持田＠NETside です。

 NetBSD/i386 での様子です。参考までに。

% uname -srm
NetBSD 1.6P i386

% cc -v
Reading specs from /usr/pkg/gcc3/lib/gcc-lib/i386--netbsdelf/3.3/specs
Configured with: ./configure --prefix=/usr/pkg/gcc3 --host=i386--netbsdelf --enable-shared --enable-languages=c
Thread model: single
gcc version 3.3

% cc -DARLEN=1000000 -Wall -o a a.c
% ./a
55, G

% cc -DARLEN=8012998 -Wall -o a a.c
% ./a 
zsh: cannot allocate memory: ./a

% cc -DARLEN=8012999 -Wall -o a a.c
a.c:13: error: size of variable `pkt' is too large

% echo '268 * 8012998' | bc
2147483464
% echo '268 * 8012999' | bc
2147483732
% echo '2^31' | bc
2147483648

% cat a.c
#include <stdio.h>
#ifndef ARLEN
#define ARLEN   1000000
#endif

struct packet {
        int nodenumber;
        int destination;
        int source;
        char body [ 256 ];
} pkt[ARLEN];

int main()
{
        pkt[0].nodenumber = 55;
        pkt[ARLEN -1].body[255] = 'G';

        printf("%d, %c\n", pkt[0].nodenumber, pkt[ARLEN -1].body[255]);
        return 0;
}

-- 
持田 修司 NETside Technologies Inc.
          -- Equal Opportunity for All Good Architectures, NetBSD. --