Re: Repost: could ia32 mmap() allocations grow downward?

Petr Vandrovec (VANDROVE@vc.cvut.cz)
Thu, 13 Dec 2001 18:36:43 MET-1

On 13 Dec 01 at 8:22, Wayne Whitney wrote:
> > So maybe MAGMA uses some API which it should not use under any
> > circumstances... Such as that you linked it with libc6 stdio.
>
> Indeed. How can I avoid the map at 0x40000000? Must I avoid using
> certain glibc2 functions, and then link the executable carefully to leave
> out their initialization routines? Or can I set some magic environment

It is caused by (I think that stupid...) code in
glibc-2.2.4/libio/libioP.h:ALLOC_BUF(), which unconditionally does
'mmap(0, ROUND_TO_PAGE(size), PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS,
-1, 0)' instead of 'malloc(size)' when it finds that underlying system
supports malloc.

If you linked Magma yourself, try adding:

---
#include <malloc.h>


void* malloc(size_t len) { return sbrk(len); }
void* __mmap(void* start, size_t len, int prot, int flags, int fd, 
        unsigned long offset) {
    if (start == 0 && fd == -1) { return malloc(len); }
    return NULL;
}

---
into your project. It forces my 'void main() { printf("X\n"); pause(); }'
to use brk() instead of mmap() for stdio buffers. Maybe we should move
to bug-glibc instead, as there is no way to force stdio to not ignore
mallopt() parameters, it still insist on using mmap, and I think that it
is a glibc2.2 bug.
                                                Petr Vandrovec
                                                vandrove@vc.cvut.cz


P.S.: I did some testing, and about 95% of mremap() allocations is
targeted to last VMA, so no VMA move is needed for them. But no Java
was part of picture, only c/c++ programs I use - gcc, mc, perl.
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