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NAME |LIBRARY |SYNOPSIS |DESCRIPTION |RETURN VALUE |NOTES |EXAMPLES |SEE ALSO |COLOPHON | |
SELECT_TUT(2) System Calls ManualSELECT_TUT(2)select, pselect - synchronous I/O multiplexing
Standard C library (libc,-lc)
Seeselect(2)
Theselect() andpselect() system calls are used to efficiently monitor multiple file descriptors, to see if any of them is, or becomes, "ready"; that is, to see whether I/O becomes possible, or an "exceptional condition" has occurred on any of the file descriptors. This page provides background and tutorial information on the use of these system calls. For details of the arguments and semantics ofselect() andpselect(), seeselect(2).Combining signal and data eventspselect() is useful if you are waiting for a signal as well as for file descriptor(s) to become ready for I/O. Programs that receive signals normally use the signal handler only to raise a global flag. The global flag will indicate that the event must be processed in the main loop of the program. A signal will cause theselect() (orpselect()) call to return witherrno set toEINTR. This behavior is essential so that signals can be processed in the main loop of the program, otherwiseselect() would block indefinitely. Now, somewhere in the main loop will be a conditional to check the global flag. So we must ask: what if a signal arrives after the conditional, but before theselect() call? The answer is thatselect() would block indefinitely, even though an event is actually pending. This race condition is solved by thepselect() call. This call can be used to set the signal mask to a set of signals that are to be received only within thepselect() call. For instance, let us say that the event in question was the exit of a child process. Before the start of the main loop, we would blockSIGCHLDusingsigprocmask(2). Ourpselect() call would enableSIGCHLDby using an empty signal mask. Our program would look like: static volatile sig_atomic_t got_SIGCHLD = 0; static void child_sig_handler(int sig) { got_SIGCHLD = 1; } int main(int argc, char *argv[]) { sigset_t sigmask, empty_mask; struct sigaction sa; fd_set readfds, writefds, exceptfds; int r; sigemptyset(&sigmask); sigaddset(&sigmask, SIGCHLD); if (sigprocmask(SIG_BLOCK, &sigmask, NULL) == -1) { perror("sigprocmask"); exit(EXIT_FAILURE); } sa.sa_flags = 0; sa.sa_handler = child_sig_handler; sigemptyset(&sa.sa_mask); if (sigaction(SIGCHLD, &sa, NULL) == -1) { perror("sigaction"); exit(EXIT_FAILURE); } sigemptyset(&empty_mask); for (;;) { /* main loop */ /* Initialize readfds, writefds, and exceptfds before the pselect() call. (Code omitted.) */ r = pselect(nfds, &readfds, &writefds, &exceptfds, NULL, &empty_mask); if (r == -1 && errno != EINTR) { /* Handle error */ } if (got_SIGCHLD) { got_SIGCHLD = 0; /* Handle signalled event here; e.g., wait() for all terminated children. (Code omitted.) */ } /* main body of program */ } }Practical So what is the point ofselect()? Can't I just read and write to my file descriptors whenever I want? The point ofselect() is that it watches multiple descriptors at the same time and properly puts the process to sleep if there is no activity. UNIX programmers often find themselves in a position where they have to handle I/O from more than one file descriptor where the data flow may be intermittent. If you were to merely create a sequence ofread(2) andwrite(2) calls, you would find that one of your calls may block waiting for data from/to a file descriptor, while another file descriptor is unused though ready for I/O.select() efficiently copes with this situation.Select law Many people who try to useselect() come across behavior that is difficult to understand and produces nonportable or borderline results. For instance, the above program is carefully written not to block at any point, even though it does not set its file descriptors to nonblocking mode. It is easy to introduce subtle errors that will remove the advantage of usingselect(), so here is a list of essentials to watch for when usingselect(). 1. You should always try to useselect() without a timeout. Your program should have nothing to do if there is no data available. Code that depends on timeouts is not usually portable and is difficult to debug. 2. The valuenfds must be properly calculated for efficiency as explained above. 3. No file descriptor must be added to any set if you do not intend to check its result after theselect() call, and respond appropriately. See next rule. 4. Afterselect() returns, all file descriptors in all sets should be checked to see if they are ready. 5. The functionsread(2),recv(2),write(2), andsend(2) donot necessarily read/write the full amount of data that you have requested. If they do read/write the full amount, it's because you have a low traffic load and a fast stream. This is not always going to be the case. You should cope with the case of your functions managing to send or receive only a single byte. 6. Never read/write only in single bytes at a time unless you are really sure that you have a small amount of data to process. It is extremely inefficient not to read/write as much data as you can buffer each time. The buffers in the example below are 1024 bytes although they could easily be made larger. 7. Calls toread(2),recv(2),write(2),send(2), andselect() can fail with the errorEINTR, and calls toread(2),recv(2),write(2), andsend(2) can fail witherrno set toEAGAIN (EWOULDBLOCK). These results must be properly managed (not done properly above). If your program is not going to receive any signals, then it is unlikely you will getEINTR. If your program does not set nonblocking I/O, you will not getEAGAIN. 8. Never callread(2),recv(2),write(2), orsend(2) with a buffer length of zero. 9. If the functionsread(2),recv(2),write(2), andsend(2) fail with errors other than those listed in7., or one of the input functions returns 0, indicating end of file, then you shouldnot pass that file descriptor toselect() again. In the example below, I close the file descriptor immediately, and then set it to -1 to prevent it being included in a set. 10. The timeout value must be initialized with each new call toselect(), since some operating systems modify the structure.pselect() however does not modify its timeout structure. 11. Sinceselect() modifies its file descriptor sets, if the call is being used in a loop, then the sets must be reinitialized before each call.Seeselect(2).
