Using macros in printf with a number sign

Question

I'm a bit confused about an explanation concerning macros in K&R 2nd Ed, p.90. Here is the paragraph:

Formal parameters are not replaced within quoted strings. If, however, a parameter name is preceded by a # in the replacement text, the combination will be expanded into a quoted string with the parameter replaced by the actual argument.

I'm not sure what that second sentence is saying. It goes on to explain a use for this with a "debugging print macro".

This can be combined with a string concatenation to make, for example, a debugging print macro:

#define dprint(expr) printf(#expr " = %g\n", expr);

Edit:

All the input was useful. Thank you guys.

jxh · Accepted Answer · 2014-08-31 01:11:56Z

5

If you define macro like this:

#define MAKE_STRING(X) #X

Then, you can do something like this:

puts(MAKE_STRING(a == b));

Which will expand into:

puts("a == b");

In the dprint() example, it is printing out a string form of the expression, as well as the expression value.

dprint(sin(x)/2);

Will expand into:

printf("sin(x)/2" " = %g\n", sin(x)/2);

String literal concatenation will treat the first parameter as a single string literal.

answered Aug 31, 2014 at 1:11

jxh

70.8k8 gold badges116 silver badges203 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Martin Liversage · Accepted Answer · 2014-08-31 01:39:14Z

3

It is just a neat feature where you can convert a macro parameter into a string literal which mainly is useful for debugging purposes. So

dprint(x + y);

is expanded by the C preprocessor to this

printf("x + y = %g\n", x + y);

Notice how the value of the parameter expr appears both inside the string literal and also in the code generated by the macro. For this to happen you need to prefix expr with # to create a string literal.

One thing worth pointing out is that adjacent string literals are combined into a single string literal, e.g. "x + y" " = %g\n" are combined into "x + y = %g\n".

edited Aug 31, 2014 at 1:39

answered Aug 31, 2014 at 1:16

Martin Liversage

107k24 gold badges216 silver badges263 bronze badges

Comments

user3920237 · Accepted Answer · 2014-08-31 01:14:20Z

2

#expr is expanded into "expr". Two string literals next to each other are automatically concatenated. We can see that invoking gcc -E for dprint(test) will give the following output:

("test" " = %g\n");

answered Aug 31, 2014 at 1:14

user3920237

Comments

Aman · Accepted Answer · 2014-09-03 09:06:18Z

0

This site may help. It describes how stringification can be implemented.

answered Sep 3, 2014 at 9:06

Aman

1,77214 silver badges21 bronze badges

Collectives™ on Stack Overflow

Using macros in printf with a number sign

4 Answers 4

Comments

Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

Comments

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related