Editing POLLHUP polling
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'''This page is a work in progress''' | |||
Recently I've been trying to listen on multiple sockets at once for events like reading or writing. This has worked fine, but as I've tried to listen for a socket close event I've found the documentation poor and often contradictory. This page is my attempt to figure this topic out. | Recently I've been trying to listen on multiple sockets at once for events like reading or writing. This has worked fine, but as I've tried to listen for a socket close event I've found the documentation poor and often contradictory. This page is my attempt to figure this topic out. | ||
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== Existing lore == | == Existing lore == | ||
Richard Kettlewell's [https://www.greenend.org.uk/rjk/tech/poll.html | Richard Kettlewell's [https://www.greenend.org.uk/rjk/tech/poll.html ll() and EOF] has a test results for the case of setting POLLIN and closing a socket with no data to read. This is the most common case of POLLHUP use I see online. | ||
The test results give us some interesting data: | The test results give us some interesting data: | ||
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* 1997: Linux 2.1.23pre1 added the poll system call, with no AF_UNIX socket support | * 1997: Linux 2.1.23pre1 added the poll system call, with no AF_UNIX socket support | ||
* 1998: Linux 2.1.106pre added AF_UNIX socket support | * 1998: Linux 2.1.106pre added AF_UNIX socket support | ||
* 2000: Linux 2.3.41pre2 returns POLLIN on empty socket close | * 2000: Linux 2.3.41pre2 returns POLLIN on empty socket close | ||
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* POLLIN if checking | * POLLIN if checking | ||
* POLLHUP always | * POLLHUP always | ||
This is regardless of whether there is data in the socket to read. | This is regardless of whether there is data in the socket to read. | ||
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It's not documented but the events bitmask can be empty. | It's not documented but the events bitmask can be empty. | ||
== NetBSD == | |||
On NetBSD 9.3 it seems that polling a closed socket will return: | |||
* POLLIN if checking | |||
* POLLOUT if checking | |||
This is regardless of whether there is data in the socket to read. | |||
The [https://man.netbsd.org/poll.2 NetBSD poll man page] specifically notes that "Sockets produce POLLIN rather than POLLHUP when the remote end is closed" | |||
Leaving the events bitmask empty causes poll to timeout. | |||
== OpenBSD == | == OpenBSD == | ||
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* POLLIN if checking | * POLLIN if checking | ||
* POLLHUP always | * POLLHUP always | ||
This is regardless of whether there is data in the socket to read. | This is regardless of whether there is data in the socket to read. | ||
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It's not documented but the events bitmask can be empty. | It's not documented but the events bitmask can be empty. | ||
== MSYS2 on Windows == | == MSYS2 on Windows == | ||
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== Conclusions == | == Conclusions == | ||
poll can be used to check for socket closure alone on some systems, but it isn't portable. | |||