IO::SigGuard - SA_RESTART in pure Perl


    IO::SigGuard::sysread( $fh, $buf, $size );
    IO::SigGuard::sysread( $fh, $buf, $size, $offset );

    IO::SigGuard::syswrite( $fh, $buf );
    IO::SigGuard::syswrite( $fh, $buf, $len );
    IO::SigGuard::syswrite( $fh, $buf, $len, $offset );

    IO::SigGuard::send( $fh, $msg, $flags );
    IO::SigGuard::send( $fh, $msg, $flags, $to );

    IO::SigGuard::select( $read, $write, $exc, $timeout );


perldoc perlipc describes how Perl versions from 5.8.0 onward disable the OS’s SA_RESTART flag when installing Perl signal handlers.

This module imitates that pattern in pure Perl: it does an automatic restart when a signal interrupts an operation so you can avoid the generally-useless EINTR error when using sysread(), syswrite(), and select().

For this to work, whatever signal handler you implement will need to break out of this module, probably via either die() or exit().

ABOUT sysread() and syswrite()

Other than that you’ll never see EINTR and that there are no function prototypes used (i.e., you need parentheses on all invocations), sysread() and syswrite() work exactly the same as Perl’s equivalent built-ins.


As of version 0.13 this module’s functions lazy-load by default. To have functionality loaded at compile time give the function name to the import logic, e.g.:

    use IO::SigGuard qw(send recv);

ABOUT select()

To handle EINTR, IO::SigGuard::select() has to subtract the elapsed time from the given timeout then repeat the internal select(). Because the select() built-in’s $timeleft return is not reliable across all platforms, we have to compute the elapsed time ourselves. By default the only means of doing this is the time() built-in, which can only measure individual seconds.

This works, but there are two ways to make it more accurate:

  • Have Time::HiRes loaded, and IO::SigGuard::select() will use that module rather than the time() built-in.

  • Set $IO::SigGuard::TIME_CR to a compatible code reference. This is useful, e.g., if you have your own logic to do the equivalent of Time::HiRes—for example, in Linux you may prefer to call the gettimeofday system call directly from Perl to avoid Time::HiRes’s XS overhead.

In scalar contact, IO::SigGuard::select() is a drop-in replacement for Perl’s 4-argument built-in.

In list context, there may be discrepancies re the $timeleft value that Perl returns from a call to select. As per Perl’s documentation this value is generally not reliable anyway, though, so that shouldn’t be a big deal. In fact, on systems like MacOS where the built-in’s $timeleft is completely useless, IO::SigGuard’s return is actually better since it does provide at least a rough estimate of how much of the given timeout value is left.

See perlport for portability notes for select.


This pattern could probably be extended to other system calls that can receive EINTR. I’ll consider adding new calls as requested.



Felipe Gasper (FELIPE)

… with special thanks to Mario Roy (MARIOROY) for extra testing and a few fixes/improvements.


Copyright 2017 by Gasper Software Consulting


This distribution is released under the same license as Perl.