Lack of Documentation about SA_RESTART...

[Paolo Ornati <ornati@fastwebnet.it>]

The documentation (man pages & info libc) doesn't cover well interaction
between various syscalls and SA_RESTART flag of sigaction()... I wonder
why!

	> MAN SIGACTION <

"SA_RESTART
Provide behaviour compatible with BSD signal semantics by
making certain system calls restartable across signals."

	certain?!? :-O!


	> INFO LIBC <

"   One important exception is `EINTR' (*note Interrupted Primitives::).
Many stream I/O implementations will treat it as an ordinary error,
which can be quite inconvenient.  You can avoid this hassle by
installing all signals with the `SA_RESTART' flag."


" -- Macro: int SA_RESTART
     This flag controls what happens when a signal is delivered during
     certain primitives (such as `open', `read' or `write'), and the
     signal handler returns normally.  There are two alternatives: the
     library function can resume, or it can return failure with error
     code `EINTR'."

	Ok, "info libc" is a bit better.


But what I'm looking for is a list of syscalls that are automatically
restarted when SA_RESTART is set, and especially in what conditions.

For example: read(), write(), open() are obviously restarted, but even
on non-blocking fd?
And what about connect() and select() for example?

There are a lot of syscalls that can fail with "EINTR"! What's the
advantage of using SA_RESTART if one doesn't know what syscalls are
restarted?

One should always check for "EINTR" or use "TEMP_FAILURE_RETRY()" macro
as suggested in "info libc" !


Looking at the source I can easly see that a syscall is retarted when it
returns "-ERESTARTSYS" and SA_RESTART flag is set. Should I look at the
code for every syscall / particular condition?


Example of behavior: according to source code it seems that "connect()"
(the "net/ipv4/af_inet.c : inet_stream_connect()" implementation)
returns -ERESTARTSYS if interrupted, but if the socket is in
non-blocking mode it returns -EINTR.


SUMMARY:

1) there is a reason for this lack of documentation?

2) what can I safely assume about syscalls restart when using SA_RESTART
flag?


Bye,

-- 
	Paolo Ornati
	Linux 2.6.12.2 on x86_64
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