How deeply does Modern C cover null pointer constant?

Defines permitted types, distinguishes compile-time constants from const objects, and traces NULL into variadic-call failures.

Null pointer constant

As explained in 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.

Consider these declarations: double const*const nix = 0; double const*const nax = nix; The literal 0 is an integer constant expression with value zero. Used where a pointer is expected, it represents a null pointer. A null pointer constant is not a const-qualified pointer object: “constant” means compile-time constant, and the permitted type is integer or void*. section 11.1.5

double const*const nix = 0;
double const*const nax = nix;


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The declarations distinguish a null pointer value from the pointer objects that contain it. section 11.1.5

The C standard permits NULL to expand to several forms:

0, 0L, or other integer constant expressions with value zero. section 11.1.5
(void*)0, which emphasizes that the value is being used as a pointer. section 11.1.5
NULL has no prescribed underlying type, so using it in a variadic function can be nonportable; implementations may provide int 0 or void* 0. section 16.5.2

Choosing a spelling

Prefer 0 when a null pointer value is sufficient. If the pointer intent must be explicit, use (void*)0 directly. Avoid NULL: its name suggests a pointer constant, but its implementation-defined expansion can obscure its type and break variadic calls such as printf("%p: %s\n", NULL, "print of NULL"). section 11.1.5 section 16.5.2

How Modern C covers it

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Also explained in

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Modern C, Third EditionA 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.

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Related concepts

missing valueproduced by · A missing value is a representation or condition indicating that an expected data value or object reference is absent, unknown, unavailable, 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 const qualifier is a language feature that restricts modification or reassignment of a value or binding 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.

This explainer was generated by Manning's knowledge graph from the text of Modern C. The section links point to the parts of the book it draws on.