Standard C Library Functions select(3C)
NAME
select, pselect, FD_SET, FD_CLR, FD_ISSET, FD_ZERO - syn-
chronous I/O multiplexing
SYNOPSIS
#include
int select(int nfds,fd_set *restrict readfds, fd_set *restrict writefds,
fd_set *restrict errorfds,
struct timeval *restrict timeout);int pselect(int nfds,
fd_set *restrict readfds, fd_set *restrict writefds,
fd_set *restrict errorfds,
const struct timespec *restrict timeout,const sigset_t *restrict sigmask);
void FD_SET(int fd, fd_set *fdset);
void FD_CLR(int fd, fd_set *fdset);
int FD_ISSET(int fd, fd_set *fdset);
void FD_ZERO(fd_set *fdset);
DESCRIPTION
The pselect() function examines the file descriptor sets
whose addresses are passed in the readfds, writefds, and errorfds parameters to see if some of their descriptors areready for reading, are ready for writing, or have an excep-
tional condition pending, respectively.The select() function is equivalent to the pselect() func-
tion, except as follows: o For the select() function, the timeout period is given in seconds and microseconds in an argument oftype struct timeval, whereas for the pselect()
function the timeout period is given in seconds and nanoseconds in an argument of type struct timespec. o The select() function has no sigmask argument. Itbehaves as pselect() does when sigmask is a null
pointer.SunOS 5.11 Last change: 1 Nov 2004 1
Standard C Library Functions select(3C) o Upon successful completion, the select() function might modify the object pointed to by the timeout argument.The select() and pselect() functions support regular files,
terminal and pseudo-terminal devices, STREAMS-based files,
FIFOs, pipes, and sockets. The behavior of select() andpselect() on file descriptors that refer to other types of
file is unspecified. The nfds argument specifies the range of file descriptors to be tested. The first nfds descriptors are checked in eachset; that is, the descriptors from zero through nfds-1 in
the descriptor sets are examined. If the readfs argument is not a null pointer, it points toan object of type fd_set that on input specifies the file
descriptors to be checked for being ready to read, and on output indicates which file descriptors are ready to read. If the writefs argument is not a null pointer, it points toan object of type fd_set that on input specifies the file
descriptors to be checked for being ready to write, and on output indicates which file descriptors are ready to write. If the errorfds argument is not a null pointer, it points toan object of type fd_set that on input specifies the file
descriptors to be checked for error conditions pending, andon output indicates which file descriptors have error condi-
tions pending. Upon successful completion, the objects pointed to by the readfs, writefs, and errorfds arguments are modified to indicate which file descriptors are ready for reading, readyfor writing, or have an error condition pending, respec-
tively, and return the total number of ready descriptors in all the output sets. For each file descriptor less thannfds, the corresponding bit will be set on successful com-
pletion if it was set on input and the associated condition is true for that file descriptor. If none of the selected descriptors are ready for therequested operation, the select() or pselect() function
blocks until at least one of the requested operations becomes ready, until the timeout occurs, or untilSunOS 5.11 Last change: 1 Nov 2004 2
Standard C Library Functions select(3C) interrupted by a signal. The timeout parameter controls howlong the select() or pselect() function takes before timing
out. If the timeout parameter is not a null pointer, it specifies a maximum interval to wait for the selection to complete. If the specified time interval expires without any requested operation becoming ready, the function returns. If the timeout parameter is a null pointer, then the call toselect() or pselect() blocks indefinitely until at least one
descriptor meets the specified criteria. To effect a poll, the timeout parameter should not be a null pointer, andshould point to a zero-valued timespec structure.
The use of a timeout does not affect any pending timers set up by alarm(2), ualarm(3C), or setitimer(2).If sigmask is not a null pointer, then the pselect() func-
tion replaces the signal mask of the process by the set ofsignals pointed to by sigmask before examining the descrip-
tors, and restores the signal mask of the process before returning. A descriptor is considered ready for reading when a call toan input function with O_NONBLOCK clear would not block,
whether or not the function would transfer data success-
fully. (The function might return data, an end-of-file indi-
cation, or an error other than one indicating that it is blocked, and in each of these cases the descriptor will be considered ready for reading.) A descriptor is considered ready for writing when a call toan output function with O_NONBLOCK clear would not block,
whether or not the function would transfer data success-
fully. If a socket has a pending error, it is considered to have an exceptional condition pending. Otherwise, what constitutesan exceptional condition is file type-specific. For a file
descriptor for use with a socket, it is protocol-specific
except as noted below. For other file types, if the opera-
tion is meaningless for a particular file type, select() orpselect() indicates that the descriptor is ready for read or
write operations and indicates that the descriptor has no exceptional condition pending.If a descriptor refers to a socket, the implied input func-
tion is the recvmsg(3XNET) function with parametersSunOS 5.11 Last change: 1 Nov 2004 3
Standard C Library Functions select(3C) requesting normal and ancillary data, such that the presence of either type causes the socket to be marked as readable.The presence of out-of-band data is checked if the socket
option SO_OOBINLINE has been enabled, as out-of-band data is
enqueued with normal data. If the socket is currentlylistening, then it is marked as readable if an incoming con-
nection request has been received, and a call to the accept function completes without blocking.If a descriptor refers to a socket, the implied output func-
tion is the sendmsg(3XNET) function supplying an amount ofnormal data equal to the current value of the SO_SNDLOWAT
