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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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