Home | Projects | Notes > C Programming > Pointers

Pointers

Pointers in C

• Pointers are one of the essential programming features which are available in C.

• Pointers make C programming more powerful.

• Pointers are heavily used in embedded C programming to

  • Configure the peripheral register addresses

  • Read/write into peripheral data registers

  • Read/write into SRAM/FLASH locations and for many other things.

Address (&var) is not a Regular Number!

• In C, an address of a variable is not a regular number. Consider the following example:

  xxxxxxxxxx
  3
  1
  /* assume 64-bit system, size of a pointer is 8 byte */
  2
  char ch = 'A';
  3
  unsigned long long int addr_ch = &ch;   // size of 'long long in' is always 8 bytes 
  

  Compiler will give you warning, because &ch is not just a number but is recognized as a pointer which is of type char *.

  If you still want to make this work, typecast the pointer into unsigned long long int.

  xxxxxxxxxx
  1
  1
  unsigned long long int addr_ch = (unsigned long long int)&ch;
  

Pointer Operators

• *: Dereference (or "value at address") operator

• &: "Address of" operator

What Significance does the Pointer Data Type Have?

• The pointer data type decides the behavior of the operations carried out on the pointer variable.

  Operations: read, write, increment, decrement

Examples

• Read operations using pointers

  xxxxxxxxxx
  3
  1
  /* Read operation on variable 'address1' yields 1 byte of data */
  2
  char *address1 = (char *)0x00007FFF8E3C3824;
  3
  /* '*address1' -> 1 byte of data
  

  The pointer data type char * decides how many byte(s) of data can be read when using that pointer.

  In this case address1 is called a "pointer variable to a char type data".

  xxxxxxxxxx
  2
  1
  /* Read operation on variable 'address1' yields 4 byte of data */
  2
  int *address1 = (int *)0x00007FFF8E3C3824;
  

  xxxxxxxxxx
  2
  1
  /* Read operation on variable 'address1' yields 8 byte of data */
  2
  long long int *address1 = (long long int *)0x00007FFF8E3C3824;
  

• Write operations using pointers

  xxxxxxxxxx
  2
  1
  char *address1 = (char *)0x00007FFF8E3C3824;
  2
  *address1 = 0x89;   /* dereferencing a pointer to write data */
  

   

Pointer Usage in Embedded Programming

• Store data into required SRAM locations.

• For copying data from peripheral register to SRAM memory and vice versa.

• To configure the peripheral registers. Because peripheral registers are memory-mapped and each register will be given unique address in the MCU memory map.

• Pointers to ISRs are stored in vector table to handle the interrupts.

• Pointers are also used to configure the memory-mapped processor specific registers like interrupt configuration registers.