Can somebody explain pointer typecasting in c.

What Is Typecasting Of Pointers
http://www.blurtit.com/q310389.html

example with explanation
http://bytes.com/topic/c/answers/594439-typecasting-pointers
http://bytes.com/topic/c/answers/622284-pointer-type-casting

Here is wiki
http://en.wikipedia.org/wiki/Pointer_%28computing%29#Typed_pointers_and_casting

Suppose you have a function that can process several different structs:
    void processSeveralDifferentStucts( void * someStruct, int messageType );

Your code might look like:

struct message1 { // ~ messageType = 1
int a;
int b;
short c;
}msg1;

struct message2{ // ~ messageType = 2
int x;
short y;
short z;
char message[20];
} msg2;

processSeveralDifferentStucts( &msg1, 1);
processSeveralDifferentStucts( &msg2, 2);

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No typecasting is required here (but it is desirable, for clarities sake) because C allows conversion of pointers to void* without typecasting. However, typecasting is required in processSeveralDifferentStucts.

void processSeveralDifferentStucts( void * someStruct, int messageType ) {
  struct message1 ptr1; 
  struct message2 ptr2;
  switch (messageType) {
   case 1:
     ptr1 = (struct message1 *)someStruct;
     printf("%d %d %d \n", ptr1->a , ptr1->b ,ptr1->c);
     break;
   case 2:
     ptrStruct = (struct message2 *)someStruct;
     printf("%d %d %d %s\n", ptr2->x , ptr2->y ,ptr2->z, ptr2->message);
     break;
  }
}

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Well, typecasting here isn't required because, again, C allows a void* pointer to be assigned to another pointer of a different type without the cast.

But suppose you were doing a TCP accept:
   int accept(int socket, struct sockaddr *address, socklen_t *restrict address_len);

Now, when using accept, you have to typecast the 2nd argument in the call because you will not get automatic conversion to the type of the passed in pointer.

    struct sockaddr_in serv_addr, cli_addr;
    newsockfd = accept(sockfd, (struct sockaddr *) &cli_addr, &clilen);

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The above two lines were take from:
    http://www.linuxhowtos.org/C_C++/socket.htm