NAME
FD_CLR
, FD_COPY
,
FD_ISSET
, FD_SET
,
FD_ZERO
, select
—
synchronous I/O multiplexing
SYNOPSIS
#include
<sys/select.h>
void
FD_CLR
(fd,
fd_set *fdset);
void
FD_COPY
(fd_set *fdset_orig,
fd_set *fdset_copy);
int
FD_ISSET
(fd,
fd_set *fdset);
void
FD_SET
(fd,
fd_set *fdset);
void
FD_ZERO
(fd_set *fdset);
int
select
(int nfds,
fd_set *restrict readfds, fd_set
*restrict writefds, fd_set *restrict errorfds,
struct timeval *restrict timeout);
DESCRIPTION
select
()
examines the I/O descriptor sets whose addresses are passed in
readfds, writefds, and
errorfds 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. (Example: If you have set two file descriptors "4"
and "17", nfds should not be "2",
but rather "17 + 1" or "18".) 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 non-zero if fd is
a member of fdset, zero otherwise.
FD_COPY
(&fdset_orig,
&fdset_copy) replaces an already allocated
&fdset_copy file descriptor set with a copy of
&fdset_orig. 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 not a 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 not be a null pointer, but it should point 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 errorfds may be given as null pointers if no descriptors are of interest.
RETURN VALUES
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 and the global variable errno will be set
to indicate the error.
ERRORS
An error return from select
()
indicates:
- [
EAGAIN
] - The kernel was (perhaps temporarily) unable to allocate the requested number of file descriptors.
- [
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.
- [
EINVAL
] - ndfs is greater than FD_SETSIZE and _DARWIN_UNLIMITED_SELECT is not defined.
LEGACY SYNOPSIS
#include
<sys/select.h>
#include <sys/types.h>
#include <sys/time.h>
#include <unistd.h>
FD_SET
(fd,
&fdset);;
FD_CLR
(fd,
&fdset);;
FD_ISSET
(fd,
&fdset);;
FD_COPY
(&fdset_orig,
&fdset_copy);;
FD_ZERO
(&fdset);;
COMPATIBILITY
select
() now returns with
errno set to EINVAL when nfds is
greater than FD_SETSIZE. Use a smaller value for nfds
or compile with -D_DARWIN_UNLIMITED_SELECT.
SEE ALSO
accept(2), connect(2), connectx(2), getdtablesize(2), gettimeofday(2), read(2), recv(2), send(2), write(2), compat(5)
BUGS
Although the provision of
getdtablesize(2) 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
size FD_SETSIZE
(currently 1024) 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⟩.
select
() should probably have been
designed to return the time remaining from the original timeout, if any, by
modifying the time value in place. However, it is unlikely this semantic
will ever be implemented, as the change would cause source code
compatibility problems. In general 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.
HISTORY
The select
() function call appeared in
4.2BSD.