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>Chapter 38. TCP/IP Library Reference</TD
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NAME="NET-COMMON-TCPIP-MANPAGES-SELECT">select</H1
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>SELECT(2) System Calls Manual SELECT(2)
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NAME
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select - synchronous I/O multiplexing
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SYNOPSIS
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#include <sys/types.h>
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#include <sys/time.h>
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#include <unistd.h>
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int
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select(int nfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds,
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struct timeval *timeout);
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FD_SET(fd, &fdset);
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FD_CLR(fd, &fdset);
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FD_ISSET(fd, &fdset);
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FD_ZERO(&fdset);
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DESCRIPTION
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select() examines the I/O descriptor sets whose addresses are passed in
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readfds, writefds, and exceptfds to see if some of their descriptors are
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ready for reading, are ready for writing, or have an exceptional condi-
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tion pending, respectively. The first nfds descriptors are checked in
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each set; i.e., the descriptors from 0 through nfds-1 in the descriptor
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sets are examined. On return, select() replaces the given descriptor
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sets with subsets consisting of those descriptors that are ready for the
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requested operation. select() returns the total number of ready descrip-
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tors in all the sets.
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The descriptor sets are stored as bit fields in arrays of integers. The
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following macros are provided for manipulating such descriptor sets:
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FD_ZERO(&fdset) initializes a descriptor set fdset to the null set.
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FD_SET(fd, &fdset) includes a particular descriptor fd in fdset.
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FD_CLR(fd, &fdset) removes fd from fdset. FD_ISSET(fd, &fdset) is non-
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zero if fd is a member of fdset, zero otherwise. The behavior of these
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macros is undefined if a descriptor value is less than zero or greater
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than or equal to FD_SETSIZE, which is normally at least equal to the max-
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imum number of descriptors supported by the system.
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If timeout is a non-null pointer, it specifies a maximum interval to wait
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for the selection to complete. If timeout is a null pointer, the select
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blocks indefinitely. To effect a poll, the timeout argument should be
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non-null, pointing to a zero-valued timeval structure. timeout is not
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changed by select(), and may be reused on subsequent calls; however, it
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is good style to re-initialize it before each invocation of select().
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Any of readfds, writefds, and exceptfds may be given as null pointers if
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no descriptors are of interest.
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RETURN VALUES
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select() returns the number of ready descriptors that are contained in
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the descriptor sets, or -1 is an error occurred. If the time limit
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expires, select() returns 0. If select() returns with an error, includ-
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ing one due to an interrupted call, the descriptor sets will be unmodi-
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fied.
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ERRORS
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An error return from select() indicates:
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[EFAULT] One or more of readfds, writefds, or exceptfds points
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outside the process's allocated address space.
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[EBADF] One of the descriptor sets specified an invalid
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descriptor.
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[EINTR] A signal was delivered before the time limit expired
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and before any of the selected events occurred.
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[EINVAL] The specified time limit is invalid. One of its com-
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ponents is negative or too large.
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SEE ALSO
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accept(2), connect(2), gettimeofday(2), poll(2), read(2), recv(2),
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send(2), write(2), getdtablesize(3)
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BUGS
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Although the provision of getdtablesize(3) was intended to allow user
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programs to be written independent of the kernel limit on the number of
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open files, the dimension of a sufficiently large bit field for select
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remains a problem. The default bit size of fd_set is based on the symbol
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FD_SETSIZE (currently 256), but that is somewhat smaller than the current
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kernel limit to the number of open files. However, in order to accommo-
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date programs which might potentially use a larger number of open files
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with select, it is possible to increase this size within a program by
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providing a larger definition of FD_SETSIZE before the inclusion of
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<sys/types.h>. The kernel will cope, and the userland libraries provided
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with the system are also ready for large numbers of file descriptors.
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Alternatively, to be really safe, it is possible to allocate fd_set bit-
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arrays dynamically. The idea is to permit a program to work properly
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even if it is execve(2)'d with 4000 file descriptors pre-allocated. The
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following illustrates the technique which is used by userland libraries:
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fd_set *fdsr;
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int max = fd;
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fdsr = (fd_set *)calloc(howmany(max+1, NFDBITS),
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sizeof(fd_mask));
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if (fdsr == NULL) {
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...
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return (-1);
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}
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FD_SET(fd, fdsr);
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n = select(max+1, fdsr, NULL, NULL, &tv);
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...
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free(fdsr);
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Alternatively, it is possible to use the poll(2) interface. poll(2) is
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more efficient when the size of select()'s fd_set bit-arrays are very
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large, and for fixed numbers of file descriptors one need not size and
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dynamically allocate a memory object.
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select() should probably have been designed to return the time remaining
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from the original timeout, if any, by modifying the time value in place.
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Even though some systems stupidly act in this different way, it is
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unlikely this semantic will ever be commonly implemented, as the change
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causes massive source code compatibility problems. Furthermore, recent
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new standards have dictated the current behaviour. In general, due to
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the existence of those brain-damaged non-conforming systems, it is unwise
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to assume that the timeout value will be unmodified by the select() call,
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and the caller should reinitialize it on each invocation. Calculating
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the delta is easily done by calling gettimeofday(2) before and after the
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call to select(), and using timersub() (as described in getitimer(2)).
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Internally to the kernel, select() works poorly if multiple processes
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wait on the same file descriptor. Given that, it is rather surprising to
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see that many daemons are written that way (i.e., httpd(8)).
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HISTORY
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The select() function call appeared in 4.2BSD.
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BSD March 25, 1994 BSD
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</PRE
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