option for the socket. If a non-blocking call to the connect
function has been made for a socket, and the connection attempt has either succeeded or failed leaving a pending error, the socket is marked as writable. A socket is considered to have an exceptional conditionpending if a receive operation with O_NONBLOCK clear for the
open file description and with the MSG_OOB flag set would
return out-of-band data without blocking. (It is protocol-
specific whether the MSG_OOB flag would be used to read
out-of-band data.) A socket will also be considered to have
an exceptional condition pending if an out-of-band data mark
is present in the receive queue.A file descriptor for a socket that is listening for connec-
tions will indicate that it is ready for reading, when con-
nections are available. A file descriptor for a socket that is connecting asynchronously will indicate that it is ready for writing, when a connection has been established. Selecting true for reading on a socket descriptor upon which a listen(3XNET) call has been performed indicates that a subsequent accept(3XNET) call on that descriptor will not block. If the timeout argument is not a null pointer, it points to an object of type struct timeval that specifies a maximum interval to wait for the selection to complete. If the timeout argument points to an object of type struct timeval whose members are 0, select() does not block. If the timeout argument is a null pointer, select() blocks until an eventcauses one of the masks to be returned with a valid (non-
zero) value. If the time limit expires before any event occurs that would cause one of the masks to be set to anon-zero value, select() completes successfully and returns
SunOS 5.11 Last change: 1 Nov 2004 4
Standard C Library Functions select(3C) 0. If the readfs, writefs, and errorfds arguments are all null pointers and the timeout argument is not a null pointer,select() or pselect() blocks for the time specified, or
until interrupted by a signal. If the readfs, writefs, and errorfds arguments are all null pointers and the timeoutargument is a null pointer, select() blocks until inter-
rupted by a signal. File descriptors associated with regular files always selecttrue for ready to read, ready to write, and error condi-
tions. On failure, the objects pointed to by the readfs, writefs, and errorfds arguments are not modified. If the timeout interval expires without the specified condition being true for any of the specified file descriptors, the objects pointed to by the readfs, writefs, and errorfds arguments have all bits set to 0.File descriptor masks of type fd_set can be initialized and
tested with the macros FD_CLR(), FD_ISSET(), FD_SET(), and
FD_ZERO().
FD_CLR(fd, &fdset) Clears the bit for the file descrip-
tor fd in the file descriptor set fdset.FD_ISSET(fd, &fdset) Returns a non-zero value if the bit
for the file descriptor fd is set in the file descriptor set pointed to by fdset, and 0 otherwise.FD_SET(fd, &fdset) Sets the bit for the file descriptor
fd in the file descriptor set fdset.FD_ZERO(&fdset) Initializes the file descriptor set
fdset to have zero bits for all file descriptors. The behavior of these macros is undefined if the fd argumentis less than 0 or greater than or equal to FD_SETSIZE, or if
SunOS 5.11 Last change: 1 Nov 2004 5
Standard C Library Functions select(3C)fd is not a valid file descriptor, or if any of the argu-
ments are expressions with side effects.RETURN VALUES
On successful completion, select() and pselect() return the
total number of bits set in the bit masks. Otherwise, -1 is
returned and errno is set to indicate the error.The FD_CLR(), FD_SET(), and FD_ZERO() macros return no
value. The FD_ISSET() macro returns a non-zero value if the
bit for the file descriptor fd is set in the file descriptor set pointed to by fdset, and 0 otherwise.ERRORS
The select() and pselect() functions will fail if:
EBADF One or more of the file descriptor sets specified a file descriptor that is not a valid open file descriptor. EINTR The function was interrupted before any of the selected events occurred and before the timeout interval expired.If SA_RESTART has been set for the interrupting
signal, it is implementation-dependent whether
select() restarts or returns with EINTR. EINVAL An invalid timeout interval was specified. EINVAL The nfds argument is less than 0 or greater thanFD_SETSIZE.
EINVAL One of the specified file descriptors refers to a STREAM or multiplexer that is linked (directly or indirectly) downstream from a multiplexer.EINVAL A component of the pointed-to time limit is out-
side the acceptable range: t_sec must be between 0
and 10^8, inclusive. t_usec must be greater than
or equal to 0, and less than 10^6.USAGE
The poll(2) function is preferred over this function. It must be used when the number of file descriptors exceedsSunOS 5.11 Last change: 1 Nov 2004 6
Standard C Library Functions select(3C)FD_SETSIZE.
The use of a timeout does not affect any pending timers set up by alarm(2), ualarm(3C) or setitimer(2). On successful completion, the object pointed to by the timeout argument may be modified.ATTRIBUTES
See attributes(5) for descriptions of the following attri-
butes:____________________________________________________________
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
|_____________________________|_____________________________|
| Interface Stability | Committed ||_____________________________|_____________________________|
| MT-Level | MT-Safe |
|_____________________________|_____________________________|
| Standard | See standards(5). ||_____________________________|_____________________________|
SEE ALSO
alarm(2), fcntl(2), poll(2), read(2), setitimer(2), write(2), accept(3SOCKET), listen(3SOCKET), ualarm(3C), attributes(5), standards(5) NOTESThe default value for FD_SETSIZE (currently 1024) is larger
than the default limit on the number of open files. Toaccommodate 32-bit applications that wish to use a larger
number of open files with select(), it is possible to increase this size at compile time by providing a largerdefinition of FD_SETSIZE before the inclusion of any
system-supplied header. The maximum supported size for
FD_SETSIZE is 65536. The default value is already 65536 for
64-bit applications.
SunOS 5.11 Last change: 1 Nov 2004 7