1SELECT_TUT(2) Linux Programmer's Manual SELECT_TUT(2)
2
6 select, pselect, FD_CLR, FD_ISSET, FD_SET, FD_ZERO - synchronous I/O
7 multiplexing
8
10 /* According to POSIX.1-2001, POSIX.1-2008 */
11 #include <sys/select.h>
12 /* According to earlier standards */
14 #include <sys/time.h>
15 #include <sys/types.h>
16 #include <unistd.h>
17 int select(int nfds, fd_set *readfds, fd_set *writefds,
19 fd_set *exceptfds, struct timeval *utimeout);
20 void FD_CLR(int fd, fd_set *set);
22 int FD_ISSET(int fd, fd_set *set);
23 void FD_SET(int fd, fd_set *set);
24 void FD_ZERO(fd_set *set);
25 #include <sys/select.h>
27 int pselect(int nfds, fd_set *readfds, fd_set *writefds,
29 fd_set *exceptfds, const struct timespec *ntimeout,
30 const sigset_t *sigmask);
31 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
33 pselect(): _POSIX_C_SOURCE >= 200112L
35
37 select() (or pselect()) is used to efficiently monitor multiple file
38 descriptors, to see if any of them is, or becomes, "ready"; that is, to
39 see whether I/O becomes possible, or an "exceptional condition" has
40 occurred on any of the file descriptors.
41 Its principal arguments are three "sets" of file descriptors: readfds,
43 writefds, and exceptfds. Each set is declared as type fd_set, and its
44 contents can be manipulated with the macros FD_CLR(), FD_ISSET(),
45 FD_SET(), and FD_ZERO(). A newly declared set should first be cleared
46 using FD_ZERO(). select() modifies the contents of the sets according
47 to the rules described below; after calling select() you can test if a
48 file descriptor is still present in a set with the FD_ISSET() macro.
49 FD_ISSET() returns nonzero if a specified file descriptor is present in
50 a set and zero if it is not. FD_CLR() removes a file descriptor from a
51 set.
52 Arguments
54 readfds
55 This set is watched to see if data is available for reading from
56 any of its file descriptors. After select() has returned,
57 readfds will be cleared of all file descriptors except for those
58 that are immediately available for reading.
59 writefds
61 This set is watched to see if there is space to write data to
62 any of its file descriptors. After select() has returned,
63 writefds will be cleared of all file descriptors except for
64 those that are immediately available for writing.
65 exceptfds
67 This set is watched for "exceptional conditions". In practice,
68 only one such exceptional condition is common: the availability
69 of out-of-band (OOB) data for reading from a TCP socket. See
70 recv(2), send(2), and tcp(7) for more details about OOB data.
71 (One other less common case where select(2) indicates an excep‐
72 tional condition occurs with pseudoterminals in packet mode; see
73 ioctl_tty(2).) After select() has returned, exceptfds will be
74 cleared of all file descriptors except for those for which an
75 exceptional condition has occurred.
76 nfds This is an integer one more than the maximum of any file
78 descriptor in any of the sets. In other words, while adding
79 file descriptors to each of the sets, you must calculate the
80 maximum integer value of all of them, then increment this value
81 by one, and then pass this as nfds.
82 utimeout
84 This is the longest time select() may wait before returning,
85 even if nothing interesting happened. If this value is passed
86 as NULL, then select() blocks indefinitely waiting for a file
87 descriptor to become ready. utimeout can be set to zero sec‐
88 onds, which causes select() to return immediately, with informa‐
89 tion about the readiness of file descriptors at the time of the
90 call. The structure struct timeval is defined as:
91 struct timeval {
93 time_t tv_sec; /* seconds */
94 long tv_usec; /* microseconds */
95 };
96 ntimeout
98 This argument for pselect() has the same meaning as utimeout,
99 but struct timespec has nanosecond precision as follows:
100 struct timespec {
102 long tv_sec; /* seconds */
103 long tv_nsec; /* nanoseconds */
104 };
105 sigmask
107 This argument holds a set of signals that the kernel should
108 unblock (i.e., remove from the signal mask of the calling
109 thread), while the caller is blocked inside the pselect() call
110 (see sigaddset(3) and sigprocmask(2)). It may be NULL, in which
111 case the call does not modify the signal mask on entry and exit
112 to the function. In this case, pselect() will then behave just
113 like select().
