https://bugzilla.kernel.org/show_bug.cgi?id=95331 Bug ID: 95331 Summary: fcntl.2 + sigaction.2 + signal.7 need further information about use of a SA_SIGINFO signal handler that uses si->si_fd Product: Documentation Version: unspecified Hardware: All OS: Linux Status: NEW Severity: normal Priority: P1 Component: man-pages Assignee: documentation_man-pages@xxxxxxxxxxxxxxxxxxxx Reporter: jason.vas.dias@xxxxxxxxx Regression: No Created attachment 171831 --> https://bugzilla.kernel.org/attachment.cgi?id=171831&action=edit File that demonstrates the problem Despite extensive reading of latest versions of the $subject manual pages over the last few days while trying to debug a program that needs to read and write pipe file descriptors, I am still none the wiser about the precise circumstances that the $signum signal, used in a : fcntl(fd, F_SETSIG, $signum ) call, will be sent for an OUTPUT file descriptor. The test program, which I will attach, which is run as : $ mkfifo -m 0600 /tmp/f.in /tmp/f.out $ ./t_sigio_rdwr </tmp/f.in >/tmp/f.out & $ echo 'hello' >/tmp/f.in ../ $ read </tmp/f.out ^C only gets a SIGIO+SIGRTMIN signal for the output file descriptor (FD 1) when a reader DISCONNECTS from its output pipe, by pressing <CTRL+C> - which is pretty useless, IMHO - the program is expecting to receive a SIGIO+RTMIN signal when a reader CONNECTs (opens) its output fifo. so that it can send the response. All the documentation states is in the sigaction manual page: * SIGIO/SIGPOLL (the two names are synonyms on Linux) fills in si_band and si_fd. The si_band event is a bit mask containing the same values as are filled in the revents field by poll(2). The si_fd field indicates the file descriptor for which the I/O event occurred. ... si_code: .. SI_SIGIO Queued SIGIO (only in kernels up to Linux 2.2; from Linux 2.4 onward SIGIO/SIGPOLL fills in si_code as described below). The following values can be placed in si_code for a SIGIO/SIGPOLL signal: POLL_IN Data input available. POLL_OUT Output buffers available. And in fcntl.2 : F_SETSIG (int) Set the signal sent when input or output becomes possible to the value given in arg. A value of zero means to send the default SIGIO signal. Any other value (including SIGIO) is the signal to send instead, and in this case additional info is available to the signal handler if installed with SA_SIGINFO. By using F_SETSIG with a nonzero value, and setting SA_SIGINFO for the signal handler (see sigaction(2)), extra information about I/O events is passed to the handler in a siginfo_t structure. If the si_code field indicates the source is SI_SIGIO, the si_fd field gives the file descriptor associated with the event. Otherwise, there is no indication which file descriptors are pending, and you should use the usual mechanisms (select(2), poll(2), read(2) with O_NONBLOCK set etc.) to determine which file descriptors are available for I/O. By selecting