Generally speaking, all operating systems that support sockets also supportselect().select() can be used to solve many problems in a portable and efficient way that naive programmers try to solve in a more complicated manner using threads, forking, IPCs, signals, memory sharing, and so on. Thepoll(2) system call has the same functionality asselect(), and is somewhat more efficient when monitoring sparse file descriptor sets. It is nowadays widely available, but historically was less portable thanselect(). The Linux-specificepoll(7) API provides an interface that is more efficient thanselect(2) andpoll(2) when monitoring large numbers of file descriptors.
Here is an example that better demonstrates the true utility ofselect(). The listing below is a TCP forwarding program that forwards from one TCP port to another. #include <arpa/inet.h> #include <errno.h> #include <netinet/in.h> #include <signal.h> #include <stdio.h> #include <stdlib.h> #include <string.h> #include <sys/select.h> #include <sys/socket.h> #include <sys/types.h> #include <unistd.h> static int forward_port; #undef max #define max(x, y) ((x) > (y) ? (x) : (y)) static int listen_socket(int listen_port) { int lfd; int yes; struct sockaddr_in addr; lfd = socket(AF_INET, SOCK_STREAM, 0); if (lfd == -1) { perror("socket"); return -1; } yes = 1; if (setsockopt(lfd, SOL_SOCKET, SO_REUSEADDR, &yes, sizeof(yes)) == -1) { perror("setsockopt"); close(lfd); return -1; } memset(&addr, 0, sizeof(addr)); addr.sin_port = htons(listen_port); addr.sin_family = AF_INET; if (bind(lfd, (struct sockaddr *) &addr, sizeof(addr)) == -1) { perror("bind"); close(lfd); return -1; } printf("accepting connections on port %d\n", listen_port); listen(lfd, 10); return lfd; } static int connect_socket(int connect_port, char *address) { int cfd; struct sockaddr_in addr; cfd = socket(AF_INET, SOCK_STREAM, 0); if (cfd == -1) { perror("socket"); return -1; } memset(&addr, 0, sizeof(addr)); addr.sin_port = htons(connect_port); addr.sin_family = AF_INET; if (!inet_aton(address, (struct in_addr *) &addr.sin_addr.s_addr)) { fprintf(stderr, "inet_aton(): bad IP address format\n"); close(cfd); return -1; } if (connect(cfd, (struct sockaddr *) &addr, sizeof(addr)) == -1) { perror("connect()"); shutdown(cfd, SHUT_RDWR); close(cfd); return -1; } return cfd; } #define SHUT_FD1 do { \ if (fd1 >= 0) { \ shutdown(fd1, SHUT_RDWR); \ close(fd1); \ fd1 = -1; \ } \ } while (0) #define SHUT_FD2 do { \ if (fd2 >= 0) { \ shutdown(fd2, SHUT_RDWR); \ close(fd2); \ fd2 = -1; \ } \ } while (0) #define BUF_SIZE 1024 int main(int argc, char *argv[]) { int h; int ready, nfds; int fd1 = -1, fd2 = -1; int buf1_avail = 0, buf1_written = 0; int buf2_avail = 0, buf2_written = 0; char buf1[BUF_SIZE], buf2[BUF_SIZE]; fd_set readfds, writefds, exceptfds; ssize_t nbytes; if (argc != 4) { fprintf(stderr, "Usage\n\tfwd <listen-port> " "<forward-to-port> <forward-to-ip-address>\n"); exit(EXIT_FAILURE); } signal(SIGPIPE, SIG_IGN); forward_port = atoi(argv[2]); h = listen_socket(atoi(argv[1])); if (h == -1) exit(EXIT_FAILURE); for (;;) { nfds = 0; FD_ZERO(&readfds); FD_ZERO(&writefds); FD_ZERO(&exceptfds); FD_SET(h, &readfds); nfds = max(nfds, h); if (fd1 > 0 && buf1_avail < BUF_SIZE) FD_SET(fd1, &readfds); /* Note: nfds is updated below, when fd1 is added to exceptfds. */ if (fd2 > 0 && buf2_avail < BUF_SIZE) FD_SET(fd2, &readfds); if (fd1 > 0 && buf2_avail - buf2_written > 0) FD_SET(fd1, &writefds); if (fd2 > 0 && buf1_avail - buf1_written > 0) FD_SET(fd2, &writefds); if (fd1 > 0) { FD_SET(fd1, &exceptfds); nfds = max(nfds, fd1); } if (fd2 > 0) { FD_SET(fd2, &exceptfds); nfds = max(nfds, fd2); } ready = select(nfds + 1, &readfds, &writefds, &exceptfds, NULL); if (ready == -1 && errno == EINTR) continue; if (ready == -1) { perror("select()"); exit(EXIT_FAILURE); } if (FD_ISSET(h, &readfds)) { socklen_t addrlen; struct sockaddr_in client_addr; int fd; addrlen = sizeof(client_addr); memset(&client_addr, 0, addrlen); fd = accept(h, (struct sockaddr *) &client_addr, &addrlen); if (fd == -1) { perror("accept()"); } else { SHUT_FD1; SHUT_FD2; buf1_avail = buf1_written = 0; buf2_avail = buf2_written = 0; fd1 = fd; fd2 = connect_socket(forward_port, argv[3]); if (fd2 == -1) SHUT_FD1; else printf("connect from %s\n", inet_ntoa(client_addr.sin_addr)); /* Skip any events on the old, closed file descriptors. */ continue; } } /* NB: read OOB data before normal reads. */ if (fd1 > 0 && FD_ISSET(fd1, &exceptfds)) { char c; nbytes = recv(fd1, &c, 1, MSG_OOB); if (nbytes < 1) SHUT_FD1; else send(fd2, &c, 1, MSG_OOB); } if (fd2 > 0 && FD_ISSET(fd2, &exceptfds)) { char c; nbytes = recv(fd2, &c, 1, MSG_OOB); if (nbytes < 1) SHUT_FD2; else send(fd1, &c, 1, MSG_OOB); } if (fd1 > 0 && FD_ISSET(fd1, &readfds)) { nbytes = read(fd1, buf1 + buf1_avail, BUF_SIZE - buf1_avail); if (nbytes < 1) SHUT_FD1; else buf1_avail += nbytes; } if (fd2 > 0 && FD_ISSET(fd2, &readfds)) { nbytes = read(fd2, buf2 + buf2_avail, BUF_SIZE - buf2_avail); if (nbytes < 1) SHUT_FD2; else buf2_avail += nbytes; } if (fd1 > 0 && FD_ISSET(fd1, &writefds) && buf2_avail > 0) { nbytes = write(fd1, buf2 + buf2_written, buf2_avail - buf2_written); if (nbytes < 1) SHUT_FD1; else buf2_written += nbytes; } if (fd2 > 0 && FD_ISSET(fd2, &writefds) && buf1_avail > 0) { nbytes = write(fd2, buf1 + buf1_written, buf1_avail - buf1_written); if (nbytes < 1) SHUT_FD2; else buf1_written += nbytes; } /* Check if write data has caught read data. */ if (buf1_written == buf1_avail) buf1_written = buf1_avail = 0; if (buf2_written == buf2_avail) buf2_written = buf2_avail = 0; /* One side has closed the connection, keep writing to the other side until empty. */ if (fd1 < 0 && buf1_avail - buf1_written == 0) SHUT_FD2; if (fd2 < 0 && buf2_avail - buf2_written == 0) SHUT_FD1; } exit(EXIT_SUCCESS); } The above program properly forwards most kinds of TCP connections including OOB signal data transmitted bytelnetservers. It handles the tricky problem of having data flow in both directions simultaneously. You might think it more efficient to use afork(2) call and devote a thread to each stream. This becomes more tricky than you might suspect. Another idea is to set nonblocking I/O usingfcntl(2). This also has its problems because you end up using inefficient timeouts. The program does not handle more than one simultaneous connection at a time, although it could easily be extended to do this with a linked list of buffers—one for each connection. At the moment, new connections cause the current connection to be dropped.accept(2),connect(2),poll(2),read(2),recv(2),select(2),send(2),sigprocmask(2),write(2),epoll(7)
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_TUT(2)Pages that refer to this page:poll(2), select(2)
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. | |