Скачать или смотреть How to Prevent Macro Expansion Inside String Literals in C Preprocessor

Learn how to avoid unwanted macro expansion within string literals by properly passing quoted strings to the compiler's -D option in C.
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This video is based on the question https://stackoverflow.com/q/79347034/ asked by the user 'ST0' ( https://stackoverflow.com/u/1007769/ ) and on the answer https://stackoverflow.com/a/79347173/ provided by the user 'Eric Postpischil' ( https://stackoverflow.com/u/298225/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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The Problem: Unexpected Macro Expansion Inside Strings

When using C preprocessor macros to create string literals, you might encounter unexpected expansions that alter the intended string. For example, defining a macro like MYPATH=/usr/linux/path and then stringifying it via macros can lead to surprising behavior. This happens because the macro inside the string is further expanded if it matches another macro (e.g., linux expanding to 1).

Example

[[See Video to Reveal this Text or Code Snippet]]

Compiled with:

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Output:

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This is because linux is expanded to 1 by the compiler's predefined macros.



The Solution: Pass Quoted Strings to the Compiler

The simplest and cleanest way to avoid macro expansion inside strings is to pass the entire string with quotes to the compiler during macro definition. Rather than relying on the macro to add quotes via stringification, provide the path as a proper string literal.

Steps

Change the code to use the macro directly without extra # stringification:

[[See Video to Reveal this Text or Code Snippet]]

Pass the path surrounded by quotes when compiling:

[[See Video to Reveal this Text or Code Snippet]]

Note:

The quotes must be escaped properly for your shell.

This way, the preprocessor replaces MYPATH with the literal string /usr/linux/path including the quotes, exactly as intended.



Why This Works

Stringizing a macro argument (# x) performs textual conversion, but after macro expansions are done on the argument.

Any macros inside the given value are expanded before stringizing.

Using quoted string literals passed as macro values preserves the exact string content without further expansions.



Summary

Avoid stringizing macros involving paths or strings that might contain macros themselves.

Directly assign const char *s = MYPATH;.

Pass macro definitions as proper quoted literals via your compiler command.

This approach is simple, portable, and reliable across different systems and compilers.