I have a registry for functions in my C code, and I have to provide an API to register custom functions at compile time to the registry.
Example: The "customer" side of code should like:
int a_test_function_a(){
return 13;
}
int init(void){
add_command(a_test_function_a);
}
In my registry code I defined this macro:
#define add_command(NAME) do{ \
command_registry = avl_tree_add( \
command_registry \
, #NAME \
, &##NAME \
, &command_handler); \
} while(0)
Here is what I do not understand. The output of gcc -E replace the string like expected:
do{ command_registry = avl_tree_add( command_registry , "a_test_function_a" , &a_test_function_a , &command_handler); } while(0);
But the compiler throw a error:
src/commands/commandRegisterTest.c:18:5: error: pasting formed '&a_test_function_a', an invalid
preprocessing token
add_command(a_test_function_a);
^
src/commands/../../../src/commands/command_api.h:18:8: note: expanded from macro 'add_command'
, &##NAME \
^
1 error generated.
How can I do this, that I do not have to call add_command('name', &function); by my own. I will register the function by it's name and just call the add_command(name) thing. Besides a solution, why is the preprocessor replace the line but fails? This doesn't make any sense for me.
Functions and function pointers are usually equivalent, i.e. you can simply omit the & ## and it'll work anyway but the preprocessor will be happy.
Anyway, you can simply remove the ## operator and it'll probably work, too. & var is fine so concatenation is not necessary.
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