Null pointer constant
A null pointer constant is a language-defined zero-valued or null-pointer expression that can be converted to a null pointer.
Modern C, Third Edition
A pre-C23 null pointer constant is a compile-time constant expression of integer type and value zero or an expression of type void* that can be converted to a null pointer.
- How it is used
- Distinguishes compile-time null pointer constants from pointer objects holding null values and guides portable use of `NULL`.
- How deep it goes
- Defines the pre-C23 forms, examines permissible `NULL` expansions, and sketches a conditional-expression test without providing its completed macro.
- Examples
- Uses two `const` pointer objects, possible `NULL` expansions, and a conditional expression whose type distinguishes qualifying expressions.
- What you can do after
- Evaluate/understand whether a pre-C23 expression qualifies as a null pointer constant and why `NULL` may hide a type choice.
Read how Modern C, Third Edition explains null pointer constant →
Modern C
A null pointer constant is an integer constant expression with value zero or such a value converted to void pointer, as permitted by the C standard.
- How it is used
- Clarifies how zero-valued constants represent null pointers and why NULL is unsafe in variadic interfaces.
- How deep it goes
- Defines permitted types, distinguishes compile-time constants from const objects, and traces NULL into variadic-call failures.
- Examples
- Uses const pointer declarations, permitted NULL expansions, and a printf call demonstrating variadic type mismatch.
- What you can do after
- Evaluate null-pointer spellings and avoid NULL where its unspecified type can compromise portability.
Related concepts
- missing valueproduced by · A missing value is a marker or condition indicating that an expected data value or reference is absent, unavailable, unknown, or inapplicable.
- macrokind of · A macro is a named construct that expands or transforms source code, commands, or templates before or during execution.
- referencecontrasts with · A reference is an indirect value or relationship that identifies or provides access to another value, object, resource, or source.
- const qualifiercontrasts with · A language feature that marks a value, binding, or access path as constant, restricting reassignment or modification according to the language’s rules.
- void pointeruses · A void pointer is a C pointer of type void* that can hold the address of an object without specifying its pointed-to object type.
- implicit conversioncontrasts with · Implicit conversion is an automatic change of a value from one type to another according to language rules, without an explicit conversion operation in the source code.
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