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NAME |LIBRARY |SYNOPSIS |DESCRIPTION |RETURN VALUE |ERRORS |VERSIONS |STANDARDS |HISTORY |NOTES |BUGS |EXAMPLES |SEE ALSO |COLOPHON | |
select(2) System Calls Manualselect(2)select, pselect, FD_CLR, FD_ISSET, FD_SET, FD_ZERO, fd_set - synchronous I/O multiplexing
Standard C library (libc,-lc)
#include <sys/select.h>typedef/* ... */fd_set;int select(intnfds, fd_set *_Nullable restrictreadfds,fd_set *_Nullable restrictwritefds,fd_set *_Nullable restrictexceptfds,struct timeval *_Nullable restricttimeout);void FD_CLR(intfd, fd_set *set);int FD_ISSET(intfd, fd_set *set);void FD_SET(intfd, fd_set *set);void FD_ZERO(fd_set *set);int pselect(intnfds, fd_set *_Nullable restrictreadfds,fd_set *_Nullable restrictwritefds,fd_set *_Nullable restrictexceptfds,const struct timespec *_Nullable restricttimeout,const sigset_t *_Nullable restrictsigmask); Feature Test Macro Requirements for glibc (seefeature_test_macros(7)):pselect(): _POSIX_C_SOURCE >= 200112L
WARNING:select() can monitor only file descriptors numbers that are less thanFD_SETSIZE(1024)—an unreasonably low limit for many modern applications—and this limitation will not change. All modern applications should instead usepoll(2) orepoll(7), which do not suffer this limitation.select() allows a program to monitor multiple file descriptors, waiting until one or more of the file descriptors become "ready" for some class of I/O operation (e.g., input possible). A file descriptor is considered ready if it is possible to perform a corresponding I/O operation (e.g.,read(2), or a sufficiently smallwrite(2)) without blocking.fd_set A structure type that can represent a set of file descriptors. According to POSIX, the maximum number of file descriptors in anfd_set structure is the value of the macroFD_SETSIZE.File descriptor sets The principal arguments ofselect() are three "sets" of file descriptors (declared with the typefd_set), which allow the caller to wait for three classes of events on the specified set of file descriptors. Each of thefd_set arguments may be specified as NULL if no file descriptors are to be watched for the corresponding class of events.Note well: Upon return, each of the file descriptor sets is modified in place to indicate which file descriptors are currently "ready". Thus, if usingselect() within a loop, the setsmust bereinitialized before each call. The contents of a file descriptor set can be manipulated using the following macros:FD_ZERO() This macro clears (removes all file descriptors from)set. It should be employed as the first step in initializing a file descriptor set.FD_SET() This macro adds the file descriptorfd toset. Adding a file descriptor that is already present in the set is a no- op, and does not produce an error.FD_CLR() This macro removes the file descriptorfd fromset. Removing a file descriptor that is not present in the set is a no-op, and does not produce an error.FD_ISSET()select() modifies the contents of the sets according to the rules described below. After callingselect(), theFD_ISSET() macro can be used to test if a file descriptor is still present in a set.FD_ISSET() returns nonzero if the file descriptorfd is present inset, and zero if it is not.Arguments The arguments ofselect() are as follows:readfds The file descriptors in this set are watched to see if they are ready for reading. A file descriptor is ready for reading if a read operation will not block; in particular, a file descriptor is also ready on end-of-file. Afterselect() has returned,readfds will be cleared of all file descriptors except for those that are ready for reading.writefds The file descriptors in this set are watched to see if they are ready for writing. A file descriptor is ready for writing if a write operation will not block. However, even if a file descriptor indicates as writable, a large write may still block. Afterselect() has returned,writefds will be cleared of all file descriptors except for those that are ready for writing.exceptfds The file descriptors in this set are watched for "exceptional conditions". For examples of some exceptional conditions, see the discussion ofPOLLPRIinpoll(2). Afterselect() has returned,exceptfds will be cleared of all file descriptors except for those for which an exceptional condition has occurred.nfds This argument should be set to the highest-numbered file descriptor in any of the three sets, plus 1. The indicated file descriptors in each set are checked, up to this limit (but see BUGS).timeout Thetimeout argument is atimeval structure (shown below) that specifies the interval thatselect() should block waiting