String concatenation using preprocessor

Question

is it possible to concatenate strings during preprocessing?

I found this example

#define H \"Hello \"
#define W \"World!\"
#define HW H W

printf(HW)         


        

Answer 1

From gcc online docs:

The '##' preprocessing operator performs token pasting. When a macro is expanded, the two tokens on either side of each '##' operator are combined into a single token, which then replaces the '##' and the two original tokens in the macro expansion.

Consider a C program that interprets named commands. There probably needs to be a table of commands, perhaps an array of structures declared as follows:

 struct command
 {
   char *name;
   void (*function) (void);
 };

 struct command commands[] =
 {
   { "quit", quit_command },
   { "help", help_command },
   ...
 };

It would be cleaner not to have to give each command name twice, once in the string constant and once in the function name. A macro which takes the name of a command as an argument can make this unnecessary. The string constant can be created with stringification, and the function name by concatenating the argument with _command. Here is how it is done:

 #define COMMAND(NAME)  { #NAME, NAME ## _command }

 struct command commands[] =
 {
   COMMAND (quit),
   COMMAND (help),
   ...
 };