SELECT(2) BSD Programmer's Manual SELECT(2)
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 condi-
tion 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 re-
quested 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 non-
zero 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 max-
imum 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 ex-
pires, 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 out-
side 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 bit-
arrays 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 fol-
lowing 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 un-
likely 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)).
MirBSD #10-current March 25, 1994 2