C Pocket Reference

Type Qualifiers and Type Definitions

The type of an object can be qualified by the keywords const and volatile in the declaration.

The type qualifier const indicates that the program can no longer modify an object after its declaration. For example:

const double pi = 3.1415927;

After this declaration, a statement that modifies the object pi, such as pi = pi+1;, is illegal and results in a compiler error.

The type qualifier volatile indicates variables that can be modified by processes other than the present program. Based on this information, the compiler may refrain from optimizing access to the variable.

The type qualifiers volatile and const can also be combined:

extern const volatile unsigned  clock_ticks;

After this declaration, clock_ticks cannot be modified by the program, but may be modified by another process, such as a hardware clock interrupt handler.

Type qualifiers are generally prefixed to the type specifier. In pointer declarations, however, type qualifiers may be applied both to the pointer itself and to the object it addresses. If the type qualifier is to be applied to the pointer itself, it must be placed immediately before the identifier.

The most common example of such a declaration is the “pointer to a constant object.” Such a pointer may point to a variable, but cannot be used to modify it. For this reason, such pointers are also called “read-only” pointers. For example:

int var1 = 1, var2 = 2, *ptr; const int cArr[2]; const int *ptrToConst;// "Read-only pointer" ...