Lack of Documentation about SA_RESTART...

Paolo Ornati
2005-07-11T10:33:45Z

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