select()

Purpose

Waits for readiness on a small set of file descriptors using fd_set bit masks.

Header

#include <sys/select.h>

Prototype

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

Parameters

• nfds: highest fd value plus one.
• readfds: fds monitored for readability.
• writefds: fds monitored for writability.
• exceptfds: exceptional conditions.
• timeout: maximum wait time, or NULL to block indefinitely.

Return Value

• Success: returns number of ready fds.
• Timeout: returns 0.
• Failure: returns -1 and sets errno.

Minimal Example

fd_set rfds;
FD_ZERO(&rfds);
FD_SET(fd, &rfds);

int ret = select(fd + 1, &rfds, NULL, NULL, NULL);

Common Pitfalls

• fd_set is modified by select(), so rebuild it before each call.
• FD_SETSIZE limits how many fd numbers can be represented.
• Prefer poll() or epoll() for scalable event loops.

Related Topics

• Event Loop