Can we declare the register variable as global?
Can We Declare the Register Variable as Global in C?
In the C programming language, a register variable is a hint to the compiler to store the variable in a CPU register for faster access. However, it's important to note that declaring a variable as register is merely a suggestion to the compiler, and it may choose to ignore it.
Local vs. Global Variables
In C, variables can be declared at different levels of scope. They can be:
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Local Variables: These are declared within a function or a block of code and are only accessible within that specific scope.
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Global Variables: These are declared outside of any function and can be accessed from anywhere in the program.
Can a register Variable be Global?
The register keyword is intended for local variables. Attempting to declare a register variable at global scope is not allowed in standard C.
register int global_variable; // Error: Cannot declare register variable at file scope
When you try to declare a register variable at global scope, most compilers will produce an error. This is because global variables have a scope that extends throughout the entire program, and trying to keep them in a CPU register could potentially conflict with the compiler's optimization strategies.
Why is it Not Allowed?
Register allocation is a complex optimization task performed by the compiler. The decision of which variables to store in registers is influenced by many factors, including the variable's usage pattern, available registers, and the compiler's optimization strategy.
Global variables, being accessible from anywhere in the program, pose a challenge for the compiler's register allocation strategy. It's not feasible for the compiler to predict all the places in the code where a global variable might be used, which makes it impractical to reserve a register for a global variable.
Conclusion
In summary, attempting to declare a register variable at global scope in C is not allowed by the language specification. It's important to use the register keyword judiciously, and even then, trust the compiler to make the best decision regarding register allocation.
Remember that modern compilers are highly optimized and will often make better decisions about register allocation than a programmer manually specifying it.