Bug 40852 - select(2): document action of close()'ing a fd during a select()
Summary: select(2): document action of close()'ing a fd during a select()
Alias: None
Product: Documentation
Classification: Unclassified
Component: man-pages (show other bugs)
Hardware: All Linux
: P1 normal
Assignee: documentation_man-pages@kernel-bugs.osdl.org
Depends on:
Reported: 2011-08-10 13:28 UTC by Stephane Fillod
Modified: 2012-08-17 06:42 UTC (History)
1 user (show)

See Also:
Kernel Version: n/a
Tree: Mainline
Regression: No


Description Stephane Fillod 2011-08-10 13:28:24 UTC
I had a hard time figuring out what is the Linux's expected behavior of select() syscall when a file descriptor is close()'d in the meantime. The man page 3.27-1ubuntu2 lacks this piece of information. This is on Ubuntu 11.04.

Like many other OS, I would have expect select() to bail out on Linux, but it is not the case. When some other thread close() a file descriptor currently watched by select(), Linux does not make the select() syscall to return with errno=EBADF, it simply ignores it and does not return.
Please note that this behavior is undefined in POSIX. 

IMO, this has to be appropriately documented in the select(2) man page.

Maybe something like "The select() system call never returns if a file descriptor is closed in the meantime. This behavior is undefined in POSIX".[3]

Rem: this case is not to be confused with a file that has already been closed before calling select().


[1] http://stackoverflow.com/questions/543541/what-does-select2-do-if-you-close2-a-file-descriptor-in-a-separate-thread/543595#543595

[2] http://lkml.indiana.edu/hypermail/linux/kernel/0106.0/0768.html

[3] https://lists.linux-foundation.org/pipermail/bugme-new/2003-April/008335.html
Comment 1 Michael Kerrisk 2012-08-17 05:45:03 UTC
Confirmed, with the program below.

#include <sys/time.h>
#include <pthread.h>
#include <sys/types.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>

#define errExit(msg) 	do { perror(msg); exit(EXIT_FAILURE); \
			} while (0)

#define errExitEN(en, msg) \
    			do { errno = en; perror(msg); \
    			     exit(EXIT_FAILURE); } while (0)

static void *
threadFunc(void *arg)
    printf("Closing file descriptor\n");

    return NULL;
} /* threadFunc */

main(int argc, char *argv[])
    pthread_t t1;
    fd_set readfds;
    int ready, s;

    FD_SET(STDIN_FILENO, &readfds);

    s = pthread_create(&t1, NULL, threadFunc, (void *) 1);
    if (s != 0)
	errExitEN(s, "pthread_create");

    printf("Main thread about to call select()\n");

    ready = select(STDIN_FILENO + 1, &readfds, NULL, NULL, NULL);
    if (ready == -1)

    s = pthread_join(t1, NULL);
    if (s != 0)
	errExitEN(s, "pthread_join");

    printf("Main thread exiting\n");

Comment 2 Michael Kerrisk 2012-08-17 06:30:36 UTC
I added some text under NOTES in select(2).

--- a/man2/select.2
+++ b/man2/select.2
@@ -411,6 +411,22 @@ when
 is defined.
 Since glibc 2.2.2 the requirements are as shown in the SYNOPSIS.
+.SS Multithreaded applications
+If a file descriptor being monitored by
+.BR select ()
+is closed in another thread, the result is unspecified.
+On some UNIX systems,
+.BR 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 the file descriptor reopened between the time
+.BR select ()
+returned and the I/O operations was performed).
+On Linux (and some other systems),
+closing the file descriptor in another thread has no effect on
+.BR select ().
+In summary, any application that relies on a particular behavior
+in this scenario must be considered buggy.
 .SS "Linux Notes"
 .BR pselect ()
Comment 3 Michael Kerrisk 2012-08-17 06:38:16 UTC
Also added a note to poll(2) that points to the discussion in select(2).
Comment 4 Michael Kerrisk 2012-08-17 06:42:06 UTC
Also added a note to epoll_wait(2) that points to the discussion in select(2).

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