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.\"     @(#)select.2    8.2 (Berkeley) 3/25/94

.\"

.Dd March 25, 1994

.Dt SELECT 2

.Os

.Sh NAME

.Nm select

.Nd synchronous I/O multiplexing

.Sh SYNOPSIS

.Fd #include 

.Fd #include 

.Fd #include 

.Ft int

.Fn select "int nfds" "fd_set *readfds" "fd_set *writefds" "fd_set *exceptfds" "struct timeval *timeout"

.Fn FD_SET fd &fdset

.Fn FD_CLR fd &fdset

.Fn FD_ISSET fd &fdset

.Fn FD_ZERO &fdset

.Sh DESCRIPTION

.Fn select

examines the I/O descriptor sets whose addresses are passed in

.Fa readfds ,

.Fa writefds ,

and

.Fa exceptfds

to see if some of their descriptors

are ready for reading, are ready for writing, or have an exceptional

condition pending, respectively.

The first

.Fa nfds

descriptors are checked in each set;

i.e., the descriptors from 0 through

.Fa nfds Ns No -1

in the descriptor sets are examined.

On return,

.Fn select

replaces the given descriptor sets

with subsets consisting of those descriptors that are ready

for the requested operation.

.Fn select

returns the total number of ready descriptors in all the sets.

.Pp

The descriptor sets are stored as bit fields in arrays of integers.

The following macros are provided for manipulating such descriptor sets:

.Fn FD_ZERO &fdset

initializes a descriptor set

.Fa fdset

to the null set.

.Fn FD_SET fd &fdset

includes a particular descriptor

.Fa fd

in

.Fa fdset .

.Fn FD_CLR fd &fdset

removes

.Fa fd

from

.Fa fdset .

.Fn FD_ISSET fd &fdset

is non-zero if

.Fa fd

is a member of

.Fa fdset ,

zero otherwise.

The behavior of these macros is undefined if

a descriptor value is less than zero or greater than or equal to

.Dv FD_SETSIZE ,

which is normally at least equal

to the maximum number of descriptors supported by the system.

.Pp

If

.Fa timeout

is a non-null pointer, it specifies a maximum interval to wait for the

selection to complete.

If

.Fa timeout

is a null pointer, the select blocks indefinitely.

To effect a poll, the

.Fa timeout

argument should be non-null, pointing to a zero-valued timeval structure.

.Fa timeout

is not changed by

.Fn select ,

and may be reused on subsequent calls; however, it is good style to

re-initialize it before each invocation of

.Fn select .

.Pp

Any of

.Fa readfds ,

.Fa writefds ,

and

.Fa exceptfds

may be given as null pointers if no descriptors are of interest.

.Sh RETURN VALUES

.Fn select

returns the number of ready descriptors that are contained in

the descriptor sets, or \-1 is an error occurred.

If the time limit expires,

.Fn select

returns 0.

If

.Fn select

returns with an error, including one due to an interrupted call,

the descriptor sets will be unmodified.

.Sh ERRORS

An error return from

.Fn select

indicates:

.Bl -tag -width Er

.It Bq Er EFAULT

One or more of

.Fa readfds ,

.Fa writefds ,

or

.Fa exceptfds

points outside the process's allocated address space.

.It Bq Er EBADF

One of the descriptor sets specified an invalid descriptor.

.It Bq Er EINTR

A signal was delivered before the time limit expired and

before any of the selected events occurred.

.It Bq Er EINVAL

The specified time limit is invalid.

One of its components is negative or too large.

.El

.Sh SEE ALSO

.Xr accept 2 ,

.Xr connect 2 ,

.Xr gettimeofday 2 ,

.Xr poll 2 ,

.Xr read 2 ,

.Xr recv 2 ,

.Xr send 2 ,

.Xr write 2 ,

.Xr getdtablesize 3

.Sh BUGS

Although the provision of

.Xr getdtablesize 3

was intended to allow user programs to be written independent

of the kernel limit on the number of open files, the dimension

of a sufficiently large bit field for select remains a problem.

The default bit size of

.Ft fd_set

is based on the symbol

.Dv FD_SETSIZE

(currently 256),

but that is somewhat smaller than the current kernel limit

to the number of open files.

However, in order to accommodate programs which might potentially

use a larger number of open files with select, it is possible

to increase this size within a program by providing

a larger definition of

.Dv FD_SETSIZE

before the inclusion of

.Aq Pa sys/types.h .

The kernel will cope, and the userland libraries provided with the

system are also ready for large numbers of file descriptors.

.Pp

Alternatively, to be really safe, it is possible to allocate

.Ft fd_set

bit-arrays dynamically.

The idea is to permit a program to work properly even if it is

.Xr execve 2 Ns 'd

with 4000 file descriptors pre-allocated.

The following illustrates the technique which is used by

userland libraries:

.Pp

.Bd -literal -offset indent -compact

        fd_set *fdsr;

        int max = fd;

        fdsr = (fd_set *)calloc(howmany(max+1, NFDBITS),

            sizeof(fd_mask));

        if (fdsr == NULL) {

                ...

                return (-1);

        }

        FD_SET(fd, fdsr);

        n = select(max+1, fdsr, NULL, NULL, &tv);

        ...

        free(fdsr);

.Ed

.Pp

Alternatively, it is possible to use the

.Xr poll 2

interface.

.Xr poll 2

is more efficient when the size of

.Fn select Ns 's

.Ft fd_set

bit-arrays are very large, and for fixed numbers of

file descriptors one need not size and dynamically allocate a

memory object.

.Pp

.Fn select

should probably have been designed to return the time remaining from the

original timeout, if any, by modifying the time value in place.

Even though some systems stupidly act in this different way, it is

unlikely this semantic will ever be commonly implemented, as the

change causes massive source code compatibility problems.

Furthermore, recent new standards have dictated the current behaviour.

In general, due to the existence of those brain-damaged

non-conforming systems, it is unwise to assume that the timeout

value will be unmodified by the

.Fn select

call, and the caller should reinitialize it on each invocation.

Calculating the delta is easily done by calling

.Xr gettimeofday 2

before and after the call to

.Fn select Ns ,

and using

.Fn timersub

(as described in

.Xr getitimer 2 ) .

.Pp

Internally to the kernel,

.Fn select

works poorly if multiple processes wait on the same file descriptor.

Given that, it is rather surprising to see that many daemons are

written that way (i.e.,

.Xr httpd 8 ) .

.Sh HISTORY

The

.Fn select

function call appeared in

.Bx 4.2 .