a real time signal (value >= SIGRTMIN), multiple I/O events may be queued using the same signal numbers. (Queuing is dependent on available mem‐ ory). Extra information is available if SA_SIGINFO is set for the signal han‐ dler, as above. Nowhere does it say WHEN or on what conditions the SIGIO will be generated, or is it discussed what happens when SIGIO for multiple FDs are queued. The following strace output of the program demonstrates the problem: [pid 11215] rt_sigprocmask(SIG_BLOCK, [PIPE RT_31], [], 8) = 0 [pid 11215] fcntl(1, F_GETFL) = 0x8001 (flags O_WRONLY|O_LARGEFILE) [pid 11215] fcntl(1, F_SETFL, O_WRONLY|O_ASYNC|O_LARGEFILE) = 0 [pid 11215] fcntl(1, 0xf /* F_??? */, 0x7fff1c940090) = 0 [pid 11215] fcntl(1, F_SETSIG, 0x3f) = 0 [pid 11215] fcntl(0, F_GETFL) = 0x8000 (flags O_RDONLY|O_LARGEFILE) [pid 11215] fcntl(0, F_SETFL, O_WRONLY|O_ASYNC|O_LARGEFILE) = 0 [pid 11215] fcntl(0, 0xf /* F_??? */, 0x7fff1c940090) = 0 [pid 11215] fcntl(0, F_SETSIG, 0x3f) = 0 [pid 11215] rt_sigaction(SIGRT_31, {0x400b4d, [], SA_RESTORER|SA_STACK|SA_NODEFER|SA_SIGINFO, 0x7fc6c47f0340}, {SIG_DFL, [], 0}, 8) = 0 [pid 11215] rt_sigaction(SIGPIPE, {0x400e49, [], SA_RESTORER|SA_STACK|SA_NODEFER|SA_SIGINFO, 0x7fc6c47f0340}, {SIG_DFL, [], 0}, 8) = 0 [pid 11215] fcntl(1, F_SETFL, O_WRONLY|O_LARGEFILE) = 0 [pid 11215] write(2, "reading\n", 8reading ) = 8 [pid 11215] read(0, "hello\n", 8192) = 6 [pid 11215] write(2, "read 6 bytes\n", 13read 6 bytes ) = 13 [pid 11215] write(2, "writing: 6 bytes\n", 17writing: 6 bytes ) = 17 [pid 11215] write(1, "hello\n", 6) = 6 [pid 11215] write(2, "wrote : 6\n", 10wrote : 6 ) = 10 [pid 11215] rt_sigprocmask(SIG_BLOCK, [PIPE RT_31], [PIPE RT_31], 8) = 0 [pid 11215] fcntl(0, F_GETFL) = 0xa000 (flags O_RDONLY|O_ASYNC|O_LARGEFILE) [pid 11215] fcntl(0, F_SETFL, O_RDONLY|O_ASYNC|O_LARGEFILE) = 0 [pid 11215] fcntl(0, 0xf /* F_??? */, 0x7fff1c940090) = 0 [pid 11215] fcntl(0, F_SETSIG, 0x3f) = 0 [pid 11215] fcntl(0, F_GETFL) = 0xa000 (flags O_RDONLY|O_ASYNC|O_LARGEFILE) [pid 11215] fcntl(0, F_SETFL, O_RDONLY|O_ASYNC|O_LARGEFILE) = 0 [pid 11215] fcntl(0, 0xf /* F_??? */, 0x7fff1c940090) = 0 [pid 11215] fcntl(0, F_SETSIG, 0x3f) = 0 [pid 11215] rt_sigaction(SIGRT_31, {0x400b4d, [], SA_RESTORER|SA_STACK|SA_NODEFER|SA_SIGINFO, 0x7fc6c47f0340}, {0x400b4d, [], SA_RESTORER|SA_STACK|SA_NODEFER|SA_SIGINFO, 0x7fc6c47f0340}, 8) = 0 [pid 11215] rt_sigaction(SIGPIPE, {0x400e49, [], SA_RESTORER|SA_STACK|SA_NODEFER|SA_SIGINFO, 0x7fc6c47f0340}, {0x400e49, [], SA_RESTORER|SA_STACK|SA_NODEFER|SA_SIGINFO, 0x7fc6c47f0340}, 8) = 0 [pid 11215] fcntl(1, F_SETFL, O_RDONLY|O_LARGEFILE) = 0 [pid 11215] write(2, "reading\n", 8reading ) = 8 [pid 11215] read(0, "", 8192) = 0 [pid 11215] write(2, "read returned 0 - waiting for SI"..., 36read returned 0 - waiting for SIGIO ) = 36 [pid 11215] fcntl(0, F_SETFL, O_RDONLY|O_ASYNC|O_LARGEFILE) = 0 [pid 11215] rt_sigprocmask(SIG_UNBLOCK, [PIPE