Common Combinations and their usage in C

Pointer to Pointer – A simple pointer to another pointer Commonly used in various scenarios where indirection or multiple levels of indirection are required.

Example:

char *ptr =(char*)malloc(sizeof(char));
char **ptr_to_ptr =&ptr;


Pointer to Arrays – A point to an array. Can be used for manipulating the array or collecting an array from a function.

Example:

char array[] ={ ‘w’,’o’,’r’,’k’,’s’};
char *ptr_to_array = array;


Pointer to multidimensional arrays – A pointer to collect a multidimensional array. Can be used to traversal and manipulation of multidimensional array values.

Example:

int multi_arr[][4] ={ {10,19,20,12}, {30,59,32,77}, {11,2578,15,117} };
int (*ptr_to_multi_arr)[4] = multi_arr;

*(ptr_to_multi_arr) gives first array
*(*(ptr_to_multi_arr) + 1) gives first array second element


Pointer to functions:  Can be uses to pass function pointers as arguments. Ex: Can be used in file operations, system calls, operations on device drivers.

Example 1:

loff_t (*llseek) (struct file *, loof_t, int);
ssize_t (*read) (struct file *, char __user *,size_t, loof_t*);

Example 2:

int fun1(int a, int b) { // Operation1…}
int fun2(int c, int d) { // Operation 2…}
void main()
{
int some_val1,some_val2,some_val3;
int store_some_val;
int (*ptr_to_function)(int,int);
ptr_to_function = fun1;
store_some_val = ptr_to_function( some_val1, some_val2);
ptr_to_function = fun2;
store_some_val = ptr_to_function( some_val3, some_val1);

}


Array of structures – Used to store multiple strcutures in continuous block of memory. Can be useful when working on multiple instances of structure and perform operations as a whole. Ex: Process Control Blocks. Scull_dev strcut to avoid race conditions.

Example:

struct scull_dev{
….
int quantum;
int qset;
unsigned long size; …
struct semaphore sem;
….
};
struct scull_dev scull_devices[10];


Pointers to structures: Used to dynamically allcoate memory for structires and access its contents indirectly. Useful to modify the structure the structure member contents without making a copy.

Example:

struct scull_pipe{
…
char *rp ,*wp;
int nreaders, nwriters;
…
struct cdev cdev; // Char driver structure
}

static ssize_t scull_p_read( struct file *filp, char __user *bud, size_t count, loff_t *f_pos)
{
Struct scull_pipe *dev = filp->private_data; // dev is a pointer to scull_pipe structure used to store private_data from filp structure.
…
}

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An Article by: Yashwanth Naidu Tikkisetty
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