Type Qualifiers in C

Many programmers may be confused about the different type-qualifiers and their meanings. Here is a brief description of some type qualifiers in C (and similar languages).

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.

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 one of the 5 basic built-in types

Further Reading

• Programming and Development (Article Index) - https://dcjtech.info/topic/programming-and-development/
• C Programming Article Forum - https://dcjtech.info/forum/articles/programming/c-c-and-c/
• Writing Your First C Program in Linux - https://dcjtech.info/topic/writing-your-first-c-program-in-linux/
• Compiler Warnings are Helpful; Do Not Ignore - https://dcjtech.info/topic/compiler-warnings-are-helpful-do-not-ignore/
• C vs C++ vs Objective-C vs etc - https://dcjtech.info/topic/c-vs-c-vs-objective-c-vs-etc/
• Machine Modes Used in C Attributes - https://dcjtech.info/topic/machine-modes-used-in-c-attributes/
• Standard C Libraries - https://dcjtech.info/topic/standard-c-libraries/
• Standard C Library Headers - https://dcjtech.info/topic/standard-c-library-headers/
• Linux Compilers - https://dcjtech.info/topic/linux-compilers/
• GNU-Toolchain - https://dcjtech.info/topic/gnu-toolchain/
• C Code Examples: Array Rotations - https://dcjtech.info/topic/c-code-examples-array-rotations/
• C Code Examples: Cyclic Redundancy Check (CRC) - https://dcjtech.info/topic/c-code-examples-cyclic-redundancy-check-crc/
• C Code Examples: File IO Tests - https://dcjtech.info/topic/c-code-examples-file-io-tests/
• C Code Examples: Stack and Queue Datatype - https://dcjtech.info/topic/c-code-examples-stack-and-queue-datatype/