man pages section 3: Basic Library Functions

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Updated: July 2014
 
 

FD_SET(3C)

Name

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

Synopsis

#include <sys/time.h>

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 are ready for reading, are ready for writing, or have an exceptional condition pending, respectively.

The select() function is equivalent to the pselect() function, except as follows:

  • For the select() function, the timeout period is given in seconds and microseconds in an argument of type struct timeval, whereas for the pselect() function the timeout period is given in seconds and nanoseconds in an argument of type struct timespec.

  • The select() function has no sigmask argument. It behaves as pselect() does when sigmask is a null pointer.

  • 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() and pselect() 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 each set; 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 to an 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 to an 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 to an object of type fd_set that on input specifies the file descriptors to be checked for error conditions pending, and on output indicates which file descriptors have error conditions 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, ready for writing, or have an error condition pending, respectively, and return the total number of ready descriptors in all the output sets. For each file descriptor less than nfds, the corresponding bit will be set on successful completion 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 the requested operation, the select() or pselect() function blocks until at least one of the requested operations becomes ready, until the timeout occurs, or until interrupted by a signal. The timeout parameter controls how long 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 to select() 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, and should 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() function replaces the signal mask of the process by the set of signals pointed to by sigmask before examining the descriptors, and restores the signal mask of the process before returning.

A descriptor is considered ready for reading when a call to an input function with O_NONBLOCK clear would not block, whether or not the function would transfer data successfully. (The function might return data, an end-of-file indication, 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 to an output function with O_NONBLOCK clear would not block, whether or not the function would transfer data successfully.

If a socket has a pending error, it is considered to have an exceptional condition pending. Otherwise, what constitutes an 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 operation is meaningless for a particular file type, select() or pselect() 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 function is the recvmsg(3XNET) function with parameters 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 currently listening, then it is marked as readable if an incoming connection request has been received, and a call to the accept function completes without blocking.

If a descriptor refers to a socket, the implied output function is the sendmsg(3XNET) function supplying an amount of normal 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 condition pending 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 connections will indicate that it is ready for reading, when connections 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 event causes 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 a non-zero value, select() completes successfully and returns 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 timeout argument is a null pointer, select() blocks until interrupted by a signal.

File descriptors associated with regular files always select true for ready to read, ready to write, and error conditions.

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 descriptor 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 argument is less than 0 or greater than or equal to FD_SETSIZE, or if fd is not a valid file descriptor, or if any of the arguments 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 than FD_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 outside the acceptable range: t_sec must be between 0 and 108, inclusive. t_usec must be greater than or equal to 0, and less than 106.

Usage

The poll(2) function is preferred over this function. It must be used when the number of file descriptors exceeds 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 attributes:

ATTRIBUTE TYPE
ATTRIBUTE VALUE
Interface Stability
Committed
MT-Level
MT-Safe
Standard

See Also

alarm(2), fcntl(2), poll(2), read(2), setitimer(2), write(2), accept(3SOCKET), listen(3SOCKET) , ualarm(3C), attributes(5) , standards(5)

Notes

The default value for FD_SETSIZE (currently 1024) is larger than the default limit on the number of open files. To accommodate 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 larger definition 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.