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From: levy@ttrdc.UUCP (Daniel R. Levy)
Newsgroups: comp.lang.c
Subject: Re: Indirection and MALLOC
Message-ID: <1690@ttrdc.UUCP>
Date: Wed, 13-May-87 14:26:22 EDT
Article-I.D.: ttrdc.1690
Posted: Wed May 13 14:26:22 1987
Date-Received: Sat, 16-May-87 07:51:48 EDT
References: <7331@brl-adm.ARPA>
Organization: AT&T, Skokie, IL
Lines: 69
Summary: bad idea to use malloc() unprotected if also used in interrupt service routine

In article <7331@brl-adm.ARPA>, SXSAW%ALASKA.BITNET@wiscvm.wisc.EDU writes:
<   Re AST re-entrency - Since the alloc/dealloc routines save the size and
< address of FREE'd blocks across calls - to manipulate later, and since the
< code can be interrupted during this manipulation via an AST routine which
< calls MALLOC - well.......  It only takes about 10 lines of code to create
< a test case for this that WILL FAIL sometimes - and sometimes not, depends
< on the timing. (with this type of coding going on in the CRTL I begin to
< wonder about other routines)  I have generally found that with VMS4.x and
< the additional features added to  LIB$GET_VM, etc. (i.e. the memory ZONE
< concepts) - that the best way to go is use the RTL procedures.  naturally
< you code will be less portable, but....

It is not a good idea to use malloc() re-entrantly in an interrupt
service routine even on a UNIX system.  The code below illustrates the
problem on a System V release 2 3B20 (will core dump with a memory fault)
when run and given rapidly repeated keyboard interrupts, unless PROTECT_MALLOC
is #define'd.  This ought to work just fine on VMS, too.

--------
#include <signal.h>
main()
{
	int astrtn();
	int i;
	char *malloc();
	char *c;
	void exit();

	(void) signal(SIGINT,astrtn);
	for (;;) {
#ifdef PROTECT_MALLOC
		(void) signal(SIGINT,SIG_IGN);
#endif
		if (!(c=malloc((unsigned)100))) {
			(void) write(1,"malloc() failure\n",(unsigned)17);
			exit(1);
		}
#ifdef PROTECT_MALLOC
		(void) signal(SIGINT,astrtn);
#endif
		for (i=0; i<100; i++)
			*c++ = 'y';
	}
}

astrtn()
{
	char *d;
	char *malloc();
	void exit();
	int i;

	(void) signal(SIGINT,SIG_IGN);
	(void) write(1,"INTERRUPT\n",(unsigned)10);
	if (!(d=malloc((unsigned)100))) {
		(void) write(1,"astrtn malloc failure\n",(unsigned)22);
		exit(2);
	}
	for (i=0; i<100; i++)
		*d++ = 'x';
	(void) signal(SIGINT,astrtn);
	return 0;
}
--------
-- 
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