Program Listing for File signal_handling.cpp

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#include "common/logging.h"
#include "signal_handling.h"

// The simplest (and recommended) way to handle signals is to simply set a flag
// in the signal handler and check that flag later.
//
// We provide setSignalFlag as the most generic signal handler. This handler uses a
// single sig_atomic_t as a bit field. On Linux, sig_atomic_t is equivalent to a signed int,
// theoretically providing 32 binary flags; in practice, most likely signals for which we may
// want to install signal handlers are
// - SIGTERM (15): which by default signals the request for a graceful shutdown
// - SIGUSR1 (10): intended for custom use, default action in Linux is termination
// - SIGUSR2 (12): intended for custom use, default action in Linux is termination
// - SIGINT (2): interrupt from the console
// Just to be safe, we accommodate signals up to signal No. 30.

// In addition, we also provide requestSaveAndExit() and saveAndExit() as a signal
// handler/checker for graceful shutdown requests during training.
constexpr int maxSignalForSetSignalFlag{30};

// Make sure sig_atomic_t is large enough as a bit field for our purposes.
// That said, I'm not aware of any platform where this would be a problem.
static_assert(SIG_ATOMIC_MAX > (1U<<maxSignalForSetSignalFlag),
              "sig_atomic_type is too small for signal flags on this platform.");

namespace marian{
volatile std::sig_atomic_t sigflags_{0};
volatile std::sig_atomic_t saveAndExit_{0};

void setSignalFlag(int sig) {
  // sigflags_ is an int type serving as a bit filed for flags corresponding
  // to signals (lower or equeal to maxSignalForSetSignalFlag). We set the
  // flag by a binary or (|=) of the bit field and an int value with exactly
  // one bit set (s^sig).
  sigflags_ |= (1<<sig);
}

// Check if the flag for the signal sig is set in the bit field sigflags_
bool getSignalFlag(const int sig) {
  ABORT_IF(sig > maxSignalForSetSignalFlag,
           "Signal out of range (must be < {}, is {}).", maxSignalForSetSignalFlag, sig);
  // Do bitwise AND between sigflags_ and an int value that has exactly one bit set that
  // corresponds to the signal in question. If the bit is set (see setSignalFlag above),
  // the bitwise AND will return a non-zero integer, if it is not set, the result will
  // be zero.
  return (sigflags_ & (1<<sig)) != 0;
}

void requestSaveAndExit(int sig) {
  setSignalFlag(sig);         // keep track of triggering signal
  saveAndExit_ = 1; // set flag to exit gracefully
}

bool saveAndExitRequested() {
  return saveAndExit_ == 1;
}

}