On Feb 25 01:17, Evgeny Stambulchik wrote:
> Hello,
>
> There is something strange about mmap(addr, ..., MAP_FIXED) under
> Cygwin. For a reason, it always fails if addr is not on a 16xPAGE_SIZE
> boundary. Here is a short demo:
>
> #include <stdio.h>
> #include <stdlib.h>
> #include <unistd.h>
> #include <errno.h>
> #include <string.h>
> #include <sys/mman.h>
>
> #define NRUNS 20
> #define CHUNK_SIZE 4096
> #define START_ADDR 0x640000
>
> int main(void)
> {
> unsigned int i, pagesize, prot, flags;
> void *addr, *maddr;
>
> pagesize = getpagesize();
> prot = PROT_READ | PROT_WRITE | PROT_EXEC;
> flags = MAP_PRIVATE | MAP_ANONYMOUS | MAP_FIXED;
>
> for (i = 0; i < NRUNS; i++) {
> addr = (void *) START_ADDR + pagesize*i;
> maddr = mmap(addr, CHUNK_SIZE, prot, flags, 0, 0);
> if (maddr != MAP_FAILED) {
> fprintf(stderr, "OK: %p -> %p\n", addr, maddr);
> } else {
> fprintf(stderr, "FAIL: %p (%s)\n", addr, strerror(errno));
> }
> }
> exit(0);
> }
>
> And the output is:
>
> OK: 0x640000 -> 0x640000
Pure coincidence. You shouldn't just use some arbitrary address and
use MAP_FIXED with it, otherwise you're making invaild assumptions
about the memory layout of your machine/os/application.
The reason that MAP_FAILED only works on 64K boundaries so far is,
that I didn't handle this case. Usually there are not many good
reasons to use MAP_FIXED. However, I've checked in a patch which
tries to handle MAP_FIXED on 4K boundaries, but only in the anonymous
case.
Corinna
--
Corinna Vinschen Please, send mails regarding Cygwin to
Cygwin Project Co-Leader mailto:cygwin@cygwin.com
Red Hat, Inc.
--
Unsubscribe info: http://cygwin.com/ml/#unsubscribe-simple
Problem reports: http://cygwin.com/problems.html
Documentation: http://cygwin.com/docs.html
FAQ: http://cygwin.com/faq/
