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Manual Reference Pages  -  SELECT (2)


select - synchronous I/O multiplexing


Return Values
See Also


.Lb libc


.In sys/select.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


The 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 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 maximum number of descriptors supported by the system.

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.


The 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.


An error return from select indicates:
  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.


accept(2), connect(2), getdtablesize(2), gettimeofday(2), kqueue(2), poll(2), read(2), recv(2), send(2), write(2), clocks(7)


The default size of FD_SETSIZE is 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_SETSIZE before the inclusion of any header which includes
.In sys/types.h .

If nfds is greater than the number of open files, select is not guaranteed to examine the unused file descriptors. For historical reasons, select will always examine the first 256 descriptors.


The select system call and FD_CLR, FD_ISSET, FD_SET, and FD_ZERO macros conform with -p1003.1-2001.


The select system call appeared in BSD 4.2 .


-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.
.Fx does not modify the return value, which can cause problems for applications ported from other systems.
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