March 20, 2006

Coverage: [USP] Chapter 8

Orientation

concurrency and communication have been the two predominant themes in this course
we have used functions such as fork, wait, and exec to achieve concurrency
we have used input and output through pipes as our main interprocess communication mechanism
signals will provide an additional way to foster interprocess communication, and it will be asynchronous

a signal is a software notification to a process of an event
the event generates the signal ([USP] §8.2)
the signal is delivered when the process takes action ([USP] §8.5)
- the process must be in the running state
the lifetime of the signal is the time between the signal generation and delivery
- a signal which has been generated, but not yet delivered, is said to be pending
what is the difference between a signal and an interrupt?
every signal is associated with a mnemonic starting with SIG
- represent small integers greater than 0
- defined in signal.h
- SIGUSR1 and SIGUSR2 are reserved for users
the process signal mask is the list of signals that the process is currently blocking ([USP] §8.3)

blocking != ignoring
- a blocked signal is not thrown away
- it is delivered when unblocked
- use sigprocmask to change the process signal mask (block the signal)

a process can catch a signal by executing a signal handler on signal delivery ([USP] §8.4)
- a program installs a signal handler by calling sigaction with the name of a user-defined function; or with
  - SIG_DFL: default action
  - SIG_IGN: ignore signal
  neither are really considered to be catching the signal

use the UNIX command kill
- kill -s signame <PID> or kill [-signame] <PID>
  - omit the leading SIG in the name
- kill [-signum] <PID>
  - -2 (SIGINT; same as <ctrl-c>)
  - -3 (SIGQUIT; same as <ctrl-\>)
  - -9 (SIGKILL; sure kill; process cannot inoculate itself against this signal)
  - -15 (SIGTERM; default)
- kill -l # lists the available symbolic signal names
int kill (pid_t pid, int sig);
- a user may only send a signal to processed that s/he owns
  - how can a child process kill its parent?
int raise (int sig);
- used to send a signal to yourself
  - why might one ever desire to do this?
unsigned alarm (unsigned seconds);
- requests to alarm are not stacked
- alarm never reports an error

blocked signals do not affect the behavior of the process until they are delivered
signal mask gives the set of signals which are currently blocked
- type sigset_t
manipulated through 5 functions
- sigaddset, sigdelset, sigemptyset, sigfillset, sigismember
use sigprocmask to examine and/or modify its process signal mask
- should only be used in a single-threaded program
- how parameter specifics how the signal mask is to be modified
  - SIG_BLOCK, SIG_UNBLOCK, SIG_SETMASK
  - recall that SIGSTOP and SIGKILL cannot be blocked
forked processes inherit the parent signal mask
examples
- blocktest.c: a program which blocks and unblocks SIGINT
- password.c: a function which blocks some signals which, if delivered, might put the terminal in an unusable state, during password entry

used to get and/or set the action associated with a signal
used like sigprocmask
struct sigaction
- uses pointers to functions
in POSIX, a signal handler is an ordinary function which returns void and has one integer parameter
- on delivery, parameter set to the signal number
two special values of sa_handler are SIG_DFL and SIG_IGN
- SIG_DFL specifies that sigaction should restore the default action
- SIG_IGN specifies that the process should handle the signal by ignoring (discarding) it
use write in signal handlers rather than fprintf or strlen
- write is async-signal-safe
  - means that it can be called safely from inside a handler
  - no such guarantees for fprintf or strlen

UNIX Systems Programming: Concurrency, Communication, and Threads