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A function definition in C provides the actual implementation of a function, binding an identifier and a signature to a block of executable statements. Unlike a function declaration (or prototype) which only informs the compiler about a function’s interface, the definition allocates memory for the function’s instructions and specifies the exact operations to be performed upon invocation. 
[storage_class_specifier] [function_specifier] return_type function_name(parameter_list) {
    // function body (compound statement)
}

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Structural Components

A standard C function definition consists of two primary parts: the function header and the function body.

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1. Function Header

The header establishes the function’s interface and consists of up to five elements:
  • storage_class_specifier (Optional): Determines the function’s linkage. The static keyword restricts linkage to the current translation unit (internal linkage), while extern explicitly declares external linkage (which is the default if omitted).
  • function_specifier (Optional): Provides specific directives to the compiler. Examples include inline (suggesting inline substitution) and _Noreturn (indicating the function never returns control to its caller).
  • return_type: Specifies the C data type of the value the function yields to the calling environment. If the function does not return a value, the void keyword must be used.
  • function_name: A unique identifier used to invoke the function. It must adhere to standard C identifier naming rules.
  • parameter_list: A comma-separated sequence of parameter declarations enclosed in parentheses. Each parameter requires both a type specifier and an identifier (e.g., int x, float y). If the function accepts no arguments, the keyword void should be explicitly placed within the parentheses.

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2. Function Body

The body is a compound statement enclosed in curly braces {}. It dictates the function’s behavior and contains:
  • Local Declarations: Variables declared within this block possess block scope and automatic storage duration by default.
  • Executable Statements: The sequence of operations the function performs.
  • return Statement: Terminates function execution and passes control back to the caller. If the return_type is not void, the return statement must evaluate to a value compatible with the return_type. Reaching the closing brace } of a non-void function without executing a return statement is permitted by the compiler, but results in undefined behavior if the caller attempts to use the return value (with the exception of main in C99 and later, which implicitly returns 0).

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Concrete Example

static inline int calculate_sum(int operand_a, int operand_b) {
    int result = operand_a + operand_b;
    return result;
}

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Technical Characteristics

  • Linkage and Definition Rules: By default, function definitions possess external linkage, meaning they are visible and callable across multiple translation units. A function with external linkage must be defined exactly once across the entire program. However, functions with internal linkage (static) and inline functions can be legally defined in multiple translation units within the same C program without violating definition rules.
  • Implicit Declaration: If a function definition appears in a translation unit before it is called, the definition inherently acts as the function’s declaration for subsequent code in that file.
  • Parameter Shadowing: Identifiers used in the parameter_list are local to the function body and will shadow any global variables sharing the same identifier.
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