Type Qualifiers in C

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There are many different type-qualifiers in the C programming language. Here is a brief description of some type qualifiers in C (and similar languages).

Do remember the “Clockwise/Spiral Rule” – Read the type qualifiers backwards

• int* – pointer to int
• int const* – pointer to const int (const int* == int const*)
• int *const – const pointer to int
• int const *const – const pointer to const int (const int* const == int const *const)
• int ** – pointer to pointer to int
• int **const – const pointer to pointer to int
• int *const * – pointer to const pointer to int
• int const ** – pointer to pointer to const int
• int *const *const – const pointer to const pointer to int
• volatile int *const – constant pointer to volatile int
• void (*signal(int, void (*fp)(int)))(int) – signal is a function passing an int and a pointer to a function passing an int returning nothing (void) returning a pointer to a function passing an int returning nothing (void)

Storage classes (listed below) come before type-qualifiers (also listed below). Only one storage class can be used when declaring a variable. Both storage and type qualifiers come before the datatype.

Storage Classes

• auto – Stored in stack during the code-block
• extern – Lasts the whole program, block, or compilation unit; globally visible
• register – Stored in stack or CPU-register during the code block
• static – Lasts the whole program, block, or compilation unit; private in program
• typedef – The data specifies a new datatype
• __thread – Thread-local-storage; one instance per thread
• _Thread_local – Thread-local data

Type-Qualifiers

• const – Value does not change; read-only
• restrict – For the lifetime of the pointer, the object can only be accessed via the pointer
• volatile – Optimizing-compilers must not change
• _Atomic – Map a variable to a basic built-in type (depending on the processor) so that reading and writing are guaranteed to happen in a single instruction

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