for a file descriptor to become ready. The call will block until either: • a file descriptor becomes ready; • the call is interrupted by a signal handler; or • the timeout expires. Note that thetimeout interval will be rounded up to the system clock granularity, and kernel scheduling delays mean that the blocking interval may overrun by a small amount. If both fields of thetimeval structure are zero, thenselect() returns immediately. (This is useful for polling.) Iftimeout is specified as NULL,select() blocks indefinitely waiting for a file descriptor to become ready.pselect() Thepselect() system call allows an application to safely wait until either a file descriptor becomes ready or until a signal is caught. The operation ofselect() andpselect() is identical, other than these three differences: •select() uses a timeout that is astruct timeval (with seconds and microseconds), whilepselect() uses astruct timespec (with seconds and nanoseconds). •select() may update thetimeout argument to indicate how much time was left.pselect() does not change this argument. •select() has nosigmask argument, and behaves aspselect() called with NULLsigmask.sigmask is a pointer to a signal mask (seesigprocmask(2)); if it is not NULL, thenpselect() first replaces the current signal mask by the one pointed to bysigmask, then does the "select" function, and then restores the original signal mask. (Ifsigmask is NULL, the signal mask is not modified during thepselect() call.) Other than the difference in the precision of thetimeout argument, the followingpselect() call: ready = pselect(nfds, &readfds, &writefds, &exceptfds, timeout, &sigmask); is equivalent toatomically executing the following calls: sigset_t origmask; pthread_sigmask(SIG_SETMASK, &sigmask, &origmask); ready = select(nfds, &readfds, &writefds, &exceptfds, timeout); pthread_sigmask(SIG_SETMASK, &origmask, NULL); The reason thatpselect() is needed is that if one wants to wait for either a signal or for a file descriptor to become ready, then an atomic test is needed to prevent race conditions. (Suppose the signal handler sets a global flag and returns. Then a test of this global flag followed by a call ofselect() could hang indefinitely if the signal arrived just after the test but just before the call. By contrast,pselect() allows one to first block signals, handle the signals that have come in, then callpselect() with the desiredsigmask, avoiding the race.)The timeout Thetimeout argument forselect() is a structure of the following type: struct timeval { time_t tv_sec; /* seconds */ suseconds_t tv_usec; /* microseconds */ }; The corresponding argument forpselect() is atimespec(3) structure. On Linux,select() modifiestimeout to reflect the amount of time not slept; most other implementations do not do this. (POSIX.1 permits either behavior.) This causes problems both when Linux code which readstimeout is ported to other operating systems, and when code is ported to Linux that reuses astruct timeval for multipleselect()s in a loop without reinitializing it. Considertimeout to be undefined afterselect() returns.On success,select() andpselect() return the number of file descriptors contained in the three returned descriptor sets (that is, the total number of bits that are set inreadfds,writefds,exceptfds). The return value may be zero if the timeout expired before any file descriptors became ready. On error, -1 is returned, anderrno is set to indicate the error; the file descriptor sets are unmodified, andtimeout becomes undefined.
EBADFAn invalid file descriptor was given in one of the sets. (Perhaps a file descriptor that was already closed, or one on which an error has occurred.) However, see BUGS.EINTRA signal was caught; seesignal(7).EINVALnfds is negative or exceeds theRLIMIT_NOFILEresource limit (seegetrlimit(2)).EINVALThe value contained withintimeout is invalid.ENOMEMUnable to allocate memory for internal tables.
On some other UNIX systems,select() can fail with the errorEAGAINif the system fails to allocate kernel-internal resources, rather thanENOMEMas Linux does. POSIX specifies this error forpoll(2), but not forselect(). Portable programs may wish to check forEAGAINand loop, just as withEINTR.
POSIX.1-2008.
select() POSIX.1-2001, 4.4BSD (first appeared in 4.2BSD). Generally portable to/from non-BSD systems supporting clones of the BSD socket layer (including System V variants). However, note that the System V variant typically sets the timeout variable before returning, but the BSD variant does not.pselect() Linux 2.6.16. POSIX.1g, POSIX.1-2001. Prior to this, it was emulated in glibc (but see BUGS).fd_setPOSIX.1-2001.
