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Preface Overview of the Compiler Getting Started with the Compiler Compiler Features Compiler Coding Practices Compiler Diagnostic Messages Using the Inline and Embedded Assemblers of the AR Compiler Command-line Options Language Extensions Preprocessor extensions #assert #include_next #unassert #warning C99 language features available in C90 // comments Subscripting struct Flexible array members C99 language features available in C++ and C90 Variadic macros long long restrict Hexadecimal floats Standard C language extensions Constant expressions Array and pointer extensions Block scope function declarations Dollar signs in identifiers Top-level declarations Benign redeclarations External entities Function prototypes Standard C++ language extensions ? operator Declaration of a class member friend Read/write constants Scalar type constants Specialization of nonmember function templates Type conversions Standard C and Standard C++ language extensions Address of a register variable Arguments to functions Anonymous classes, structures and unions Assembler labels Empty declaration Hexadecimal floating-point constants Incomplete enums Integral type extensions Label definitions Long float Nonstatic local variables Structure, union, enum, and bitfield extensions GNU extensions to the C and C++ languages Compiler-specific Features C and C++ Implementation Details What is Semihosting? Via File Syntax Summary Table of GNU Language Extensions Standard C Implementation Definition Standard C++ Implementation Definition C and C++ Compiler Implementation Limits


8.13 restrict

The restrict keyword is a C99 feature. It enables you to convey a declaration of intent to the compiler that different pointers and function parameter arrays do not point to overlapping regions of memory at runtime.

This enables the compiler to perform optimizations that can otherwise be prevented because of possible aliasing.


The keywords __restrict and __restrict__ are supported as synonyms for restrict and are always available.
You can specify --restrict to allow the use of the restrict keyword in C90 or C++.


The declaration of intent is effectively a promise to the compiler that, if broken, results in undefined behavior.


The following example shows use of the restrict keyword applied to function parameter arrays.
void copy_array(int n, int *restrict a, int *restrict b)
    while (n-- > 0)
        *a++ = *b++;
The following example shows use of the restrict keyword applied to different pointers that exist in the form of local variables.
void copy_bytes(int n, int *a, int *b)
    int *restrict x;
    int *restrict y;
    x = a;
    y = b;
    while (n-- > 0)
        *q++ = *s++;
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