When is the \'register\' keyword in C merely a suggestion, and when is it an instruction?

Register Keyword in C : Unveiling the Difference

In C , the syntaxes "int x=7" and "register int x=7" may seem similar, but they signify a subtle distinction. The "register" keyword hints to the compiler that a variable should be stored in a processor register rather than memory.

This distinction arises from the concept of storage duration. By default, variables declared without the "register" keyword are stored in memory, which involves slower access times. In contrast, variables marked with "register" can potentially reside in registers, granting faster access speeds.

However, it's crucial to note that the "register" keyword is merely a hint. The compiler may or may not adhere to this request based on factors such as:

• Availability of registers: If all registers are occupied, the compiler may ignore the "register" specifier.
• Variable's behavior: If a variable is frequently modified or passed by reference, the compiler may decide against storing it in a register.

Herb Sutter, a renowned C expert, highlights this in his book, "Keywords That Aren't (or, Comments by Another Name)":

"A register specifier has the same semantics as an auto specifier..."

This quote implies that the "register" keyword behaves similarly to the "auto" specifier, which instructs the compiler to determine the variable's storage duration based on its usage patterns.

Ultimately, utilizing the "register" keyword is a subtle optimization technique that may not always result in the desired performance gains. However, it serves as a valuable tool to inform the compiler about the programmer's intentions regarding variable storage.

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