RT_31], [PIPE RT_31], 8) = 0 [pid 11215] pause() = ? ERESTARTNOHAND (To be restarted if no handler) [pid 11215] --- SIGRT_31 {si_signo=SIGRT_31, si_code=0x1, si_pid=65, si_uid=0} --- [pid 11215] --- SIGRT_31 {si_signo=SIGRT_31, si_code=0x1, si_pid=65, si_uid=0} --- [pid 11215] write(2, "IO: SI_FD: 0\n", 13IO: SI_FD: 0 ) = 13 [pid 11215] rt_sigreturn() = 0 [pid 11215] write(2, "IO: SI_FD: 0\n", 13IO: SI_FD: 0 ) = 13 [pid 11215] rt_sigreturn() = -1 EINTR (Interrupted system call) [pid 11215] rt_sigprocmask(SIG_BLOCK, [PIPE RT_31], [], 8) = 0 [pid 11215] write(2, "after SIGIO - retrying read \n", 29after SIGIO - retrying read ) = 29 [pid 11215] fcntl(0, F_SETFL, O_RDONLY|O_LARGEFILE) = 0 [pid 11215] write(2, "reading\n", 8reading ) = 8 [pid 11215] read(0, "hello\n", 8192) = 6 [pid 11215] write(2, "read 6 bytes\n", 13read 6 bytes ) = 13 [pid 11215] write(2, "writing: 6 bytes\n", 17writing: 6 bytes ) = 17 [pid 11215] write(1, "hello\n", 6) = -1 EPIPE (Broken pipe) [pid 11215] rt_sigprocmask(SIG_UNBLOCK, [PIPE RT_31], [PIPE RT_31], 8) = 0 [pid 11215] --- SIGPIPE {si_signo=SIGPIPE, si_code=SI_USER, si_pid=11215, si_uid=1001} --- [pid 11215] write(2, "PIPE: SI_FD: 0\n", 15PIPE: SI_FD: 0 ) = 15 [pid 11215] rt_sigreturn() = 0 [pid 11215] write(2, "PIPE - waiting for connection\n", 30PIPE - waiting for connection ) = 30 [pid 11215] fcntl(1, F_SETFL, O_RDONLY|O_ASYNC|O_LARGEFILE) = 0 [pid 11215] pause( Now I have to press <CTRL+C>, and only AFTER the reader has disonnected does the t_sigio_rdwr process get a SIGIO: ) = ? ERESTARTNOHAND (To be restarted if no handler) [pid 11215] --- SIGRT_31 {si_signo=SIGRT_31, si_code=0x2, si_pid=772, si_uid=0, si_value={int=1, ptr=0x1}} --- [pid 11215] write(2, "IO: SI_FD: 1\n", 13IO: SI_FD: 1 ) = 13 [pid 11215] rt_sigreturn() = -1 EINTR (Interrupted system call) [pid 11215] fcntl(1, F_SETFL, O_RDONLY|O_LARGEFILE) = 0 [pid 11215] rt_sigprocmask(SIG_BLOCK, [PIPE RT_31], [], 8) = 0 [pid 11215] write(2, "AFTER SIGIO : 1\n", 16AFTER SIGIO : 1 ) = 16 [pid 11215] write(2, "writing: 6 bytes\n", 17writing: 6 bytes ) = 17 [pid 11215] write(1, "hello\n", 6) = -1 EPIPE (Broken pipe) [pid 11215] rt_sigprocmask(SIG_UNBLOCK, [PIPE RT_31], [PIPE RT_31], 8) = 0 [pid 11215] --- SIGPIPE {si_signo=SIGPIPE, si_code=SI_USER, si_pid=11215, si_uid=1001} --- [pid 11215] write(2, "PIPE: SI_FD: 1\n", 15PIPE: SI_FD: 1 ) = 15 [pid 11215] rt_sigreturn() = 0 [pid 11215] write(2, "PIPE - waiting for connection\n", 30PIPE - waiting for connection ) = 30 [pid 11215] fcntl(1, F_SETFL, O_RDONLY|O_ASYNC|O_LARGEFILE) = 0 [pid 11215] pause( But since the reader had to disconnect, it gets SIGPIPE trying to write, so never leaves the state of trying to write back the second input line. -- You are receiving this mail because: You are watching the assignee of the bug.-- To unsubscribe from this list: send the line "unsubscribe linux-man" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
