Tag: c/c++

Today I came across an annoying problem: how do I expand a C macro into a string?

One of C’s preprocessor operators is #, which surrounds the token that follows it in the replacement text with double quotes (“). So, at first the solution sounds pretty simple: just define

#define STR(tok) #tok

and things will work. However, there is one caveat: it will not work if it is passed another macro. For example,

#define BUF_LEN 100
#define STR(tok) #tok

STR(BUF_LEN)

will produce, after going through the preprocessor,

"BUF_LEN"

instead of "100", which is undesired. This behavior is due to the C standard noting that no macro expansions should happen to a token preceded by #.

However, after reconsidering the source of the problem, I’ve found the following workaround: define another macro that will expand the argument and only then call the macro that does the quoting.

#define STR_EXPAND(tok) #tok
#define STR(tok) STR_EXPAND(tok)

#define BUF_LEN 100

STR(BUF_LEN)

will produce

"100"

as desired.

Explanation: The STR macro calls the STR_EXPAND macro with its argument. Unlike in the first example, this time the parameter is checked for macro expansions and evaluated by the preprocessor before being passed to STR_EXPAND, which quotes it, thus giving the desired behavior.

Well, it isn’t really an operator, but this is a nice C code construct I ran into. It doesn’t seem to have any benefit except as a nice way to create a reverse loop:

int count = 100;
while (count-->0) {
        //some work
}

As I said, I don’t think I’ll find any performance benefit in using this code snippet. On the other hand, it is always fun to see the puzzled face other programmers have the first time they see the code.