C Enum Declaration

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Syntax

enum [tag_name] {
    enumerator_1 [= constant_expression],
    enumerator_2 [= constant_expression],
    /* ... */
} [variable_list];

• tag_name: An optional identifier that names the enumeration type.
• enumerator_list: A comma-separated list of identifiers. A trailing comma is permitted in modern C (C99 onwards).
• variable_list: An optional comma-separated list of variable instances declared immediately alongside the type definition.

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Declaration Patterns

enum State {
    IDLE,
    RUNNING,
    HALTED
};

enum State current_state;

enum {
    READ_MODE,
    WRITE_MODE
} file_mode;

typedef enum {
    TCP,
    UDP
} Protocol;

Protocol network_proto;

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Enumerator Value Assignment

enum Configuration {
    MAX_CONNECTIONS = 100,  // Explicitly 100
    TIMEOUT = 50,           // Explicitly 50
    RETRY_LIMIT,            // Implicitly 51 (TIMEOUT + 1)
    DEFAULT_PORT = 8080,    // Explicitly 8080
    ALT_PORT                // Implicitly 8081 (DEFAULT_PORT + 1)
};

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Scope and Namespace Rules

• Enumerator Scope: Enumerators are injected directly into the enclosing scope of the enum declaration. C does not support scoped enumerations; accessing an enumerator via its tag (e.g., State.IDLE) is a syntax error. The identifier is simply IDLE.
• Identifier Uniqueness: Because enumerators leak into the enclosing scope, two different enumerations within the same scope cannot share an enumerator identifier.
• Tag Namespace: The tag_name occupies the C tag namespace (shared with struct and union). It can share an identifier with a variable or function, but it cannot share an identifier with another struct, union, or enum tag in the same scope.