Wednesday, April 23, 2014

kmalloc() V/S other page allocation methods

If you are developing anthing in Kernel, I hope you have used kmalloc() many times till now. But if you see there are other function also to allocate physical memory. Those are :

__get_free_page(unsigned int gfp_mask)
__get_free_pages(unsinged int gfp_mask, int order)
get_zeroed_page(unsigned int gfp_mask)

So when we use kmalloc() and when we use the above functions ?
Answer lies in your requirement, if you need very small memory to allocate, go for kmalloc, because there you define the minimum no of bytes(let say n) you need to allocate. But remember it just ensure that atleast n bytes will be allocated, but in reality, it allocates more than it, sometimes even double of the size. So in short there is no fine granuality with kmalloc().

while get_free_page family is used when you want to allocate memory in terms of pages, as for kernel, page is the smallest memory unit. So you will pass the order x, and it will return you 2 power x pages. So here Kernel ensures that only requested no of pages will be allocated. So there will not be any memory waste.

And if you want to allocate memory from high memory, then there is a seperate function : [Although these function can be used for normal zones too]

alloc_pages(unsinged int gfp_mask, unsigned int order)
alloc_pages(unsinged int gfp_mask)

Sunday, April 20, 2014

Short Note on Android App Signing

I know you will find many articles for mentioned title. But I thought of summarizing it , So Here are the steps :

1. You need Keytool and Jarsigner. If you have java installed in your system, then you have these utilities.

2. Now generate a private key for your app, So here is the command :
 keytool -genkey -v -keystore my-release-key.keystore -alias ANY_NAME -keyalg RSA -keysize 2048 -validity 10000

This will ask you few basic question, answer those. finally it will ask the password, give it, this will encrypt your keystore file.

3. I hope as you are an android developer, you must be having Eclipse with you. So Go to File -> export -> Android -> Export Android Application.

4. Select the project from which you want to export the application.

5.  Select keystore, remember in step 2 you created my-release-key.keystore, give path to this file. And entered the password you have entered at the time of creation.

6. Next it will ask key alias selection, So whatever ANY_NAME you given in step2, choose that and again give the password.

7. Finally it will ask you the location, where the signed application will be exported. So just give the path and click finish.

And you are done.

Monday, April 14, 2014

Create a structure which can contain any type of data

Generally whenever we create a structure, we define that what kind of data it will be storing. But think it this way, what if you don't know what kind of data structure it can use in future, to avoid that situation. Here is the way :

struct node
void *ptr;
struct node *link;
int main(void) {
// your code goes here
struct node *head,*temp;
float var = 10.5f;
head = (struct node *)malloc(sizeof(struct node));
*(int *)(head->ptr) =10;
head->link = NULL;

temp = (struct node *)malloc(sizeof(struct node));
*(float *)(temp->ptr) = var;
temp->link = NULL;

head->link = temp;

printf("Value = %d",*(int *)(head->ptr));
printf("Value = %f",*(float *)(temp->ptr));
return 0;

Hope it helps you in your next interview :)