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.TH SELECT 2
.CT 2 file_io comm_proc
.SH NAME
select \(mi synchronous input/output multiplexing
.SH SYNOPSIS
.nf
.B #include <sys/types.h>
.PP
.B int select(nfds, readfds, writefds, milli);
.B fd_set *readfds, *writefds;
.fi
.SH DESCRIPTION
.I Select
examines a set of file descriptors
to see if they will block if read or written.
.I Readfds
points to an object of type
.BR fd_set ,
which contains a set of descriptors to be checked for reading;
.I writefds
similarly for writing.
Only descriptors
0 through
.IR nfds \-1
are considered.
The number of ready descriptors is returned,
and the
.B fd_set
pointed to by
.I readfds
.RI ( writefds )
is overwritten with a set of descriptors
ready to be read
(written).
The call waits until at least one descriptor is ready,
or until
.I milli
milliseconds have passed.
.PP
Either
.I readfds
or
.I writefds
may be 0
if no descriptors are interesting.
.PP
The
.BR fd_set
type looks like
.RS
.EX
typedef struct {
unsigned int fds_bits[FDWORDS];
} fd_set;
.EE
.RE
.B FDWORDS
is sufficient to contain as many file descriptors as the system will allow
(currently 128).
If there are
.I B
bits in an
.BR "unsigned int" ,
file descriptor
.I n
is represented by
.BI "1<<((" n % B )-1)
in word
.BI fds_bits[ n / B ]\c
\&.
.PP
These macros should be used
to manipulate the contents
of an
.BR fd_set :
.TF FD_ISSET(n,\0s)
.TP
.PD 0
.B FD_ZERO(s)
clear all bits
in set
.I s
.TP
.B "FD_SET(n, s)
set bit for file descriptor
.I n
in set
.I s
.TP
.B "FD_CLR(n, s)
clear bit for file descriptor
.I n
in s
.TP
.B "FD_ISSET(n, s)
return a value of 1
if bit for file descriptor
.I n
is set in
.IR s ,
0 otherwise
.PD
.SH EXAMPLES
.EX
int p[2];
fd_set wfs;
pipe(p);
do {
FD_SET(p[1], wfs);
write(p[1], ".", 1);
i++;
} while(select(p[1]+1, (fd_set*)0, wfs, 0) == 1);
printf("Pipe capacity = %d\en", i);
.EE
.SH "SEE ALSO"
.IR read (2)
.SH DIAGNOSTICS
.BR EBADF ,
.BR EFAULT ,
.BR EINTR
.SH BUGS
.I Milli
is rounded up to the nearest second.
.br
.I Select
is intended for use with streams;
file descriptors referring to ordinary files
or to non-stream special files
always appear ready.
This is a lie for some special files.
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