https://bugzilla.kernel.org/show_bug.cgi?id=94271 Bug ID: 94271 Summary: fcntl.2 and pipe.7 need to say more about use of O_ASYNC 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 169011 --> https://bugzilla.kernel.org/attachment.cgi?id=169011&action=edit program that demonstrates the problem. This problem lost me a couple of hours debugging today, so I thought I should report it so that others do not waste time. It is correct, but not entirely sufficient, to say in pipe.7 : " Setting the O_ASYNC flag for the read end of a pipe causes a signal (SIGIO by default) to be generated when new input becomes available on the pipe (see fcntl(2) for details) " and in fcntl.2: " If you set the O_ASYNC status flag on a file descriptor by using the F_SETFL command of fcntl(), a SIGIO signal is sent whenever input or output becomes possible on that file descriptor. " In fact, unless you have used : fnctl(fd, F_SETOWN, getpid()); for the fd that you have set the O_ASYNC flag for, no SIGIO signals are ever sent. Please could those lines be modified to say something like : pipe.7 : " Setting the O_ASYNC flag for the read end of a pipe causes a signal (SIGIO by default) to be generated when new input becomes available on the pipe (see fcntl(2) for details), and you have set the receiver of the signal with fcntl(2)'s F_SETOWN command. No signal will be sent for an FD when new input becomes available unless both the O_ASYNC flag has been set and the receiver of the signal has been set with F_SETOWN. " fnctl.2: ' F_SETOWN (int) ... If you set the O_ASYNC status flag on a file descriptor by using the F_SETFL command of fcntl(), a SIGIO signal is sent whenever input or output becomes possible on that file descriptor, if and only if the fnctl F_SETOWN command has also been issued for that file descriptor. ' There was nothing in the pipe.7 manual page or in the fcntl.2 manual page to say that without calling fcntl(fd, F_SETOWN, getpid()), no IO signals are ever sent for that fd. I had expected that in the absence of a fcntl(fd, F_SETOWN, getpid()) call, the default receiver of IO signals would be the process that issued the fcntl(fd, F_SETFL, O_ASYNC) or which did fd = open(... O_ASYNC ...) ; This is illustrated by the attached test program . Compiling the program with no -D flags allows it to wait for new input to become available on its input pipe : $ gcc -o -std=gnu99 pio pio.c $ mkfifo /tmp/p.in /tmp/p.out $ ./pio </tmp/p.in >/tmp/p.out & [7] 10059 $ echo 'line1' > /tmp/p.out ; read line </tmp/p.in; echo "$line"; read nothing - waiting for SIGIO signal read: line1 $ echo 'line2' >/tmp/p.in; read line < /tmp/p.out; echo "$line" read nothing - waiting for SIGIO signal read: line2 But if the program is compiled with -DNO_SETOWN flag, preventing it from issuing the fcntl(0, F_SETOWN, getpid()); call: $ gcc -std=gnu99 -DNO_SETOWN -o pio pio.c $ ./pio </tmp/p.in >/tmp/p.out & [7] 10135 $ echo 'line1' >/tmp/p.in; read line < /tmp/p.out; echo "$line" read nothing - waiting for SIGIO signal read: line1 $ echo 'line2' >/tmp/p.in; read line < /tmp/p.out; echo "$line" ^C Now it just hangs because it never gets a SIGIO signal. It is very difficult to write a program to use named fifos for input and output unless one appreciates that one must call fcntl(fd,F_SETOWN,getpid()) - please document this fact. -- 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
