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Aucbvax.5205 fa.unix-wizards utzoo!decvax!ucbvax!unix-wizards Sun Nov 15 23:26:37 1981 Bug in sleep >From harpo!jerry@Berkeley Sun Nov 15 22:21:30 1981

Yes, an interrupt at an inoportune time could cause the V7 sleep to "pause" forever. It can be corrected (without kernel changes ) by making the following changes at two places. First at the point where pause is called:

wakecnt = 0 ; /* Added. This is a new global (declared static in this file) that will count how many times awake is called */

alarm(sleep_tm); /* Unchanged */ pause(); /* Unchanged */ unslept = alarm(0) ; /* Unchanged */

if ( unslept >= wakecnt ) unslept -= wakecnt ; else unslept = 0 ; /* Added. Corrects for the effect of awake reseting the alarm. In normal circumstances unslept will be at least one and wakecnt will be exactly one so the test isn't required. But that may not hold in some pathological cases. If the else is taken some ticks will have been lost, but that's life. */

Second redefine awake:

static awake() { ++ wakecnt ; signal(SIGALRM, awake ) ; alarm(1) ; }

I haven't tested this code, so it may not be right, but the idea is sound. It adds two system calls for every invocation of sleep and so may not be worth installing.

Incidentally, there is another subtle bug in sleep.c. It arises in the following circumstance. Define

caught() {

signal(SIGALRM,caught) ; alarm(10) ; }

Somewhere else execute:

signal(SIGALRM,caught) ; alarm(10) ; sleep(20) ;

It's probably easier to avoid doing this sort of thing than to fix the bug in sleep. If you really care, the problem is at the places where signal(SIGALRM,alrm_sig) is called.

Jerry Schwarz Bell Labs, Whippany

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