Date: Thu, 9 May 1996 12:47:19 +1000 From: Bruce Evans <bde@zeta.org.au> To: alk@Think.COM, hackers@freebsd.org Subject: Re: SIGFPE Message-ID: <199605090247.MAA03905@godzilla.zeta.org.au>
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>Does anyone have an example of code which handles SIGFPEs?
>I do not understand how to discriminate causes and rectify
>problems in a handler, and there does not appear to be any
>code in the os which does this, to serve as an example.
It's not supported. The best method of handling a SIGFPE is:
sig_atomic_t saw_sigfpe;
jmp_buf sigfpe_jb;
void sigfpe_handler(int s)
{
saw_sigfpe = 1;
longjmp(sigfpe_jb, 1);
}
Use of floating point in a signal handler (not just the SIGFPE
handler) gives undefined behaviour (in FreeBSD-current it trashes
the FPU state and may cause a SIGFPE).
Something like the following might work in FreeBSD-current. Signal handlers
see the process' FPU state directly, so it is easy to change it directly if
you know what to change.
static struct save87 fpu_state;
void sigfpe_handler(int s, int code, struct sigcontext *scp)
{
switch (code)
{
case FPE_INTDIV_TRAP: ... break;
... other integer traps ...
case 0: /* XXX code for all h/w FP traps. Emulators may be different. */
asm("fnsave _fpu_state"); /* XXX */
fpu_state.sv_ex_sw =
ptrace_or_something_to_read_saved_exception_status_from_user_area();
code = decode(fpu_state.sv_ex_sw);
... adjust fpu_state ...
asm("frstor _fpu_state"); /* XXX */
break;
}
Bruce
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