114 Combining signal and data events
116 pselect() is useful if you are waiting for a signal as well as for file
117 descriptor(s) to become ready for I/O. Programs that receive signals
118 normally use the signal handler only to raise a global flag. The
119 global flag will indicate that the event must be processed in the main
120 loop of the program. A signal will cause the select() (or pselect())
121 call to return with errno set to EINTR. This behavior is essential so
122 that signals can be processed in the main loop of the program, other‐
123 wise select() would block indefinitely. Now, somewhere in the main
124 loop will be a conditional to check the global flag. So we must ask:
125 what if a signal arrives after the conditional, but before the select()
126 call? The answer is that select() would block indefinitely, even
127 though an event is actually pending. This race condition is solved by
128 the pselect() call. This call can be used to set the signal mask to a
129 set of signals that are to be received only within the pselect() call.
130 For instance, let us say that the event in question was the exit of a
131 child process. Before the start of the main loop, we would block
132 SIGCHLD using sigprocmask(2). Our pselect() call would enable SIGCHLD
133 by using an empty signal mask. Our program would look like:
134 static volatile sig_atomic_t got_SIGCHLD = 0;
136 static void
138 child_sig_handler(int sig)
139 {
140 got_SIGCHLD = 1;
141 }
142 int
144 main(int argc, char *argv[])
145 {
146 sigset_t sigmask, empty_mask;
147 struct sigaction sa;
148 fd_set readfds, writefds, exceptfds;
149 int r;
150 sigemptyset(&sigmask);
152 sigaddset(&sigmask, SIGCHLD);
153 if (sigprocmask(SIG_BLOCK, &sigmask, NULL) == -1) {
154 perror("sigprocmask");
155 exit(EXIT_FAILURE);
156 }
157 sa.sa_flags = 0;
159 sa.sa_handler = child_sig_handler;
160 sigemptyset(&sa.sa_mask);
161 if (sigaction(SIGCHLD, &sa, NULL) == -1) {
162 perror("sigaction");
163 exit(EXIT_FAILURE);
164 }
165 sigemptyset(&empty_mask);
167 for (;;) { /* main loop */
169 /* Initialize readfds, writefds, and exceptfds
170 before the pselect() call. (Code omitted.) */
171 r = pselect(nfds, &readfds, &writefds, &exceptfds,
173 NULL, &empty_mask);
174 if (r == -1 && errno != EINTR) {
175 /* Handle error */
176 }
177 if (got_SIGCHLD) {
179 got_SIGCHLD = 0;
180 /* Handle signalled event here; e.g., wait() for all
182 terminated children. (Code omitted.) */
183 }
184 /* main body of program */
186 }
187 }
188 Practical
190 So what is the point of select()? Can't I just read and write to my
191 file descriptors whenever I want? The point of select() is that it
192 watches multiple descriptors at the same time and properly puts the
193 process to sleep if there is no activity. UNIX programmers often find
194 themselves in a position where they have to handle I/O from more than
195 one file descriptor where the data flow may be intermittent. If you
196 were to merely create a sequence of read(2) and write(2) calls, you
197 would find that one of your calls may block waiting for data from/to a
198 file descriptor, while another file descriptor is unused though ready
199 for I/O. select() efficiently copes with this situation.
200 Select law
202 Many people who try to use select() come across behavior that is diffi‐
203 cult to understand and produces nonportable or borderline results. For
204 instance, the above program is carefully written not to block at any
205 point, even though it does not set its file descriptors to nonblocking
206 mode. It is easy to introduce subtle errors that will remove the
207 advantage of using select(), so here is a list of essentials to watch
208 for when using select().
209 1. You should always try to use select() without a timeout. Your pro‐
211 gram should have nothing to do if there is no data available. Code
212 that depends on timeouts is not usually portable and is difficult
213 to debug.
214 2. The value nfds must be properly calculated for efficiency as
216 explained above.
217 3. No file descriptor must be added to any set if you do not intend to
219 check its result after the select() call, and respond appropri‐
220 ately. See next rule.
221 4. After select() returns, all file descriptors in all sets should be
223 checked to see if they are ready.
224 5. The functions read(2), recv(2), write(2), and send(2) do not neces‐
226 sarily read/write the full amount of data that you have requested.
