linux-kernel - Re: Questions about mmap

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Message-Id: <200810080302.11618.nickpiggin@yahoo.com.au>
Date:	Wed, 8 Oct 2008 03:02:11 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	"Thiago Lacerda" <thiagotbl@...il.com>
Cc:	linux-kernel@...r.kernel.org,
	"Stefan Richter" <stefanr@...6.in-berlin.de>
Subject: Re: Questions about mmap

On Wednesday 08 October 2008 00:40, Thiago Lacerda wrote:
> Thank you Stefan.... by I'd something more concrete.
>
> I'm trying to do like this:
>
> //Code of the char device
> unsigned int **test;
> static int device_mmap(struct file *filp, struct vm_area_struct *vma) {
>       printk(KERN_INFO"Calling mmap\n");
>       vma->vm_flags |= VM_LOCKED;
>       if(remap_pfn_range(vma, vma->vm_start,
> virt_to_phys((void*)((unsigned long)test)) >> PAGE_SHIFT, vma->vm_end
> - vma->vm_start, PAGE_SHARED))

I guess it is more usual to use vma->vm_page_prot for the last argument.
Unless you really specifically want to override it.

>                 return -EAGAIN;
>       printk(KERN_INFO"mmap returned\n");
>       return 0;
> }
>
> static int __init testeInit(void) {
> /* after creating char device and registering it*/
> ...
> ....
> test = (unsigned int**) kmalloc(sizeof(unsigned int*)*1024, GFP_KERNEL);
> int i;
> for(i = 0; i < 1024; i++)
>      test[i] = NULL;
>
> unsigned int* temp1 = (unsigned int*) kmalloc(sizeof(unsigned int),
> GFP_KERNEL); (*temp1) = 9;
> test[0] = temp1;
> return 0;
> }

The idea seems strange because the userspace program will never be able
to dereference these pointers.


> and at user program:
>
> int main() {
>     int fd;
>
>     unsigned int **mptr;
>     size_t size = 1024*sizeof(unsigned int*);
>     fd = open("/dev/myDev", O_RDWR);
>     if( fd == -1) {
>         printf("open error...\n");
>         exit(0);
>     }
>
>     mptr = mmap(0, sizeof(unsigned int*)*1024, PROT_READ | PROT_WRITE,
> MAP_FILE | MAP_SHARED, fd, 0);
>     if(mptr == MAP_FAILED) {
>         printf("mmap() failed\n");
>         exit(1);
>     }
>     printf("teste[0]: %u\n", *mptr[0]);
>     munmap(mptr, size);
>     close(fd);
>     return 0;
> }
>
> It's not working, could you tell me what am I doing wrong? It is for
> my undergraduate thesis ans it is really driving me mad.
>
> I hope that you can help me.

What is not working? What is failing or going wrong for you?
Can't really help without knowing that.
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