proper use of (volatile unsigned short *)

[CptDondo]

I'm trying to write what should be a simple program, and it keeps
hanging if I use volatile....

The program, stripped of its error checking, is this:

unsigned short * start;
unsigned short * ctl;

int fd = open("/dev/mem", O_RDWR);
start = mmap(0, getpagesize()*2, PROT_READ|PROT_WRITE, MAP_SHARED, fd,
0x72000000);

ctl = (volatile unsigned short *)(start + 0x0410);
printf("got my pointer %p\n",ctl);
sleep(1);
printf("got ctl reg: 0x%02x\n",*ctl);

The program hangs as soon as it hits the last printf - it prints the
"got my pointer" message but hard-locks before printing the "got ctl
reg" message.

Before I start digging into various other issues, is this a proper use
of volatile?

--Yan

[Walter Roberson]

In article <13*************@corp.supernews.com>,
CptDondo <ya*@NsOeSiPnAeMr.comwrote:

>I'm trying to write what should be a simple program, and it keeps
hanging if I use volatile....

>The program, stripped of its error checking, is this:

unsigned short * start;
unsigned short * ctl;

> int fd = open("/dev/mem", O_RDWR);

open() is not part of the C language; it is an OS extension.

> start = mmap(0, getpagesize()*2, PROT_READ|PROT_WRITE, MAP_SHARED, fd,
0x72000000);

mmap() is not part of the C language; it is an OS extension.

> ctl = (volatile unsigned short *)(start + 0x0410);

Note that start + 0x0410 means the address of the 0x0410'th
unsigned short past start, not the address of the unsigned
short 0x0410 bytes after short. If you intend a byte offset
rather than a short offset, then you will need to cast start
to be a character pointer before doing the pointer arithmetic.

> printf("got my pointer %p\n",ctl);
sleep(1);
printf("got ctl reg: 0x%02x\n",*ctl);

>The program hangs as soon as it hits the last printf - it prints the
"got my pointer" message but hard-locks before printing the "got ctl
reg" message.

>Before I start digging into various other issues, is this a proper use
of volatile?

In itself, it looks okay, but the pointer arithmetic is questionable.
--
"It is important to remember that when it comes to law, computers
never make copies, only human beings make copies. Computers are given
commands, not permission. Only people can be given permission."
-- Brad Templeton

[CptDondo]

Walter Roberson wrote:

In article <13*************@corp.supernews.com>,
CptDondo <ya*@NsOeSiPnAeMr.comwrote:

>I'm trying to write what should be a simple program, and it keeps
hanging if I use volatile....

>The program, stripped of its error checking, is this:

unsigned short * start;
unsigned short * ctl;

> int fd = open("/dev/mem", O_RDWR);

open() is not part of the C language; it is an OS extension.

> start = mmap(0, getpagesize()*2, PROT_READ|PROT_WRITE, MAP_SHARED, fd,
0x72000000);

mmap() is not part of the C language; it is an OS extension.

> ctl = (volatile unsigned short *)(start + 0x0410);

Note that start + 0x0410 means the address of the 0x0410'th
unsigned short past start, not the address of the unsigned
short 0x0410 bytes after short. If you intend a byte offset
rather than a short offset, then you will need to cast start
to be a character pointer before doing the pointer arithmetic.

OK, thanks for the explanation.... I got too hung up on the volatile
stuff to check the basic stuff....

>

> printf("got my pointer %p\n",ctl);
sleep(1);
printf("got ctl reg: 0x%02x\n",*ctl);

>The program hangs as soon as it hits the last printf - it prints the
"got my pointer" message but hard-locks before printing the "got ctl
reg" message.

>Before I start digging into various other issues, is this a proper use
of volatile?

In itself, it looks okay, but the pointer arithmetic is questionable.

The pointer arithmatic is what got me....

--Yan