IEEE_HANDLER(3M)

NAME
ieee_handler - IEEE exception trap handler function

SYNOPSIS
#include <floatingpoint.h>

int ieee_handler(action,exception,hdl)
char action[], exception[];
sigfpe_handler_type hdl;

DESCRIPTION

This function provides easy exception handling to exploit ANSI/IEEE Std 754-1985 arithmetic in a C program. The first two arguments are pointers to strings. Results arising from invalid arguments and invalid combinations are undefined for efficiency.

There are three types of action : get, set, and clear. There are five types of exception :
    inexact
    division
        ... division by zero exception
    underflow
    overflow
    invalid
    all ... all five exceptions above
    common
        ... invalid, overflow, and division exceptions

Note: all and common only make sense with set or clear.

hdl contains the address of a signal-handling routine. <floatingpoint.h> defines sigfpe_handler_type.

get will return the location of the current handler routine for exception cast to an int. set will set the routine pointed at by hdl to be the handler routine and at the same time enable the trap on exception, except when hdl == SIGFPE_DEFAULT or SIGFPE_IGNORE; then ieee_handler() will disable the trap on exception. When hdl == SIGFPE_ABORT, any trap on exception will dump core using abort(3). clear all disables trapping on all five exceptions.

Two steps are required to intercept an IEEE-related SIGFPE code with ieee_handler:

1)  Set up a handler with ieee_handler.

2)  Perform a floating-point operation that generates the intended IEEE exception.

Unlike sigfpe(3), ieee_handler() also adjusts floating-point hardware mode bits affecting IEEE trapping. For clear, set SIGFPE_DEFAULT, or set SIGFPE_IGNORE, the hardware trap is disabled. For any other set , the hardware trap is enabled.

SIGFPE signals can be handled using sigvec(2), signal(3V), sigfpe(3), or ieee_handler(3M). In a particular program, to avoid confusion, use only one of these interfaces to handle SIGFPE signals.

DIAGNOSTICS
ieee_handler() normally returns 0 for set . 1 will be returned if the action is not available (for instance, not supported in hardware). For get , the address of the current handler is returned, cast to an int.

EXAMPLE

A user-specified signal handler might look like this:

    void sample_handler(sig, code, scp, addr)
    int sig;               /* sig == SIGFPE always */
    int code;
    struct sigcontext *scp;
    char *addr;
    {
        /*
         * Sample user-written sigfpe code handler.
         * Prints a message and continues.
         * struct sigcontext is defined in <signal.h>.
         */
        printf("ieee exception code %x occurred at pc %X \n", code, scp->sc_pc);
    }

and it might be set up like this:

    extern void sample_handler();
    main()
    {
        sigfpe_handler_type hdl, old_handler1, old_handler2;
        /*
         * save current overflow and invalid handlers
         */
        old_handler1 = (sigfpe_handler_type) ieee_handler("get", "overflow", old_handler1);
        old_handler2 = (sigfpe_handler_type) ieee_handler("get", "invalid", old_handler2);
        /*
         * set new overflow handler to sample_handler() and set new
         * invalid handler to SIGFPE_ABORT (abort on invalid)
         */
        hdl = (sigfpe_handler_type) sample_handler;
        if (ieee_handler("set", "overflow", hdl) != 0)
            printf("ieee_handler can't set overflow \n");
        if (ieee_handler("set", "invalid", SIGFPE_ABORT) != 0)
            printf("ieee_handler can't set invalid \n");
        ...
        /*
         * restore old overflow and invalid handlers
         */
        ieee_handler("set", "overflow", old_handler1);
        ieee_handler("set", "invalid", old_handler2);
    }

SEE ALSO
sigvec(2), abort(3), floatingpoint(3), sigfpe(3), signal(3V)