2024-04-01

From wikipedia

register is a reserved keyword, type modifier, storage class, and hint.

The keyword hints the compiler to use a register to store a particular variable rather than using random-access memory (RAM).

In C the location of a variable declared with register cannot be accessed, but the sizeof operator can be applied.

Aside from this limitation, register is essentially meaningless in modern compilers due to optimization which will place variables in a register if appropriate regardless of whether the hint is given.

For programming of embedded systems register may still be significant; for example the Microchip MPLAB XC32 compiler allows the programmer to specify a particular register with the keyword; however, this is discouraged in favor of the compiler's optimizations. When used, register is typically for loop counters, or possibly for other very frequently used variables in the code.

Syntax:

/* store integer variable "i" in RAM, register, or other location as compiler sees fit */
int i;

/* suggests storing integer variable "i" in a CPU register or other fast location */
register int i;

source

Limitations

• The register keyword can only be applied to variables declared within a function (local variables). It cannot be used with global variables.
• You cannot take the address of a register variable because it doesn't have a memory location.
• It's also not possible to apply the register keyword to structure or union members.
• In practice, it's rare to see significant performance gains from using register, and its use is largely considered unnecessary in modern C programming.

Background

The "register" keyword tends to be ignored by modern compilers, because it messes with the compilers' optimized register assignment algorithms (which algorithms didn't exist in 1972, when the keyword was created).

Other uses

You can use the C register keyword to make sure your variable will not be accessed by address and changed outside of the scope that you expect.

For example:

#include <stdio.h>

int main()
{
    register int number = 1;
    scanf("%d", &number);
    printf("%d", number);

    return 0;
}

Compiles into an error:

/tmp/main.c:6:5: error: address of register variable ‘number’ requested.

Exception: If you're using a C++ compiler, it won't result in an error because C++ allows access to the address of register variables.