227 If they do read/write the full amount, it's because you have a low
228 traffic load and a fast stream. This is not always going to be the
229 case. You should cope with the case of your functions managing to
230 send or receive only a single byte.
231 6. Never read/write only in single bytes at a time unless you are
233 really sure that you have a small amount of data to process. It is
234 extremely inefficient not to read/write as much data as you can
235 buffer each time. The buffers in the example below are 1024 bytes
236 although they could easily be made larger.
237 7. Calls to read(2), recv(2), write(2), send(2), and select() can fail
239 with the error EINTR, and calls to read(2), recv(2) write(2), and
240 send(2) can fail with errno set to EAGAIN (EWOULDBLOCK). These
241 results must be properly managed (not done properly above). If
242 your program is not going to receive any signals, then it is
243 unlikely you will get EINTR. If your program does not set non‐
244 blocking I/O, you will not get EAGAIN.
245 8. Never call read(2), recv(2), write(2), or send(2) with a buffer
247 length of zero.
248 9. If the functions read(2), recv(2), write(2), and send(2) fail with
250 errors other than those listed in 7., or one of the input functions
251 returns 0, indicating end of file, then you should not pass that
252 file descriptor to select() again. In the example below, I close
253 the file descriptor immediately, and then set it to -1 to prevent
254 it being included in a set.
255 10. The timeout value must be initialized with each new call to
257 select(), since some operating systems modify the structure. pse‐
258 lect() however does not modify its timeout structure.
259 11. Since select() modifies its file descriptor sets, if the call is
261 being used in a loop, then the sets must be reinitialized before
262 each call.
263 Usleep emulation
265 On systems that do not have a usleep(3) function, you can call select()
266 with a finite timeout and no file descriptors as follows:
267 struct timeval tv;
269 tv.tv_sec = 0;
270 tv.tv_usec = 200000; /* 0.2 seconds */
271 select(0, NULL, NULL, NULL, &tv);
272 This is guaranteed to work only on UNIX systems, however.
274
276 On success, select() returns the total number of file descriptors still
277 present in the file descriptor sets.
278 If select() timed out, then the return value will be zero. The file
280 descriptors set should be all empty (but may not be on some systems).
281 A return value of -1 indicates an error, with errno being set appropri‐
283 ately. In the case of an error, the contents of the returned sets and
284 the struct timeout contents are undefined and should not be used. pse‐
285 lect() however never modifies ntimeout.
286
288 Generally speaking, all operating systems that support sockets also
289 support select(). select() can be used to solve many problems in a
290 portable and efficient way that naive programmers try to solve in a
291 more complicated manner using threads, forking, IPCs, signals, memory
292 sharing, and so on.
293 The poll(2) system call has the same functionality as select(), and is
295 somewhat more efficient when monitoring sparse file descriptor sets.
296 It is nowadays widely available, but historically was less portable
297 than select().
298 The Linux-specific epoll(7) API provides an interface that is more
300 efficient than select(2) and poll(2) when monitoring large numbers of
301 file descriptors.
302
304 Here is an example that better demonstrates the true utility of
305 select(). The listing below is a TCP forwarding program that forwards
306 from one TCP port to another.