The following header also provides thefd_set type:<sys/time.h>. Anfd_set is a fixed size buffer. ExecutingFD_CLR() orFD_SET() with a value offd that is negative or is equal to or larger thanFD_SETSIZEwill result in undefined behavior. Moreover, POSIX requiresfd to be a valid file descriptor. The operation ofselect() andpselect() is not affected by theO_NONBLOCKflag.The self-pipe trick On systems that lackpselect(), reliable (and more portable) signal trapping can be achieved using the self-pipe trick. In this technique, a signal handler writes a byte to a pipe whose other end is monitored byselect() in the main program. (To avoid possibly blocking when writing to a pipe that may be full or reading from a pipe that may be empty, nonblocking I/O is used when reading from and writing to the pipe.)Emulating usleep(3) Before the advent ofusleep(3), some code employed a call toselect() with all three sets empty,nfds zero, and a non-NULLtimeout as a fairly portable way to sleep with subsecond precision.Correspondence between select() and poll() notifications Within the Linux kernel source, we find the following definitions which show the correspondence between the readable, writable, and exceptional condition notifications ofselect() and the event notifications provided bypoll(2) andepoll(7): #define POLLIN_SET (EPOLLRDNORM | EPOLLRDBAND | EPOLLIN | EPOLLHUP | EPOLLERR) /* Ready for reading */ #define POLLOUT_SET (EPOLLWRBAND | EPOLLWRNORM | EPOLLOUT | EPOLLERR) /* Ready for writing */ #define POLLEX_SET (EPOLLPRI) /* Exceptional condition */Multithreaded applications If a file descriptor being monitored byselect() is closed in another thread, the result is unspecified. On some UNIX systems,select() unblocks and returns, with an indication that the file descriptor is ready (a subsequent I/O operation will likely fail with an error, unless another process reopens the file descriptor between the timeselect() returned and the I/O operation is performed). On Linux (and some other systems), closing the file descriptor in another thread has no effect onselect(). In summary, any application that relies on a particular behavior in this scenario must be considered buggy.C library/kernel differences The Linux kernel allows file descriptor sets of arbitrary size, determining the length of the sets to be checked from the value ofnfds. However, in the glibc implementation, thefd_set type is fixed in size. See also BUGS. Thepselect() interface described in this page is implemented by glibc. The underlying Linux system call is namedpselect6(). This system call has somewhat different behavior from the glibc wrapper function. The Linuxpselect6() system call modifies itstimeout argument. However, the glibc wrapper function hides this behavior by using a local variable for the timeout argument that is passed to the system call. Thus, the glibcpselect() function does not modify itstimeout argument; this is the behavior required by POSIX.1-2001. The final argument of thepselect6() system call is not asigset_t * pointer, but is instead a structure of the form: struct { const kernel_sigset_t *ss; /* Pointer to signal set */ size_t ss_len; /* Size (in bytes) of object pointed to by 'ss' */ }; This allows the system call to obtain both a pointer to the signal set and its size, while allowing for the fact that most architectures support a maximum of 6 arguments to a system call. Seesigprocmask(2) for a discussion of the difference between the kernel and libc notion of the signal set.Historical glibc details glibc 2.0 provided an incorrect version ofpselect() that did not take asigmask argument. From glibc 2.1 to glibc 2.2.1, one must define_GNU_SOURCEin order to obtain the declaration ofpselect() from<sys/select.h>.POSIX allows an implementation to define an upper limit, advertised via the constantFD_SETSIZE, on the range of file descriptors that can be specified in a file descriptor set. The Linux kernel imposes no fixed limit, but the glibc implementation makesfd_set a fixed-size type, withFD_SETSIZEdefined as 1024, and theFD_*() macros operating according to that limit. To monitor file descriptors greater than 1023, usepoll(2) orepoll(7) instead. The implementation of thefd_set arguments as value-result arguments is a design error that is avoided inpoll(2) andepoll(7). According to POSIX,select() should check all specified file descriptors in the three file descriptor sets, up to the limitnfds-1. However, the current implementation ignores any file descriptor in these sets that is greater than the maximum file descriptor number that the process currently has open. According to POSIX, any such file descriptor that is specified in one of the sets should result in the errorEBADF. Starting with glibc 2.1, glibc provided an emulation ofpselect() that was implemented usingsigprocmask(2) andselect(). This implementation remained vulnerable to the very race condition thatpselect() was designed to prevent. Modern versions of glibc use the (race-free)pselect() system call on kernels where it is provided. On Linux,select() may report a socket file descriptor as "ready for reading", while nevertheless a subsequent read blocks. This could for example happen when data has arrived but upon examination has the wrong checksum and is discarded. There may be other circumstances in which a file descriptor is spuriously reported as ready. Thus it may be safer to useO_NONBLOCKon sockets that should not block. On Linux,select() also modifiestimeout if the call is interrupted by a signal handler (i.e., theEINTRerror return). This is not permitted by POSIX.1. The Linuxpselect() system call has the same behavior, but the glibc wrapper hides this behavior by internally copying thetimeout to a local variable and passing that variable to the system call.
