Can anyone explain what the code putchar('/' //*/ 1) mean

Question

I was doing the exercise of the K&R2. When i was reading the code by Ben Pfaff in this page http://clc-wiki.net/wiki/K%26R2_solutions:Chapter_1:Exercise_23 I coudn't understand what the single code putchar('/' //*/ 1) mean. While in my compiler, it is a syntax error. So can anyone explain this to me.

Answer 1

If you read the comments at the beginning of the solution it explains why you're seeing that error:

It also contains examples of a comment that ends in a star and a comment preceded by a slash. Note that the latter will break C99 compilers and C89 compilers with // comment extensions.

In a compiler that does not support // style comments, this:

putchar('/' //**/ 
        1) 

Is equivalent to:

putchar('/'/1)

Which is legal -- though odd -- expression (remember that in C a char is a numeric type, so '/'/1 is the same as /). This happens because the sequence /**/ is an empty comment.

In a modern compiler with // style comments, the expression ends up being equivalent to:

puchar('/' 1)

Which is simply an error.

Answer 2

To make it clear, the original code is placed in multiple lines, like so:

    putchar('/' //**/
            1);

From here, /**/ part is a comment, so after pre-processing, the code would look like this:

putchar('/' / 1);

Which is equal to putchar('/');

You are getting compiler error because you are compiling this code either as C99 or, most likely, as C++, where // is a single-line comment. Compile as C89 instead.

Sorry for bad formatting - writing from my phone...