select(2) BSD System Calls Manual select(2)

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 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. (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 consist-
 ing 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 unde-
 fined 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-nil pointer, it specifies a maximum interval to wait
 for the selection to complete. If timeout is a nil pointer, the select
 blocks indefinitely. To effect a poll, the timeout argument should be
 non-nil, 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 errorfds may be given as nil 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, includ-
 ing one due to an interrupted call, the descriptor sets will be unmodi-
 fied 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 allo-
 cate 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 com-
 ponents is negative or too large.
 [EINVAL] ndfs is greater than FD_SETSIZE and _DARWIN_UNLIM-
 ITED_SELECT is not defined.

LEGACY SYNOPSIS

 #include <sys/select.h>
 - or -
 #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_DAR-
 WIN_UNLIMITED_SELECT.

SEE ALSO

 accept(2) , connect(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 some-
 what 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.
4.2 Berkeley Distribution March 25, 1994 4.2 Berkeley Distribution