Convert a preprocessor token to a string

Question

I'm looking for a way to convert a preprocessor token to a string.

Specifically, I've somewhere got:

#define MAX_LEN 16

and I want to use it to prevent buffer overrun:

char val[MAX_LEN+1]; // room for \0
sscanf(buf, "%"MAX_LEN"s", val);

I'm open to other ways to accomplish the same thing, but standard library only.

Solution 1

see http://www.decompile.com/cpp/faq/file_and_line_error_string.htm specifically:

#define STRINGIFY(x) #x
#define TOSTRING(x) STRINGIFY(x)
#define AT __FILE__ ":" TOSTRING(__LINE__)

so your problem can be solved by doing sscanf(buf, "%" TOSTRING(MAX_LEN) "s", val);

Solution 2

I found an answer online.

#define VERSION_MAJOR 4
#define VERSION_MINOR 47

#define VERSION_STRING "v" #VERSION_MAJOR "." #VERSION_MINOR

The above does not work but hopefully illustrates what I would like to do, i.e. make VERSION_STRING end up as "v4.47".

To generate the proper numeric form use something like

#define VERSION_MAJOR 4
#define VERSION_MINOR 47

#define STRINGIZE2(s) #s
#define STRINGIZE(s) STRINGIZE2(s)
#define VERSION_STRING "v" STRINGIZE(VERSION_MAJOR) \
"." STRINGIZE(VERSION_MINOR)

#include <stdio.h>
int main() {
    printf ("%s\n", VERSION_STRING);
    return 0;
}

Solution 3

It's been a while, but this should work:

 sscanf(buf, "%" #MAX_LEN "s", val);

If not, it'll need to "double expansion" trick:

 #define STR1(x)  #x
 #define STR(x)  STR1(x)
 sscanf(buf, "%" STR(MAX_LEN) "s", val);

Solution 4

You should use the double-expansion stringification macro trick. Or just have a

#define MAX_LEN    16
#define MAX_LEN_S "16"

char val[MAX_LEN+1];
sscanf(buf, "%"MAX_LEN_S"s", val);

and keep it in sync. (That's a bit of a bother, but as long as the definitions are right next to each other, you'll probably remember.)

Actually, in this particular case, wouldn't strncpy suffice?

strncpy(val, buf, MAX_LEN);
val[MAX_LEN] = '\0';

If it were printf, though, this would be easier:

sprintf(buf, "%.*s", MAX_LEN, val);

Solution 5

While some of the above "work", personally I'd recommend just using a simple string API instead of the dreck that comes in libc. There are a number of portable APIs, some of which are also optimized for ease of inclusion in your project ... and some like ustr have tiny space overhead and support for stack variables.

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Comments

[Jonathan Leffler]

The first won't work; # stringizes macro arguments in macro expansions. The second will work.

[davenpcj]

Yeah, I wish the * or another symbol were available for length-limiting scanf routines with an argument. sprintf in that form works better than strncpy, imo, because strncpy won't terminate the string if it overruns the buffer, and writes extra data if the string is too short.

[feedc0de]

why cascading 2 macros? Wouldn't be one TOSTRING enough?

[webbo]

See answer below about double-expansion stringification. The C/C++ preprocessor is dirty and ugly and I believe without a standard. So why it requires two instead of one I could not say, because I don't want to do the research.

[Tim Sylvester]

@Daniel Brunner A single macro will paste the token itself, yielding literally "%" "MAX_LEN" "%" a second macro causes the token value to be pasted, e.g., "16" because the TOSTRING macro makes the final code equivalent to STRINGIFY(16).

[Matthieu]

@TimSylvester I wish I could give you rep for that comment! :)

[Jonathan Leffler]

@RaviRaj — because # only stringizes macro arguments and the first line is not in the body of a macro.