test but just
           before the call.  By contrast, **pselect**() allows one to first block
           signals, handle the signals that have come in, then call **pselect**()
           with the desired _sigmask_ , avoiding the race.)
    
       **The timeout**
           The _timeout_ argument for **select**() is a structure of the following
           type:
    
               struct timeval {
                   time_t      tv_sec;         /* seconds */
                   suseconds_t tv_usec;        /* microseconds */
               };
    
           The corresponding argument for **pselect**() is a **timespec**(3)
           structure.
    
           On Linux, **select**() modifies _timeout_ to reflect the amount of time
           not slept; most other implementations do not do this.  (POSIX.1
           permits either behavior.)  This causes problems both when Linux
           code which reads _timeout_ is ported to other operating systems, and
           when code is ported to Linux that reuses a _struct timeval_ for
           multiple **select**()s in a loop without reinitializing it.  Consider
           _timeout_ to be undefined after **select**() returns.
    


## RETURN VALUE



           On success, **select**() and **pselect**() return the number of file
           descriptors contained in the three returned descriptor sets (that
           is, the total number of bits that are set in _readfds_ , _writefds_ ,
           _exceptfds_).  The return value may be zero if the timeout expired
           before any file descriptors became ready.
    
           On error, -1 is returned, and _errno_ is set to indicate the error;
           the file descriptor sets are unmodified, and _timeout_ becomes
           undefined.
    


## ERRORS



           **EBADF** An invalid file descriptor was given in one of the sets.
                  (Perhaps a file descriptor that was already closed, or one
                  on which an error has occurred.)  However, see BUGS.
    
           **EINTR** A signal was caught; see signal(7).
    
           **EINVAL** _nfds_ is negative or exceeds the **RLIMIT_NOFILE** resource
                  limit (see getrlimit(2)).
    
           **EINVAL** The value contained within _timeout_ is invalid.
    
           **ENOMEM** Unable to allocate memory for internal tables.
    


## VERSIONS



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


## STANDARDS



           POSIX.1-2008.
    


## NOTES



           The following header also provides the _fd_set_ type: _< sys/time.h>_.
    
           An _fd_set_ is a fixed size buffer.  Executing **FD_CLR**() or **FD_SET**()
           with a value of _fd_ that is negative or is equal to or larger than
           **FD_SETSIZE** will result in undefined behavior.  Moreover, POSIX
           requires _fd_ to be a valid file descriptor.
    
           The operation of **select**() and **pselect**() is not affected by the
           **O_NONBLOCK** flag.
    
       **The self-pipe trick**
           On systems that lack **pselect**(), reliable (and more portable)
           signal trapping can be achieved using the self-pipe trick.  In
           this technique, a signal handler writes a byte to a pipe whose
           other end is monitored by **select**() in the main program.  (To avoid
           possibly blocking when writing to a pipe that may be full or
           reading from a pipe that may be empty, nonblocking I/O is used
           when reading from and writing to the pipe.)
    
       **Emulating usleep(3)**
           Before the advent of usleep(3), some code employed a call to
           **select**() with all three sets empty, _nfds_ zero, and a non-NULL
           _timeout_ as a fairly portable way to sleep with subsecond
           precision.
    
       **Correspondence between select() and poll() notifications**
           Within the Linux kernel source, we find the following definitions
           which show the correspondence between the readable, writable, and
           exceptional condition notifications of **select**() and the event
           notifications provided by poll(2) and epoll(7):
    
               #define POLLIN_SET  (EPOLLRDNORM | EPOLLRDBAND | EPOLLIN |
                                    EPOLLHUP | EPOLLERR)
                                  /* Ready for reading */
               #define POLLOUT_SET (EPOLLWRBAND | EPOLLWRNORM | EPOLLOUT |
                                    EPOLLERR)
                                  /* Ready for writing */
               #define POLLEX_SET  (EPOLLPRI)
                                  /* Exceptional condition */
    
       **Multithreaded applications**
           If a file descriptor being monitored by **select**() is closed in
           another thread, the result is unspecified.  On some UNIX systems,
           **select**() unblocks and returns, with an indication that the file
           descriptor is ready (a subsequent I/O operation will likely fail
           with an error, unless another process reopens the file descriptor
           between the time **select**() returned and the I/O operation is
           performed).  On Linux (and some other systems), closing the file
           descriptor in another thread has no effect on **select**().  In
           summary, any application that relies on a particular behavior in
           this scenario must be considered buggy.
    
