Select (Unix)
Encyclopedia
select is a system call
and application programming interface
(API) in Unix-like
and POSIX
-compliant operating systems for examining the status of file descriptors of open input/output channels. The select system call is similar to the poll facility introduced in UNIX System V and later operating systems.
In the C programming language source code, the select system call is declared in the header file sys/select.h or unistd.h
, and has the following syntax:
Select returns the total number of bits set in readfds, writefds and errorfds, or zero if the timeout expired, and -1 on error.
The sets of file descriptor used in select are finite in size, depending on the operating system. The newer system call poll provides a more flexible solution.
System call
In computing, a system call is how a program requests a service from an operating system's kernel. This may include hardware related services , creating and executing new processes, and communicating with integral kernel services...
and application programming interface
Application programming interface
An application programming interface is a source code based specification intended to be used as an interface by software components to communicate with each other...
(API) in Unix-like
Unix-like
A Unix-like operating system is one that behaves in a manner similar to a Unix system, while not necessarily conforming to or being certified to any version of the Single UNIX Specification....
and POSIX
POSIX
POSIX , an acronym for "Portable Operating System Interface", is a family of standards specified by the IEEE for maintaining compatibility between operating systems...
-compliant operating systems for examining the status of file descriptors of open input/output channels. The select system call is similar to the poll facility introduced in UNIX System V and later operating systems.
In the C programming language source code, the select system call is declared in the header file sys/select.h or unistd.h
Unistd.h
In the C and C++ programming languages, unistd.h is the name of the header file that provides access to the POSIX operating system API. It is defined by the POSIX.1 standard, the base of the Single Unix Specification, and should therefore be available in any conforming operating system/compiler .On...
, and has the following syntax:
int select(int nfds, fd_set* readfds, fd_set* writefds, fd_set* errorfds, struct timeval* timeout);
| argument | description |
|---|---|
| nfds | the maximum file descriptor across all the sets, plus 1 |
| readfds | fd_set type holding the file descriptors to be checked for being ready to read, and on output indicates which file descriptors are ready to read. Can be NULL. |
| writefds | fd_set type holding the file descriptors to be checked for being ready to write, and on output indicates which file descriptors are ready to write. Can be NULL. |
| errorfds | fd_set type holding the file descriptors to be checked for error conditions pending, and on output indicates which file descriptors have error conditions pending. Can be NULL. |
| timeout | structure of type struct timeval that specifies a maximum interval to wait for the selection to complete. If the timeout argument points to an object of type struct timeval whose members are 0, select does not block. If the timeout argument is NULL, select blocks until an event causes one of the masks to be returned with a valid (non-zero) value. |
fd_set type arguments may be manipulated with four utility macros: FD_SET, FD_CLR, FD_ZERO, and FD_ISSET.Select returns the total number of bits set in readfds, writefds and errorfds, or zero if the timeout expired, and -1 on error.
The sets of file descriptor used in select are finite in size, depending on the operating system. The newer system call poll provides a more flexible solution.