In C programming, a register variable is a type of local variable that is stored in the CPU registers rather than in memory. CPU registers are much faster to access than memory, so using register variables can result in faster program execution.

Here is an example of declaring and using a register variable:

#include <stdio.h>

int main() {
  register int num = 10; // declaring and initializing a register variable
  
  printf("The value of num is %d\n", num); // accessing the value of the register variable
  
  return 0;
}

In this example, we declared a register variable named num inside the main() function. The register keyword tells the compiler to try to store the variable in a CPU register rather than in memory. The variable is only accessible within the main() function, and its value cannot be accessed or modified from outside the function.

It is important to note that the use of register variables is optional, and the compiler may choose to store the variable in memory instead of in a CPU register. The compiler may also choose to ignore the register keyword entirely if it determines that storing the variable in a register will not result in faster program execution.

In general, register variables should only be used for variables that are frequently accessed in performance-critical sections of the program. They should not be used for variables that are only accessed occasionally or for variables that are too large to fit in a CPU register.

In summary, a register variable in C programming is a type of local variable that is stored in a CPU register rather than in memory. Register variables can result in faster program execution, but their use is optional and the compiler may choose to store the variable in memory instead of in a CPU register. Register variables should only be used for performance-critical variables that are frequently accessed.