Hello,
the following short programm stops (no more input, no telnet login, etc...)
my linux system (kernel 2.6.5) immediately (this is the short version of what
xfree does on XF86DGADirectVideo(), mmap the video memory).

SysRq-t tells that bash hangs with the following call trace:

handle_mm_fault
do_page_fault
sys_wait4
sigprocmask
default_wake_function
sys_rt_sigprocmask
default_ware_function
syscall_call

Thanks in advance for your help,
Peter

#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <sys/mman.h>
#include <assert.h>

int main(int argc, char *argv[]) {
        int fd;
        assert((fd = open("/dev/mem", O_RDWR)) != (-1));

        size_t s = 67108864;
        void *m;
        assert((m = mmap(NULL, s, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0xd0000000)) != NULL);

        char *c = 0;
        char a = *c;
        return 0;
}

-- 
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Peter Seiderer                     E-Mail:  seiderer123@ciselant.de