307 #include <stdlib.h>
309 #include <stdio.h>
310 #include <unistd.h>
311 #include <sys/time.h>
312 #include <sys/types.h>
313 #include <string.h>
314 #include <signal.h>
315 #include <sys/socket.h>
316 #include <netinet/in.h>
317 #include <arpa/inet.h>
318 #include <errno.h>
319 static int forward_port;
321 #undef max
323 #define max(x,y) ((x) > (y) ? (x) : (y))
324 static int
326 listen_socket(int listen_port)
327 {
328 struct sockaddr_in addr;
329 int lfd;
330 int yes;
331 lfd = socket(AF_INET, SOCK_STREAM, 0);
333 if (lfd == -1) {
334 perror("socket");
335 return -1;
336 }
337 yes = 1;
339 if (setsockopt(lfd, SOL_SOCKET, SO_REUSEADDR,
340 &yes, sizeof(yes)) == -1) {
341 perror("setsockopt");
342 close(lfd);
343 return -1;
344 }
345 memset(&addr, 0, sizeof(addr));
347 addr.sin_port = htons(listen_port);
348 addr.sin_family = AF_INET;
349 if (bind(lfd, (struct sockaddr *) &addr, sizeof(addr)) == -1) {
350 perror("bind");
351 close(lfd);
352 return -1;
353 }
354 printf("accepting connections on port %d\n", listen_port);
356 listen(lfd, 10);
357 return lfd;
358 }
359 static int
361 connect_socket(int connect_port, char *address)
362 {
363 struct sockaddr_in addr;
364 int cfd;
365 cfd = socket(AF_INET, SOCK_STREAM, 0);
367 if (cfd == -1) {
368 perror("socket");
369 return -1;
370 }
371 memset(&addr, 0, sizeof(addr));
373 addr.sin_port = htons(connect_port);
374 addr.sin_family = AF_INET;
375 if (!inet_aton(address, (struct in_addr *) &addr.sin_addr.s_addr)) {
377 perror("bad IP address format");
378 close(cfd);
379 return -1;
380 }
381 if (connect(cfd, (struct sockaddr *) &addr, sizeof(addr)) == -1) {
383 perror("connect()");
384 shutdown(cfd, SHUT_RDWR);
385 close(cfd);
386 return -1;
387 }
388 return cfd;
389 }
390 #define SHUT_FD1 do { \
392 if (fd1 >= 0) { \
393 shutdown(fd1, SHUT_RDWR); \
394 close(fd1); \
395 fd1 = -1; \
396 } \
397 } while (0)
398 #define SHUT_FD2 do { \
400 if (fd2 >= 0) { \
401 shutdown(fd2, SHUT_RDWR); \
402 close(fd2); \
403 fd2 = -1; \
404 } \
405 } while (0)
406 #define BUF_SIZE 1024
408 int
410 main(int argc, char *argv[])
411 {
412 int h;
413 int fd1 = -1, fd2 = -1;
414 char buf1[BUF_SIZE], buf2[BUF_SIZE];
415 int buf1_avail = 0, buf1_written = 0;
416 int buf2_avail = 0, buf2_written = 0;
417 if (argc != 4) {
419 fprintf(stderr, "Usage\n\tfwd <listen-port> "
420 "<forward-to-port> <forward-to-ip-address>\n");
421 exit(EXIT_FAILURE);
422 }
423 signal(SIGPIPE, SIG_IGN);
425 forward_port = atoi(argv[2]);
427 h = listen_socket(atoi(argv[1]));
429 if (h == -1)
430 exit(EXIT_FAILURE);
431 for (;;) {
433 int ready, nfds = 0;
434 ssize_t nbytes;
435 fd_set readfds, writefds, exceptfds;
436 FD_ZERO(&readfds);
438 FD_ZERO(&writefds);
439 FD_ZERO(&exceptfds);
440 FD_SET(h, &readfds);
441 nfds = max(nfds, h);
442 if (fd1 > 0 && buf1_avail < BUF_SIZE)
444 FD_SET(fd1, &readfds);
445 /* Note: nfds is updated below, when fd1 is added to
446 exceptfds. */
447 if (fd2 > 0 && buf2_avail < BUF_SIZE)
448 FD_SET(fd2, &readfds);
449 if (fd1 > 0 && buf2_avail - buf2_written > 0)
451 FD_SET(fd1, &writefds);
452 if (fd2 > 0 && buf1_avail - buf1_written > 0)
453 FD_SET(fd2, &writefds);
454 if (fd1 > 0) {
456 FD_SET(fd1, &exceptfds);
457 nfds = max(nfds, fd1);
458 }
459 if (fd2 > 0) {
460 FD_SET(fd2, &exceptfds);
461 nfds = max(nfds, fd2);
462 }
463 ready = select(nfds + 1, &readfds, &writefds, &exceptfds, NULL);
465 if (ready == -1 && errno == EINTR)
467 continue;
468 if (ready == -1) {
470 perror("select()");
471 exit(EXIT_FAILURE);
472 }
473 if (FD_ISSET(h, &readfds)) {
475 socklen_t addrlen;
