Hi Rajat,

On Tue, Jan 11, 2011 at 9:26 PM, Rajat Sharma [off-list ref] wrote:

Hi Dave,

My understanding was based out of linux/errno.h. Maybe the below
comment is in context of glibc abstracting out ERESTART* error codes.

#ifdef __KERNEL__

/*
?* These should never be seen by user programs. ?To return one of ERESTART*
?* codes, signal_pending() MUST be set. ?Note that ptrace can observe these
?* at syscall exit tracing, but they will never be left for the debugged user
?* process to see.
?*/
#define ERESTARTSYS ? ? 512
#define ERESTARTNOINTR ?513
#define ERESTARTNOHAND ?514 ? ? /* restart if no handler.. */
#define ENOIOCTLCMD ? ? 515 ? ? /* No ioctl command */
#define ERESTART_RESTARTBLOCK 516 /* restart by calling sys_restart_syscall */

Yeah - that's consistent with my understanding. The code flow would be
something like this

user-mode program calls ioctl
    ioctl does some stuff
    signal happens
    ioctl detects signal (typically by a call returning -EINTR) and
decides to return -ERESTARTSYS
    sys handler detects pending signal
        user-mode signal handler is called
    sys handler re-issues ioctl
    ioctl does some stuff
    ioctl returns normally
user mode program sees normal reutrn code

So ioctl returns -ERESTARTSYS, but the user mode program is completely
oblivious to the fact that this happened.

glibc doesn't even get to know that the -ERESTARTSYS code was returned
by ioctl. It gets intercepted and processed by the kernel.

Dave Hylands