In the

C programming language C (''pronounced'' '' – like the letter c'') is a general-purpose programming language. It was created in the 1970s by Dennis Ritchie and remains very widely used and influential. By design, C's features cleanly reflect the capabilities of ...

, register is a

reserved word In a programming language, a reserved word (sometimes known as a reserved identifier) is a word that cannot be used by a programmer as an identifier, such as the name of a variable, function, or label – it is "reserved from use". In brief, an '' ...

(or keyword), type modifier,

storage class The syntax of the C programming language is the set of rules governing writing of software in C. It is designed to allow for programs that are extremely terse, have a close relationship with the resulting object code, and yet provide relatively ...

, and hint. The register keyword was deprecated in C++, until it became reserved and unused in

C++17 C17, C-17 or C.17 may refer to: Transportation * , a 1917 British C-class submarine Air * Boeing C-17 Globemaster III, a military transport aircraft * Lockheed Y1C-17 Vega, a six-passenger monoplane * Cierva C.17, a 1928 English experimental ...

. It ''suggests'' that the

compiler In computing, a compiler is a computer program that Translator (computing), translates computer code written in one programming language (the ''source'' language) into another language (the ''target'' language). The name "compiler" is primaril ...

stores a declared variable in a

CPU A central processing unit (CPU), also called a central processor, main processor, or just processor, is the primary processor in a given computer. Its electronic circuitry executes instructions of a computer program, such as arithmetic, log ...

register Register or registration may refer to: Arts, entertainment, and media Music * Register (music), the relative "height" or range of a note, melody, part, instrument, etc. * ''Register'', a 2017 album by Travis Miller * Registration (organ), ...

(or some other faster location) instead of in

random-access memory Random-access memory (RAM; ) is a form of Computer memory, electronic computer memory that can be read and changed in any order, typically used to store working Data (computing), data and machine code. A random-access memory device allows ...

. If possible depending on the type of

and complexity of the program code, it will optimize access to that variable and hence improve the execution time of a program. In C (but not C++ where the keyword is essentially ignored) the location of a variable declared with register cannot be accessed, but the

sizeof 


sizeof is a  unary operator in the  C and  C++ programming languages that evaluates to the  storage size of an  expression or a data type, measured in units sized as char. Consequently, the expression sizeof(char) evaluates to 1. The number of  b ...

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 counter In computer science, a for-loop or for loop is a control flow statement for specifying iteration. Specifically, a for-loop functions by running a section of code repeatedly until a certain condition has been satisfied. For-loops have two parts: ...

s, or possibly for other very frequently used variables in the code.

Examples

/* 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;

References

{{Reflist C (programming language) Variable (computer science)

Examples

See also

References