select - synchronous I/O multiplexing
#include <sys/types.h>
#include <sys/time.h>
#include <string.h>
#include <unistd.h>
int
select(int nfds, fd_set *readfds, fd_set *writefds, fd_set
*exceptfds,
struct timeval *timeout);
FD_SET(fd, &fdset);
FD_CLR(fd, &fdset);
FD_ISSET(fd, &fdset);
FD_ZERO(&fdset);
select() examines the I/O descriptor sets whose addresses
are passed in
readfds, writefds, and 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 nfds descriptors are
checked in
each set; i.e., the descriptors from 0 through nfds-1 in the
descriptor
sets are examined. On return, select() replaces the given
descriptor
sets with subsets consisting of those descriptors that are
ready for the
requested operation. select() returns the total number of
ready descriptors
in all the sets.
The descriptor sets are stored as bit fields in arrays of
integers. The
following macros are provided for manipulating such descriptor sets:
FD_ZERO(&fdset) initializes a descriptor set fdset to the
null set.
FD_SET(fd, &fdset) includes a particular descriptor fd in
fdset.
FD_CLR(fd, &fdset) removes fd from fdset. FD_ISSET(fd,
&fdset) is nonzero
if fd is a member of fdset, zero otherwise. The behavior of these
macros is undefined if a descriptor value is less than zero
or greater
than or equal to FD_SETSIZE, which is normally at least
equal to the maximum
number of descriptors supported by the system.
If timeout is a non-null pointer, it specifies a maximum interval to wait
for the selection to complete. If timeout is a null pointer, the select
blocks indefinitely. To effect a poll, the timeout argument
should be
non-null, pointing to a zero-valued timeval structure.
timeout is not
changed by select(), and may be reused on subsequent calls;
however, it
is good style to re-initialize it before each invocation of
select().
Any of readfds, writefds, and exceptfds may be given as null
pointers if
no descriptors are of interest.
select() returns the number of ready descriptors that are
contained in
the descriptor sets, or -1 if an error occurred. If the
time limit expires,
select() returns 0. If select() returns with an error, including
one due to an interrupted call, the descriptor sets will be
unmodified.
An error return from select() indicates:
[EFAULT] One or more of readfds, writefds, or exceptfds
points outside
the process's allocated address space.
[EBADF] One of the descriptor sets specified an invalid descriptor.
[EINTR] A signal was delivered before the time limit
expired and
before any of the selected events occurred.
[EINVAL] The specified time limit is invalid. One of
its components
is negative or too large.
accept(2), connect(2), gettimeofday(2), poll(2), read(2),
recv(2),
send(2), write(2), getdtablesize(3)
The select() function call appeared in 4.2BSD.
Although the provision of 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 fd_set is based
on the symbol
FD_SETSIZE (currently 1024), 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 FD_SETSIZE before the
inclusion of
<sys/types.h>. The kernel will cope, and the userland libraries provided
with the system are also ready for large numbers of file descriptors.
Alternatively, to be really safe, it is possible to allocate
fd_set bitarrays
dynamically. The idea is to permit a program to work
properly
even if it is execve(2)'d with 4000 file descriptors pre-allocated. The
following illustrates the technique which is used by userland libraries:
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);
Alternatively, it is possible to use the poll(2) interface.
poll(2) is
more efficient when the size of select()'s fd_set bit-arrays
are very
large, and for fixed numbers of file descriptors one need
not size and
dynamically allocate a memory object.
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
select() call,
and the caller should reinitialize it on each invocation.
Calculating
the delta is easily done by calling gettimeofday(2) before
and after the
call to select(), and using timersub() (as described in
getitimer(2)).
Internally to the kernel, 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., httpd(8)).
OpenBSD 3.6 March 25, 1994
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