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Stringification operator

As explained in Modern C, Third Edition

Macro argument stringification is the C preprocessing technique of converting a macro argument's textual spelling into a string literal.

In a function-like macro, placing # before a parameter converts the argument's textual spelling into a string literal. For example, #X records the expression passed as X rather than its evaluated value. The preprocessing step is purely textual, allowing whitespace adjustments but not interpreting the expression. section 17.3

Use stringification with these rules:

  • Stringification does not expand macros in the argument. Therefore, STRINGIFY(__LINE__) produces "__LINE__" rather than the current line number. section 17.3
  • Add an extra expansion layer when the argument must expand first: STRGY(X) expands to STRINGIFY(X), so STRGY(__LINE__) can produce "25" on line 25. section 17.3
  • Stringify __VA_ARGS__ like any other macro parameter; the resulting text includes the commas separating the variadic arguments. section 17.4.1

Tracing the original expression

A trace macro can include the expression itself in diagnostic output: TRACE_PRINT5("my favorite variable: %g", sum) produces a line such as main:25:(sum): my favorite variable: 889, while TRACE_PRINT5("a good expression: %g", sum*argc) records (sum*argc).

How Modern C, Third Edition covers it

How deep it goes
Explains textual conversion, suppressed argument expansion, and the two-layer workaround, but stops short of preprocessing implementation internals.
Examples
Uses STRINGIFY, STRGY, TRACE_PRINT5, __LINE__, expressions, and variadic arguments as concrete examples.
What it assumes
Assumes familiarity with C function-like macros and macro parameters while establishing stringification and delayed expansion directly from examples.
What you can do after
Apply it via C macros to emit expression text and safely stringify values such as expanded line numbers.
How it connects
Connects stringification with function-like macros, macro expansion, variadic macros, __VA_ARGS__, and __LINE__.
Cover of Modern C, Third Edition
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