How to access a physical memory address in C++

QNX Newsgroups (Archives) qnx.development_tools
1

QNX 6.x…

Can anyone provide sample C++ code to access an absolute memory address?

This won’t work because it yields the data at location 0x100 in the virtual
address space of the process, correct?

unsigned char *ptr = 0x100; // address 0x100
unsigned char chr;
chr = *ptr;


Thanks,

Bill
Cincinnati, OH USA

2

Dirk Gently wrote:

QNX 6.x…

Can anyone provide sample C++ code to access an absolute memory address?

This won’t work because it yields the data at location 0x100 in the virtual
address space of the process, correct?

unsigned char *ptr = 0x100; // address 0x100
unsigned char chr;
chr = *ptr;


Thanks,

Bill
Cincinnati, OH USA

You need to map that physical address+range into the address space. You
can do so via:

vaddr = mmap64(0, , PROT_READ|PROT_WRITE, MAP_PHYS|MAP_SHARED,
NOFD, );

Now the pointer ‘vaddr’ will allow you to access that physical location.
Check the docs on mmap() for more flags (like PROT_NOCACHE etc) and
information.

–
Cheers,
Adam

QNX Software Systems Ltd.
[ amallory@qnx.com ]

With a PC, I always felt limited by the software available.
On Unix, I am limited only by my knowledge.
–Peter J. Schoenster <pschon@baste.magibox.net>

3

Thanks!

“Adam Mallory” <amallory@qnx.com> wrote in message
news:cdr9jr$m23$1@inn.qnx.com…

Dirk Gently wrote:
QNX 6.x…

Can anyone provide sample C++ code to access an absolute memory address?

This won’t work because it yields the data at location 0x100 in the
virtual
address space of the process, correct?

unsigned char *ptr = 0x100; // address 0x100
unsigned char chr;
chr = *ptr;


Thanks,

Bill
Cincinnati, OH USA



You need to map that physical address+range into the address space. You
can do so via:

vaddr = mmap64(0, , PROT_READ|PROT_WRITE, MAP_PHYS|MAP_SHARED,
NOFD, );

Now the pointer ‘vaddr’ will allow you to access that physical location.
Check the docs on mmap() for more flags (like PROT_NOCACHE etc) and
information.

–
Cheers,
Adam

QNX Software Systems Ltd.
[ > amallory@qnx.com > ]

With a PC, I always felt limited by the software available.
On Unix, I am limited only by my knowledge.
–Peter J. Schoenster <> pschon@baste.magibox.net