476 struct sockaddr_in client_addr;
477 int fd;
478 addrlen = sizeof(client_addr);
480 memset(&client_addr, 0, addrlen);
481 fd = accept(h, (struct sockaddr *) &client_addr, &addrlen);
482 if (fd == -1) {
483 perror("accept()");
484 } else {
485 SHUT_FD1;
486 SHUT_FD2;
487 buf1_avail = buf1_written = 0;
488 buf2_avail = buf2_written = 0;
489 fd1 = fd;
490 fd2 = connect_socket(forward_port, argv[3]);
491 if (fd2 == -1)
492 SHUT_FD1;
493 else
494 printf("connect from %s\n",
495 inet_ntoa(client_addr.sin_addr));
496 /* Skip any events on the old, closed file descriptors. */
498 continue;
499 }
500 }
501 /* NB: read OOB data before normal reads */
503 if (fd1 > 0 && FD_ISSET(fd1, &exceptfds)) {
505 char c;
506 nbytes = recv(fd1, &c, 1, MSG_OOB);
508 if (nbytes < 1)
509 SHUT_FD1;
510 else
511 send(fd2, &c, 1, MSG_OOB);
512 }
513 if (fd2 > 0 && FD_ISSET(fd2, &exceptfds)) {
514 char c;
515 nbytes = recv(fd2, &c, 1, MSG_OOB);
517 if (nbytes < 1)
518 SHUT_FD2;
519 else
520 send(fd1, &c, 1, MSG_OOB);
521 }
522 if (fd1 > 0 && FD_ISSET(fd1, &readfds)) {
523 nbytes = read(fd1, buf1 + buf1_avail,
524 BUF_SIZE - buf1_avail);
525 if (nbytes < 1)
526 SHUT_FD1;
527 else
528 buf1_avail += nbytes;
529 }
530 if (fd2 > 0 && FD_ISSET(fd2, &readfds)) {
531 nbytes = read(fd2, buf2 + buf2_avail,
532 BUF_SIZE - buf2_avail);
533 if (nbytes < 1)
534 SHUT_FD2;
535 else
536 buf2_avail += nbytes;
537 }
538 if (fd1 > 0 && FD_ISSET(fd1, &writefds) && buf2_avail > 0) {
539 nbytes = write(fd1, buf2 + buf2_written,
540 buf2_avail - buf2_written);
541 if (nbytes < 1)
542 SHUT_FD1;
543 else
544 buf2_written += nbytes;
545 }
546 if (fd2 > 0 && FD_ISSET(fd2, &writefds) && buf1_avail > 0) {
547 nbytes = write(fd2, buf1 + buf1_written,
548 buf1_avail - buf1_written);
549 if (nbytes < 1)
550 SHUT_FD2;
551 else
552 buf1_written += nbytes;
553 }
554 /* Check if write data has caught read data */
556 if (buf1_written == buf1_avail)
558 buf1_written = buf1_avail = 0;
559 if (buf2_written == buf2_avail)
560 buf2_written = buf2_avail = 0;
561 /* One side has closed the connection, keep
563 writing to the other side until empty */
564 if (fd1 < 0 && buf1_avail - buf1_written == 0)
566 SHUT_FD2;
567 if (fd2 < 0 && buf2_avail - buf2_written == 0)
568 SHUT_FD1;
569 }
570 exit(EXIT_SUCCESS);
571 }
572 The above program properly forwards most kinds of TCP connections
574 including OOB signal data transmitted by telnet servers. It handles
575 the tricky problem of having data flow in both directions simultane‐
576 ously. You might think it more efficient to use a fork(2) call and
577 devote a thread to each stream. This becomes more tricky than you
578 might suspect. Another idea is to set nonblocking I/O using fcntl(2).
579 This also has its problems because you end up using inefficient time‐
580 outs.
581 The program does not handle more than one simultaneous connection at a
583 time, although it could easily be extended to do this with a linked
584 list of buffers—one for each connection. At the moment, new connec‐
585 tions cause the current connection to be dropped.
586
588 accept(2), connect(2), ioctl(2), poll(2), read(2), recv(2), select(2),
589 send(2), sigprocmask(2), write(2), sigaddset(3), sigdelset(3), sigemp‐
590 tyset(3), sigfillset(3), sigismember(3), epoll(7)
591
593 This page is part of release 4.16 of the Linux man-pages project. A
594 description of the project, information about reporting bugs, and the
595 latest version of this page, can be found at
596 https://www.kernel.org/doc/man-pages/.
597Linux 2017-09-15 SELECT_TUT(2)