select (Unix)

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select is a system call for polling multiple file descriptors. In C programming, it is declared in the header file sys/select.h or unistd.h. Those files and sys/time.h must be included to use select.

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

argument	description
nfds	the highest numbered file descriptor of 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 a null pointer, select() blocks until an event causes one of the masks to be returned with a valid (non-zero) value.