select(2)

DESCRIPTION

The select() function indicates which of the specified file descriptors is ready for reading, ready for writing, or has an error condition pending. If the specified condition is false for all of the specified file descriptors, select() blocks, up to the specified timeout interval, until the specified condition is true for at least one of the specified file descriptors.

The select() function supports regular files, terminal and pseudo-terminal devices, STREAMS-based files, FIFOs and pipes. The behaviour of select() on file descriptors that refer to other types of file is unspecified.

The nfds argument specifies the range of file descriptors to be tested. The select() function tests file descriptors in the range of 0 to nfds -1.

If the readfds argument is not a null pointer, it points to an object of type fd_set that on input specifies the file descriptors to be checked for being ready to read, and on output indicates which file descriptors are ready to read.

If the writefds argument is not a null pointer, it points to an object of type fd_set that on input specifies the file descriptors to be checked for being ready to write, and on output indicates which file descriptors are ready to write.

If the errorfds argument is not a null pointer, it points to an object of type fd_setthatoninput specifies the file descriptors to be checked for error conditions pending, and on output indicates which file descriptors have error conditions pending.

On successful completion, the objects pointed to by the readfds, writefds, and errorfds arguments are modified to indicate which file descriptors are ready for reading, ready for writing, or have an error condition pending, respectively. For each file descriptor less than nfds, the corresponding bit will be set on successful completion if it was set on input and the associated condition is true for that file descriptor.

If the timeout argument is not a null pointer, it points to an object of type structtimeval that specifies a maximum interval to wait for the selection to complete. If the timeout argument points to an object of type structtimeval whose members are 0, select() does not block. If the timeout argument is a null pointer, select() blocks until an event causes one of the masks to be returned with a valid (non-zero) value. If the time limit expires before any event occurs that would cause one of the masks to be set to a non-zero value, select() completes successfully and returns 0.

Implementations may place limitations on the maximum timeout interval supported. On all implementations, the maximum timeout interval supported will be at least 31 days. If the timeout argument specifies a timeout interval greater than the implementation- dependent maximum value, the maximum value will be used as the actual timeout value. Implementations may also place limitations on the granularity of timeout intervals. If the requested timeout interval requires a finer granularity than the implementation supports, the actual timeout interval will be rounded up to the next supported value.

If the readfds, writefds, and errorfds arguments are all null pointers and the timeout argument is not a null pointer, select() blocks for the time specified, or until interrupted by a signal. If the readfds, writefds, and errorfds arguments are all null pointers and the timeout argument is a null pointer, select() blocks until interrupted by a signal.

File descriptors associated with regular files always select true for ready to read, ready to write, and error conditions.

On failure, the objects pointed to by the readfds, writefds, and errorfds arguments are not modified. If the timeout interval expires without the specified condition being true for any of the specified file descriptors, the objects pointed to by the readfds, writefds, and errorfds arguments have all bits set to 0.

File descriptor masks of type fd_set can be initialised and tested with FD_CLR(), FD_ISSET(), FD_SET(), and FD_ZERO(). It is unspecified whether each of these is a macro or a function. If a macro definition is suppressed in order to access an actual function, or a program defines an external identifier with any of these names, the behaviour is undefined.

RETURN VALUE

FD_CLR(), FD_SET(), and FD_ZERO() return no value. FD_ISSET() returns a non-zero value if the bit for the file descriptor fd is set in the file descriptor set pointed to by fdset, and 0 otherwise.

On successful completion, select() returns the total number of bits set in the bit masks. Otherwise, -1 is returned, and errno is set to indicate the error.

ERRORS

Under the following conditions, select() fails and sets errno to:

APPLICATION USAGE

The use of a timeout does not affect any pending timers set up by alarm(), ualarm(), or setitimer().

On successful completion, the object pointed to by the timeout argument may be modified.

The FD_SETSIZE is used in the definition of fd_set structure. It is set to a value of 2048 to accommodate 2048 file descriptors. Any user code that uses FD_SETSIZE or the structure fd_set should redefine FD_SETSIZE to a smaller value (greater than or equal to the number of open files the process will have) in order to save space. Similarly, any user code that wants to test more than 2048 file descriptors should redefine FD_SETSIZE to the required higher value.

The user can also allocate the space for fd_set structure dynamically, depending upon the number of file descriptors to be tested. The following code segment illustrates the basic concepts.

SEE ALSO

fcntl(2), poll(2), read(2), write(2), <sys/time.h>.

CHANGE HISTORY

First released in Issue 4, Version 2.

select HP-UX EXTENSIONS

SYNOPSIS (HP-UX)

DESCRIPTION

This select() function prototype is provided for backward compatibility only. For this prototype to be used, <time.h>, instead of <sys/time.h>, must be included and the symbol _XOPEN_SOURCE_EXTENDED must not be defined in the compilation time. Otherwise, the X/Open compliant version will be used.

select() examines the files or devices associated with the file descriptors specified by the bit masks readfds, writefds, and exceptfds. The bits from 0 through nfds-1 are examined. File descriptor f is represented by the bit 1<<f in the masks. More formally, a file descriptor is represented by:

Ttys and sockets are ready for reading or writing, respectively, if a read() or write() would not block for one or more of the following reasons:

Sockets select true on reads and/or exceptions if out-of-band data is available.

Pipes are ready for reading if there is any data in the pipe, or if there are no writers left for the pipe. Pipes are ready for writing if there is room for more data in the pipe AND there are one or more readers for the pipe, OR there are no readers left for the pipe. select() returns the same results for a pipe whether a file descriptor associated with the read-only end or the write-only end of the pipe is used, since both file descriptors refer to the same underlying pipe. So a select() of a read-only file descriptor that is associated with a pipe can return ready to write, even though that particular file descriptor cannot be written to.

ERRORS

EXAMPLES

The following call to select() checks if any of 4 terminals are ready for reading. select() times out after 5 seconds if no terminals are ready for reading. Note that the code for opening the terminals or reading from the terminals is not shown in this example. Also, note that this example must be modified if the calling process has more than 32 file descriptors open. Following this first example is an example of select with more than 32 file descriptors.

The following example is the same as the previous example, except that it works for more than 32 open files. Definitions for howmany, fd_set, and NFDBITS are in <sys/types.h>.

WARNINGS

Check all references to signal(5) for appropriateness on systems that support sigvector(). sigvector() can affect the behavior described on this manpage.

The file descriptor masks are always modified on return, even if the call returns as the result of a timeout.

DEPENDENCIES

select() supports the following devices and file types:

AUTHOR

select() was developed by HP and the University of California, Berkeley.

SEE ALSO

fcntl(2), read(2), write(2).