linux-kernel - Re: Questions about mmap

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Date:	Tue, 7 Oct 2008 10:40:10 -0300
From:	"Thiago Lacerda" <thiagotbl@...il.com>
To:	linux-kernel@...r.kernel.org
Cc:	"Stefan Richter" <stefanr@...6.in-berlin.de>
Subject: Re: Questions about mmap

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))
                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;
}

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.

best regards.

On Sun, Oct 5, 2008 at 11:01 AM, Stefan Richter
<stefanr@...6.in-berlin.de> wrote:
>
> Thiago Lacerda wrote:
> > typedef struct hashtable {
> >    flow_t *hashBuckets[X]; // X is a natural number
> >    unsigned int size;
> > } Hastable;
> >
> >
> > So, my question is: Can I mmap the hashtable struct and access
> > directly from userspace? I'm afraid that this could turn on mess
> > because of the array of pointers.
> > If it's possible, does anyone know a piece of code that can match my
> > problem? I've been googling and I just find codes dealing with structs
> > itself, not pointers.
>
> If you require portability, you can only use integer types of fixed size
> in kernel<->userspace ABIs.  Pointers can then be exchanged as __u64
> under the assumption that pointers not wider than 64 bits.  See for
> example the FW_CDEV_IOC_QUEUE_ISO ioctl in include/linux/firewire-cdev.h.
> --
> Stefan Richter
> -=====-==--- =-=- --=-=
> http://arcgraph.de/sr/


--
Thiago de Barros Lacerda
Computer Science Undergraduate Student - CIn/UFPE - 2004.2
Researcher/Software Developer - GPRT - Networking and
Telecommunications Research Group
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