Signal

• Semantics

unreliable

• signal handler does not remain installed once called(reset signal behaviour to SIG_DFL upon invocation of the signal handler)
  • there is a race condition in which the signal can arrive again before the handler is re-installed, which can cause the signal to either be lost or for the original behavior of the signal to be triggered

• there is no counter
  • the second one will be lost if it arrives before the signal handler is called
• In earlier versions of the UNIX System (such as Version 7), signals were unreliable.

reliable

• Changes were made with 4.2BSD to provide what are called reliable signals.
• A different set of changes was then made in SVR3 to provide reliable signals under System V.
• POSIX.1 chose the BSD model to standardize.
• queued signals

• Functions

SVR4

• signal
  • unreliable-signal semantics
• sigset
• sighold
• sigrelse
• sigignore
• sigpause

BSD

• BSD Function to Establish a Handler
• BSD Functions for Blocking Signals

• signal
  • it is defined in terms of the sigaction function
  • reliable-signal semantics
• sigvec
  • the equivalent of sigaction
• sigblock
  • the equivalent of sigprocmask with a how argument of SIG_BLOCK
• sigsetmask
  • the equivalent of sigprocmask with a how argument of SIG_SETMASK
• sigpause
  • the equivalent of sigsuspend

POSIX.1

• sigaction
• sigprocmask
• sigpending
• sigsuspend

signal

• Under Linux, the signal function is equivalent to using sigaction with the SA_ONESHOT and SA_NOMASK options
• If you wish signal to use BSD semantics, most Linux systems provide a BSD compatibility library which can be linked with. To use this library, you could add the options

-I/usr/include/bsd -lbsd

sigaction

struct sigaction sa;

sa.sa_handler = test_interrupt;
sa.sa_flags = SA_INTERRUPT;
sa.sa_mask = 0;
sa.sa_restorer = NULL;
irqaction(irq, &sa);

sigsuspend

• Problem with pause

if (!usr_interrupt)
  pause ();

The signal could arrive after the variable usr_interrupt is checked, but before the call to pause. If no further signals arrive, the process would never wake up again.

• Remedy

• Specific Issues

signal number

• <bits/signum.h>

sa_flags(signal options)

• SA_NOCLDSTOP: Don't send SIGCHLD when a child process is stopped.
• SA_RESTART: Force restart of certain system calls when interrupted by a signal handler
  • SA_RESTART is suppressed
    • System V
    • explicitly check the return value of all interruptible system calls for -EINTR and, where this value is true, to restart the call repeatedly until it is no longer interrupted by a signal
  • SA_RESTART is alwayse set
    • 4.2BSD
    • the system call does not issue a return value but is restarted automatically by the kernel once the signal handler has terminated.
  • SA_RESTART is set as default
    • Linux, 4.3BSD, 4.4BSD, Mac OS X
• SA_NOMASK: Disable signal mask (which blocks signals during execution of a signal handler).
• SA_ONESHOT: Clear signal handler after execution. Note that SVR4 uses SA_RESETHAND to mean the same thing.
• SA_INTERRUPT: Defined under Linux, but unused. Under SunOS, system calls were automatically restarted, and this flag disabled that behavior.
• SA_STACK: Currently a no-op, to be used for signal stacks.

Reentrant Functions

• Use reentrant functions for safer signal handling