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+NAME="NET-COMMON-TCPIP-MANPAGES-SELECT">select</H1
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+>SELECT(2) System Calls Manual SELECT(2)
+
+NAME
+ select - synchronous I/O multiplexing
+
+SYNOPSIS
+ #include <sys/types.h>
+ #include <sys/time.h>
+ #include <unistd.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);
+
+DESCRIPTION
+ select() 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 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. 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 descrip-
+ tors 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 max-
+ imum number of descriptors supported by the system.
+
+ If timeout is a non-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
+ non-null, 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 exceptfds 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 is 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.
+
+ERRORS
+ An error return from select() indicates:
+
+ [EFAULT] One or more of readfds, writefds, or exceptfds points
+ outside the process's allocated address space.
+
+ [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.
+
+SEE ALSO
+ accept(2), connect(2), gettimeofday(2), poll(2), read(2), recv(2),
+ send(2), write(2), getdtablesize(3)
+
+BUGS
+ Although the provision of getdtablesize(3) 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 bit size of fd_set is based on the symbol
+ FD_SETSIZE (currently 256), but that is somewhat smaller than the current
+ kernel limit to the number of open files. However, in order to accommo-
+ date 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>. The kernel will cope, and the userland libraries provided
+ with the system are also ready for large numbers of file descriptors.
+
+ Alternatively, to be really safe, it is possible to allocate fd_set bit-
+ arrays dynamically. The idea is to permit a program to work properly
+ even if it is execve(2)'d with 4000 file descriptors pre-allocated. The
+ following illustrates the technique which is used by userland libraries:
+
+ fd_set *fdsr;
+ int max = fd;
+
+ fdsr = (fd_set *)calloc(howmany(max+1, NFDBITS),
+ sizeof(fd_mask));
+ if (fdsr == NULL) {
+ ...
+ return (-1);
+ }
+ FD_SET(fd, fdsr);
+ n = select(max+1, fdsr, NULL, NULL, &tv);
+ ...
+ free(fdsr);
+
+ Alternatively, it is possible to use the poll(2) interface. poll(2) is
+ more efficient when the size of select()'s fd_set bit-arrays are very
+ large, and for fixed numbers of file descriptors one need not size and
+ dynamically allocate a memory object.
+
+ select() should probably have been designed to return the time remaining
+ from the original timeout, if any, by modifying the time value in place.
+ Even though some systems stupidly act in this different way, it is
+ unlikely this semantic will ever be commonly implemented, as the change
+ causes massive source code compatibility problems. Furthermore, recent
+ new standards have dictated the current behaviour. In general, due to
+ the existence of those brain-damaged non-conforming systems, 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. Calculating
+ the delta is easily done by calling gettimeofday(2) before and after the
+ call to select(), and using timersub() (as described in getitimer(2)).
+
+ Internally to the kernel, select() works poorly if multiple processes
+ wait on the same file descriptor. Given that, it is rather surprising to
+ see that many daemons are written that way (i.e., httpd(8)).
+
+HISTORY
+ The select() function call appeared in 4.2BSD.
+
+BSD March 25, 1994 BSD
+ </PRE
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