What does the restrict keyword mean in C?

Take this example:

char* realpath(const char *restrict, char *restrict);

realpath(path, realpath) canonicalizes the filepath path and puts the result in the buffer realpath. If realpath == NULL, it allocates a new string and returns a pointer to it.

In C, restrict is a “type qualifier”. (Other things in this category are const and volatile). This means, for some type T, we can write T restrict to get another type. For example, char const * restrict is a type.

Actually, it only applies to pointer types, i.e. T * restrict. So int restrict is invalid, but int * restrict is valid.

In a function, a parameter T * restrict p means that the allocated object pointed at by p is only pointed at by p. That is, during the execution of the function body, the only way to access *p is via p itself (also allowing for pointer manipulation like p++). Other variables in scope, such as other function parameters, or global variables, do not point at *p; nor does the memory graph available from those variables contain any pointers to *p.

The compiler can then make some optimizations.

Ref: https://en.wikipedia.org/wiki/Restrict

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