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select(2)
NAME
select - Synchronous I/O multiplexing
SYNOPSIS
#include <sys/time.h>
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);
STANDARDS
Interfaces documented on this reference page conform to industry standards
as follows:
select(): XPG4-UNIX
Refer to the standards(5) reference page for more information about
industry standards and associated tags.
PARAMETERS
nfds 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.
readfds 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.
writefds 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.
exceptfds 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.
timeout Points to a type timeval structure that specifies the 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.
fd Specifies a file descriptor.
fdset Points to an I/O descriptor set.
DESCRIPTION
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.
On return, the select() function 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. The total number of ready objects represented by set bits in all
the I/O descriptor sets is returned by the select() function.
[Digital] Objects ready for the requested operation are those which the
operation would not block and not necessarily those which have data
available for the operation.
After an I/O descriptor set is created, it may be modified with the
following macros:
FD_CLR(fd, &fdset)
Clears the I/O descriptor bit specified by file descriptor fd in
the I/O descriptor set addressed by fdset.
FD_ISSET(fd, &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.
FD_SET(fd, &fdset)
Includes the particular I/O descriptor bit specified by fd in the
I/O descriptor set addressed by fdset.
FD_ZERO(&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.
NOTES
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.
RETURN VALUES
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 successfully
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.
ERRORS
The select() function sets errno to the specified values for the following
conditions:
[EBADF] One or more of the I/O descriptor sets specified an invalid file
descriptor.
[EINTR] A signal was delivered before the time limit specified by the
timeout parameter expired and before any of the selected events
occurred.
[EINVAL] 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.
[EAGAIN] [Digital] Allocation of internal data structures failed. A
later call to the select() function may complete successfully.
RELATED INFORMATION
Functions: accept(2), connect(2), send(2), getdtablesize(2), poll(2)
read(2), recv(2), setsysinfo(2), write(2)
Standards: standards(5)