       **C library/kernel differences**
           The Linux kernel allows file descriptor sets of arbitrary size,
           determining the length of the sets to be checked from the value of
           _nfds_.  However, in the glibc implementation, the _fd_set_ type is
           fixed in size.  See also BUGS.
    
           The **pselect**() interface described in this page is implemented by
           glibc.  The underlying Linux system call is named **pselect6**().
           This system call has somewhat different behavior from the glibc
           wrapper function.
    
           The Linux **pselect6**() system call modifies its _timeout_ argument.
           However, the glibc wrapper function hides this behavior by using a
           local variable for the timeout argument that is passed to the
           system call.  Thus, the glibc **pselect**() function does not modify
           its _timeout_ argument; this is the behavior required by
           POSIX.1-2001.
    
           The final argument of the **pselect6**() system call is not a
           _sigset_t *_ pointer, but is instead a structure of the form:
    
               struct {
                   const kernel_sigset_t *ss;   /* Pointer to signal set */
                   size_t ss_len;               /* Size (in bytes) of object
                                                   pointed to by 'ss' */
               };
    
           This allows the system call to obtain both a pointer to the signal
           set and its size, while allowing for the fact that most
           architectures support a maximum of 6 arguments to a system call.
           See sigprocmask(2) for a discussion of the difference between the
           kernel and libc notion of the signal set.
    
       **Historical glibc details**
           glibc 2.0 provided an incorrect version of **pselect**() that did not
           take a _sigmask_ argument.
    
           From glibc 2.1 to glibc 2.2.1, one must define **_GNU_SOURCE** in
           order to obtain the declaration of **pselect**() from _< sys/select.h>_.
    


## BUGS



           POSIX allows an implementation to define an upper limit,
           advertised via the constant **FD_SETSIZE** , on the range of file
           descriptors that can be specified in a file descriptor set.  The
           Linux kernel imposes no fixed limit, but the glibc implementation
           makes _fd_set_ a fixed-size type, with **FD_SETSIZE** defined as 1024,
           and the **FD_***() macros operating according to that limit.  To
           monitor file descriptors greater than 1023, use poll(2) or
           epoll(7) instead.
    
           The implementation of the _fd_set_ arguments as value-result
           arguments is a design error that is avoided in poll(2) and
           epoll(7).
    
           According to POSIX, **select**() should check all specified file
           descriptors in the three file descriptor sets, up to the limit
           _nfds-1_.  However, the current implementation ignores any file
           descriptor in these sets that is greater than the maximum file
           descriptor number that the process currently has open.  According
           to POSIX, any such file descriptor that is specified in one of the
           sets should result in the error **EBADF**.
    
           Starting with glibc 2.1, glibc provided an emulation of **pselect**()
           that was implemented using sigprocmask(2) and **select**().  This
           implementation remained vulnerable to the very race condition that
           **pselect**() was designed to prevent.  Modern versions of glibc use
           the (race-free) **pselect**() system call on kernels where it is
           provided.
    
           On Linux, **select**() may report a socket file descriptor as "ready
           for reading", while nevertheless a subsequent read blocks.  This
           could for example happen when data has arrived but upon
           examination has the wrong checksum and is discarded.  There may be
           other circumstances in which a file descriptor is spuriously
           reported as ready.  Thus it may be safer to use **O_NONBLOCK** on
           sockets that should not block.
    
           On Linux, **select**() also modifies _timeout_ if the call is
           interrupted by a signal handler (i.e., the **EINTR** error return).
           This is not permitted by POSIX.1.  The Linux **pselect**() system call
           has the same behavior, but the glibc wrapper hides this behavior
           by internally copying the _timeout_ to a local variable and passing
           that variable to the system call.
    


## EXAMPLES



           #include <stdio.h>
           #include <stdlib.h>
           #include <sys/select.h>
           #include <sys/time.h>
    
           int
           main(void)
           {
               int             retval;
               fd_set          rfds;
               struct timeval  tv;
    
               /* Watch stdin (fd 0) to see when it has input. */
    
               FD_ZERO(&rfds);
               FD_SET(0, &rfds);
    
               /* Wait up to five seconds. */
    
               tv.tv_sec = 5;
               tv.tv_usec = 0;
    
               retval = select(1, &rfds, NULL, NULL, &tv);
               /* Don't rely on the value of tv now! */
    
               if (retval == -1)
                   perror("select()");
               else if (retval)
                   printf("Data is available now.\n");
                   /* FD_ISSET(0, &rfds) will be true. */
               else
                   printf("No data within five seconds.\n");
    
               exit(EXIT_SUCCESS);
           }
    


* * *