select(2)select(2)Nameselect - synchronous I/O multiplexing
Syntax
#include <sys/types.h>
#include <sys/time.h>
int select (nfds, readfds, writefds, exceptfds, timeout)
int nfsd;
fd_set *readfds, *writefds, *exceptfds;
struct timeval *timeout;
FD_SET (fd, fdset)
FD_CLR (fd, fdset)
FD_ISSET (fd, fdset)
FD_ZERO (fdset)
int fd;
fd_set *fdset;
Description
The system call examines the I/O descriptor sets whose addresses are
passed in readfds, writefds, and exceptfds to see if some of their
descriptors are ready for reading, ready for writing, or have an excep‐
tion condition pending. The nfds parameter is the number of bits to be
checked in each bit mask that represent a file descriptor; the descrip‐
tors from 0 through nfds-1 in the descriptor sets are examined. Typi‐
cally nfds has the value returned by for the maximum number of file
descriptors. On return, replaces the given descriptor sets with subsets
consisting of those descriptors that are ready for the requested opera‐
tion. The total number of ready descriptors in all the sets is
returned.
The descriptor sets are stored as bit fields in arrays of integers. The
following macros are provided for manipulating such descriptor sets:
FD_ZERO(fdset) Initializes a descriptor set fdset to the
null set.
FD_SET(fd, fdset) Includes a particular descriptor fd in fdset.
FD_CLR(fd, fdset) Removes fd from fdset.
FD_ISSET(fd, fdset) Is nonzero if fd is a member of fdset; other‐
wise the value is set to zero.
The behavior of these macros is undefined if a descriptor value is less
than zero or greater than or equal to FD_SETSIZE, which is equal to the
maximum number of descriptors that can be supported by the system.
If timeout is not a NULL pointer, it specifies a maximum interval to
wait for the selection to complete. If timeout is a NULL pointer, the
call blocks indefinitely. To effect a poll, the timeout argument should
be a non-NULL pointer, pointing to a zero-valued timeval structure.
Any of readfds, writefds, and exceptfds may be given as NULL pointers
if no descriptors are of interest.
Selecting true for reading on a socket descriptor upon which a call has
been performed indicates that a subsequent call on that descriptor will
not block.
Restrictions
The call may indicate that a descriptor is ready for writing when in
fact an attempt to write would block. This can happen if system
resources necessary for a write are exhausted or otherwise unavailable.
If an application deems it critical that writes to a file descriptor
not block, it should set the descriptor for non-blocking I/O using the
F_SETFL request to the call.
Return Values
The call returns a non-negative value on success. A positive value
indicates the number of ready descriptors in the descriptor sets. A 0
indicates that the time limit referred to by timeout expired. On fail‐
ure, returns -1, sets errno to indicate the error, and does not change
the descriptor sets.
Diagnostics
[EBADF] One of the descriptor sets specified an invalid descriptor.
[EFAULT] One of the pointers given in the call referred to a non-
existent portion of the process's address space.
[EINTR] A signal was delivered before any of the selected events
occurred, or before the time limit expired.
[EINVAL] A component of the pointed-to time limit is outside the
acceptable range. The t_sec range must be between 0 and
10^8, inclusive; t_usec must be greater than or equal to 0,
and less than 10^6.
See Alsoaccept(2), connect(2), fcntl(2), gettimeofday(2), listen(2), read(2),
recv(2), send(2), write(2), getdtablesize(2)select(2)
