On 11/11/2013 09:56 AM, Anthony Liguori wrote: >> Here's a hint: ioctl() can clobber errno. But if a signal handler is >> called in the middle of other code that is using errno, then the handler >> MUST restore the value of errno before returning, if it is to guarantee >> that the interrupted context won't be corrupted. > > Isn't this precisely why EINTR exists?
That's part of the equation, but not everything. EINTR exists for a system call that was cut short by the delivery of a signal; if you check for errno==EINTR after a call that is documented to support it (such as write() or poll()), then you know that the call was interrupted; use of SA_RESTART with sigaction() can also control whether you will even see EINTR in the first place for some functions. But consider what happens when the system call completes normally, and the signal handler then gets invoked in between the syscall completion and the later code that checks the value of errno. There, errno will NOT be EINTR, and it is vital that the signal handler not corrupt errno prior to returning control to normal execution context. -- Eric Blake eblake redhat com +1-919-301-3266 Libvirt virtualization library http://libvirt.org
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