select(2) System Calls Manual select(2)
select - Synchronous I/O multiplexing
int select( int nfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds, struct timeval *timeout) ;
void FD_CLR( int fd, fd_set *fdset);
int FD_ISSET( int fd, fd_set *fdset);
void FD_SET( int fd, fd_set *fdset);
void FD_ZERO( fd_set *fdset);
Interfaces documented on this reference page conform to industry standards as follows:
select(): XSH5.0, XNS5.0
Refer to the standards(5) reference page for more information about industry standards and associated tags.
Specifies the number of open objects that may be ready for reading or writing or that have exceptions pending. The nfds parameter cannot
be greater than FD_SETSIZE. Points to an I/O descriptor set consisting of file descriptors of objects opened for reading. When the readfds
parameter is a null pointer, the read I/O descriptor set is ignored by the select() function. Points to an I/O descriptor set consisting
of file descriptors for objects opened for writing. When the writefds parameter is a null pointer, the write I/O descriptor set is
ignored. Points to an I/O descriptor set consisting of file descriptors for objects opened for reading or writing that have an exception
pending. When the exceptfds parameter is a null pointer, the exception I/O descriptor set is ignored. Points to a type timeval structure
that specifies the maximum time to wait for a response to a select() function. When the timeout parameter has a nonzero value, the maximum
time interval to wait for the select() function to complete is specified by values stored in space reserved by the type timeval structure
pointed to by the timeout parameter.
When the timeout parameter is a null pointer, the select() function blocks indefinitely. To poll, the timeout parameter should be
specified as a nonzero value and point to a zero-valued timeval structure.
The use of a timeout does not affect any pending timers set up by alarm(, ualarm(, or settimer(. Specifies a file descriptor.
Points to an I/O descriptor set.
The select() function checks the status of objects identified by bit masks called I/O descriptor sets. Each I/O descriptor set consists of
an array of bits whose relative position and state represent a file descriptor and the status of its corresponding object. There is an I/O
descriptor set for reading, writing, and for pending exceptions. These I/O descriptor sets are pointed to by the readfds, writefds, and
exceptfds parameters, respectively. The I/O descriptor sets provide a means of monitoring the read, write, and exception status of objects
represented by file descriptors.
The status of nfds-1 file descriptors in each referenced I/O descriptor set is checked when the select() function is called. The select()
function returns a modified I/O descriptor set, which has the following characteristics: for any selected I/O descriptor set pointed to by
the readfds, writefds, and exceptfds parameters, if the state of any bit corresponding with an active file descriptor is set on entry, when
the object represented by the set bit is ready for reading, writing, or its exception condition has been satisfied, a corresponding bit
position is also set in the returned I/O descriptor set pointed to by the readfds, writefds, or exceptfds parameters.
When it returns successfully, select() does the following: First, it replaces the original I/O descriptor sets with the corresponding I/O
descriptor sets (that have a set bit for each file descriptor representing those objects that are ready for the requested operation). Sec-
ond, it returns the total number of ready objects represented by set bits in all the I/O descriptor sets.
Note: When objects are ready for the requested operation, the operation will not block. This does not necessarily mean, however, that
data is available for the operation. See read(2) and write(2) for information on when these calls would block.
After an I/O descriptor set is created, it may be modified with the following macros: Clears the I/O descriptor bit specified by file
descriptor fd in the I/O descriptor set addressed by fdset. Returns a nonzero value when the I/O descriptor bit for fd is included in the
I/O descriptor set addressed by fdset. Otherwise 0 (zero) is returned. Includes the particular I/O descriptor bit specified by fd in the
I/O descriptor set addressed by fdset. Initializes the I/O descriptor set addressed by fdset to a null value.
The behavior of these macros is undefined when parameter fd has a value less than 0 (zero) or greater than or equal to FD_SETSIZE, which is
normally at least equal to the maximum number of file descriptors supported by the system.
The select( function supports regular files, terminal and pseudo-terminal devices, STREAMS-based files, FIFOs, and pipes. The behavior of
the select( function on file descriptors that refer to other types of files is unspecified.
For sockets, a file descriptor for a socket that is listening for connections indicates that it is ready for reading when connections are
available. A file descriptor for a socket that is connecting asynchronously indicates that it is ready for writing after a connection is
This function supports up to 64K open file descriptors per process if that capability is enabled. If not already defined, FD_SETSIZE is
set in the <sys/select.h> header file to be 4K.
New applications can use any of these features for processes using more than 4K open file descriptors. When they do so, however, they must
specify an alternate value for FD_SETSIZE--one that does not exceed 64K--before they include <sys/select.h> in a program to be compiled.
Upon successful completion, the select() function returns the number of ready objects represented by corresponding file descriptor bits in
the I/O descriptor sets. When an error occurs, -1 is returned and errno is set to indicate the error.
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 suc-
cessfully and returns 0 (zero).
When select() returns an error, including a process interrupt, the I/O descriptor sets pointed to by the readfds, writefds, and exceptfds
parameters remain unmodified.
The select() function sets errno to the specified values for the following conditions:
One or more of the I/O descriptor sets specified an invalid file descriptor. A signal was delivered before the time limit specified by the
timeout parameter expired and before any of the selected events occurred. The time limit specified by the timeout parameter is invalid.
The nfds parameter is less than 0, or greater than or equal to FD_SETSIZE.
One of the specified file descriptors refers to a STREAM or multiplexer that is linked (directly or indirectly) downstream from a
multiplexer. [Tru64 UNIX] Allocation of internal data structures failed. A later call to the select() function may complete suc-
Functions: accept(2), connect(2), getdtablesize(2), poll(2) read(2), recv(2), send(2), setsysinfo(2), write(2)
Standards: standards(5) delim off