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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The history of FreeBSD's behaviour is as follows: | The history of FreeBSD's behaviour is as follows: | ||
- implement poll from netbsd 42d1175732fdd | |||
- 7f5dff50646e2 adds POLLHUP | |||
- POLLOUT FIX f2159cc7908d0 | |||
- bug ad7eb8cad5320 | |||
- 74d1c4927a3b7ce785e41e97e33a804f4f8c62ce always return POLLIN | |||
- f2159cc7908d09fcb3e11c23a28760dc96ccfe8e fix conformance | |||
== 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. | ||
== NetBSD == | == NetBSD == | ||
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== Conclusions == | == Conclusions == | ||
poll can be used to check for socket closure alone on some systems, but it isn't portable. | |||