Start mentionning macros arguments expansion rules

book/pages/03_log.md

@@ -327,3 +327,29 @@ Let's finish this feature and integrate it with the logging macros:
 :::{preprocessed} 03_eval3
 :output: markdown
 :::
+
+## Convert the line number to a string literal
+
+Currently, the line number of each log entry is placed at run-time, using the `"%i"`{l=C} printf format.
+
+But why do it at run-time ? `__LINE__` is a macro that expands to an integer literal, meaning its value is accessible at compile-time. It just isn't a string, so we cannot concatenate it as-is.
+
+We've just seen how the `#` operator can create a string literal from arbitrary code, let's use it !
+
+`__LINE__` is not an argument of our `log_log` macro, so we can't write `# __LINE__` directly. Instead we'll define a `STRINGIZE` macro that applies `#` to its argument:
+
+:::{preprocessed} 03_stringline1
+:no-compiler-view:
+:::
+
+Wait, that is not what we wanted ! We wanted `__LINE__` to be expanded to `8` __before__ being converted to a string !
+
+> Macro arguments are completely macro-expanded before they are substituted into a macro body, __unless they are stringized or pasted with other tokens__.
+
+
+:::{preprocessed} 03_stringline2
+:no-compiler-view:
+:::
+
+Success !
+

book/samples/03_stringline1.c

@@ -0,0 +1,10 @@
+#include <stdio.h>    // printf
+
+#define STRINGIZE(ARG) # ARG
+
+int main(int arg_count, char** arg_values)
+{
+	printf("Line as integer: %i\n", __LINE__);
+	printf("Line as string: %s\n", STRINGIZE(__LINE__));
+	printf("Concatenated: " STRINGIZE(__LINE__) "\n");
+}

book/samples/03_stringline2.c

@@ -0,0 +1,11 @@
+#include <stdio.h>    // printf
+
+#define STRINGIZE(ARG) # ARG
+#define STRINGIZE_WRAPPER(ARG) STRINGIZE(ARG)
+
+int main(int arg_count, char** arg_values)
+{
+	printf("Line as integer: %i\n", __LINE__);
+	printf("Line as string: %s\n", STRINGIZE_WRAPPER(__LINE__));
+	printf("Concatenated: " STRINGIZE_WRAPPER(__LINE__) "\n");
+}