Select (Unix)

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 ' facility introduced in UNIX System V and later operating systems. However, with the c10k problem, both select and poll have been superseded by the likes of kqueue, epoll, /dev/poll and I/O completion ports.

One common use of select outside of its stated use of waiting on filehandles is to implement a portable sub-second sleep. This can be achieved by passing NULL for all three fd_set arguments, and the duration of the desired sleep as the timeout argument.

In the C programming language, the select system call is declared in the header file sys/select.h or unistd.h, and has the following syntax:

int select(int nfds, fd_set *readfds, fd_set *writefds, fd_set *errorfds, struct timeval *timeout);

fd_set type arguments may be manipulated with four utility macros: , and .

Select returns the total number of bits set in and , 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 ' provides a more flexible solution.

Example

#include
#include
#include

#include
#include
#include
#include

#include
#include
#include
#include
#include

#define PORT "9421"

/* function prototypes */
void die(const char*);

int main(int argc, char **argv)

void die(const char *msg)

See also

*Berkeley sockets
*Polling
* epoll
*kqueue
*Input/output completion port (IOCP)

References

External links

*
* {{URL, http://mdoc.su/-/select.2, man-pages for select(2) in FreeBSD, NetBSD, OpenBSD and DragonFly BSD

C POSIX library
Events (computing)
System calls
Articles with example C code