|SELECT(2)||System Calls Manual||SELECT(2)|
LIBRARY¶Standard C Library (libc, -lc)
select() system call 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 only exceptional condition detectable is out-of-band data received on a socket. 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. The
select() system call 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
initializes a descriptor set fdset to the null set.
&fdset) includes a particular descriptor
fd in fdset.
&fdset) removes fd from
&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 maximum number of descriptors supported by the
If timeout is not a null pointer, it specifies the maximum interval to wait for the selection to complete. System activity can lengthen the interval by an indeterminate amount.
If timeout is a null pointer, the select blocks indefinitely.
To effect a poll, the timeout argument should not be a null pointer, but it should point to a zero-valued timeval structure.
Any of readfds, writefds, and exceptfds may be given as null pointers if no descriptors are of interest.
select() system call 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 system call, the descriptor sets will be unmodified.
ERRORS¶An error return from
- One of the descriptor sets specified an invalid descriptor.
- One of the arguments readfds, writefds, exceptfds, or timeout points to an invalid address.
- A signal was delivered before the time limit expired and before any of the selected events occurred.
- The specified time limit is invalid. One of its components is negative or too large.
- The nfds argument was invalid.
SEE ALSO¶accept(2), connect(2), getdtablesize(2), gettimeofday(2), kqueue(2), poll(2), read(2), recv(2), send(2), write(2), clocks(7)
NOTES¶The default size of
FD_SETSIZEis currently 1024. In order to accommodate programs which might potentially use a larger number of open files with
select(), it is possible to increase this size by having the program define
FD_SETSIZEbefore the inclusion of any header which includes
If nfds is greater than the number of open
select() is not guaranteed to examine the
unused file descriptors. For historical reasons,
select() will always examine the first 256
select() system call and
FD_ZERO() macros conform with IEEE Std 1003.1-2001 (“POSIX.1”).
select() system call appeared in 4.2BSD.
BUGS¶Version 2 of the Single UNIX Specification (“SUSv2”) allows systems to modify the original timeout in place. Thus, it is unwise to assume that the timeout value will be unmodified by the
select() system call. FreeBSD does not modify the return value, which can cause problems for applications ported from other systems.
|November 17, 2002||Linux 4.19.0-6-amd64|