#include <stdio.h> #include <stdlib.h> #include <sys/select.h> #include <sys/time.h> int main(void) { int retval; fd_set rfds; struct timeval tv; /* Watch stdin (fd 0) to see when it has input. */ FD_ZERO(&rfds); FD_SET(0, &rfds); /* Wait up to five seconds. */ tv.tv_sec = 5; tv.tv_usec = 0; retval = select(1, &rfds, NULL, NULL, &tv); /* Don't rely on the value of tv now! */ if (retval == -1) perror("select()"); else if (retval) printf("Data is available now.\n"); /* FD_ISSET(0, &rfds) will be true. */ else printf("No data within five seconds.\n"); exit(EXIT_SUCCESS); }accept(2),connect(2),poll(2),read(2),recv(2),restart_syscall(2),send(2),sigprocmask(2),write(2),timespec(3),epoll(7),time(7) For a tutorial with discussion and examples, seeselect_tut(2).
This page is part of theman-pages (Linux kernel and C library user-space interface documentation) project. Information about the project can be found at ⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report for this manual page, see ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩. This page was obtained from the tarball man-pages-6.15.tar.gz fetched from ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on 2025-08-11. If you discover any rendering problems in this HTML version of the page, or you believe there is a better or more up- to-date source for the page, or you have corrections or improvements to the information in this COLOPHON (which isnot part of the original manual page), send a mail to man-pages@man7.orgLinux man-pages 6.15 2025-05-17select(2)Pages that refer to this page:pmdaamdgpu(1), pmdanvidia(1), pmdapipe(1), pmdasystemd(1), strace(1), accept(2), alarm(2), connect(2), epoll_wait(2), eventfd(2), F_GETSIG(2const), FUTEX_FD(2const), io_uring_enter2(2), io_uring_enter(2), migrate_pages(2), open(2), pause(2), perf_event_open(2), perfmonctl(2), personality(2), pidfd_open(2), poll(2), PR_SET_TIMERSLACK(2const), read(2), recv(2), restart_syscall(2), seccomp_unotify(2), select_tut(2), send(2), signalfd(2), socket(2), syscalls(2), timerfd_create(2), TIOCPKT(2const), userfaultfd(2), write(2), avc_netlink_loop(3), ldap_get_option(3), ldap_result(3), pcap(3pcap), pcap_get_required_select_timeout(3pcap), pcap_get_selectable_fd(3pcap), pmrecord(3), pmtime(3), rpc(3), sctp_connectx(3), timeval(3type), ualarm(3), usleep(3), random(4), rtc(4), ldap.conf(5), proc_pid_mounts(5), slapd-asyncmeta(5), slapd-ldap(5), slapd-meta(5), systemd.exec(5), epoll(7), fanotify(7), inotify(7), mq_overview(7), pipe(7), pty(7), signal(7), signal-safety(7), socket(7), system_data_types(7), tcp(7), time(7), udp(7), setarch(8)
HTML rendering created 2025-09-06 byMichael Kerrisk, author ofThe Linux Programming Interface. For details of in-depthLinux/UNIX system programming training courses that I teach, lookhere. Hosting byjambit GmbH. | |