--- name: "select" section: "2" platform: "macos" package: "xnu" version: "12377.121.6" release: "macOS 26.5" platforms: ["linux", "freebsd", "macos"] url: "https://new.carta.tech/select" --- # select(2) synchronous I/O multiplexing ## NAME **FD\_CLR**, **FD\_COPY**, **FD\_ISSET**, **FD\_SET**, **FD\_ZERO**, **select** – synchronous I/O multiplexing ## SYNOPSIS **#include \** *void*\ **FD\_CLR**(*fd*, *fd\_set \*fdset*); *void*\ **FD\_COPY**(*fd\_set \*fdset\_orig*, *fd\_set \*fdset\_copy*); *int*\ **FD\_ISSET**(*fd*, *fd\_set \*fdset*); *void*\ **FD\_SET**(*fd*, *fd\_set \*fdset*); *void*\ **FD\_ZERO**(*fd\_set \*fdset*); *int*\ **select**(*int nfds*, *fd\_set \*restrict readfds*, *fd\_set \*restrict writefds*, *fd\_set \*restrict errorfds*, *struct timeval \*restrict timeout*); ## DESCRIPTION **select**() examines the I/O descriptor sets whose addresses are passed in *readfds*, *writefds*, and *errorfds* to see if some of their descriptors are ready for reading, are ready for writing, or have an exceptional condition pending, respectively. The first *nfds* descriptors are checked in each set; i.e., the descriptors from 0 through *nfds*-1 in the descriptor sets are examined. (Example: If you have set two file descriptors "4" and "17", *nfds* should not be "2", but rather "17 + 1" or "18".) On return, **select**() replaces the given descriptor sets with subsets consisting of those descriptors that are ready for the requested operation. **select**() returns the total number of ready descriptors in all the sets. The descriptor sets are stored as bit fields in arrays of integers. The following macros are provided for manipulating such descriptor sets: **FD\_ZERO**(*&fdset*) initializes a descriptor set *fdset* to the null set. **FD\_SET**(*fd*, *&fdset*) includes a particular descriptor *fd* in *fdset*. **FD\_CLR**(*fd*, *&fdset*) removes *fd* from *fdset*. **FD\_ISSET**(*fd*, *&fdset*) is non-zero if *fd* is a member of *fdset*, zero otherwise. **FD\_COPY**(*&fdset\_orig*, *&fdset\_copy*) replaces an already allocated *&fdset\_copy* file descriptor set with a copy of *&fdset\_orig*. The behavior of these macros is undefined if a descriptor value is less than zero or greater than or equal to FD\_SETSIZE, which is normally at least equal to the maximum number of descriptors supported by the system. If *timeout* is not a null pointer, it specifies a maximum interval to wait for the selection to complete. If *timeout* is a null pointer, the select blocks indefinitely. To effect a poll, the *timeout* argument should be not be a null pointer, but it should point to a zero-valued timeval structure. *timeout* is not changed by **select**(), and may be reused on subsequent calls, however it is good style to re-initialize it before each invocation of **select**(). Any of *readfds*, *writefds*, and *errorfds* may be given as null pointers if no descriptors are of interest. ## RETURN VALUES **select**() returns the number of ready descriptors that are contained in the descriptor sets, or -1 if an error occurred. If the time limit expires, **select**() returns 0. If **select**() returns with an error, including one due to an interrupted call, the descriptor sets will be unmodified and the global variable *errno* will be set to indicate the error. ## ERRORS An error return from **select**() indicates: - \[EAGAIN\] The kernel was (perhaps temporarily) unable to allocate the requested number of file descriptors. - \[EBADF\] One of the descriptor sets specified an invalid descriptor. - \[EINTR\] A signal was delivered before the time limit expired and before any of the selected events occurred. - \[EINVAL\] The specified time limit is invalid. One of its components is negative or too large. - \[EINVAL\] *ndfs* is greater than FD\_SETSIZE and \_DARWIN\_UNLIMITED\_SELECT is not defined. ## LEGACY SYNOPSIS **#include \** \- or - **#include \** **#include \** **#include \** **FD\_SET**(*fd*, *&fdset*); **FD\_CLR**(*fd*, *&fdset*); **FD\_ISSET**(*fd*, *&fdset*); **FD\_COPY**(*&fdset\_orig*, *&fdset\_copy*); **FD\_ZERO**(*&fdset*); ## COMPATIBILITY **select**() now returns with *errno* set to EINVAL when *nfds* is greater than FD\_SETSIZE. Use a smaller value for *nfds* or compile with -D\_DARWIN\_UNLIMITED\_SELECT. ## SEE ALSO [accept(2)](/accept/2.md), [connect(2)](/connect/2.md), [connectx(2)](/connectx/2.md), [getdtablesize(2)](/getdtablesize/2.md), [gettimeofday(2)](/gettimeofday/2.md), [read(2)](/read/2.md), [recv(2)](/recv/2.md), [send(2)](/send/2.md), [write(2)](/write/2.md), [compat(5)](/compat/5.md) ## BUGS Although the provision of [getdtablesize(2)](/getdtablesize/2.md) was intended to allow user programs to be written independent of the kernel limit on the number of open files, the dimension of a sufficiently large bit field for select remains a problem. The default size FD\_SETSIZE (currently 1024) is somewhat smaller than the current kernel limit to the number of open files. However, in order to accommodate programs which might potentially use a larger number of open files with select, it is possible to increase this size within a program by providing a larger definition of FD\_SETSIZE before the inclusion of ⟨*sys/types.h*⟩. **select**() should probably have been designed to return the time remaining from the original timeout, if any, by modifying the time value in place. However, it is unlikely this semantic will ever be implemented, as the change would cause source code compatibility problems. In general it is unwise to assume that the timeout value will be unmodified by the **select**() call, and the caller should reinitialize it on each invocation. ## HISTORY The **select**() function call appeared in 4.2BSD.