How to append to the name of an argument in C macro - c

Question

I have a weird use case in which I need to modify the name of a variable that was passed in C
#define SET_PROTOBUF_REPEATED(msg_ptr, field_name, src_data, src_data_length)
{
/*
msg_ptr->field_name.len = src_data_length;
*/
// I want to do this
// msg_ptr->n_field_name = src_data_length
/*
msg_ptr->field_name.data = malloc(src_data_length);
memcpy(msg_ptr->field_name.data, src_data, src_data_length);
*/
msg_ptr->field_name = malloc(src_data_length);
memcpy(msg_ptr->field_name, src_data, src_data_length);
}
Because I am using repeated type in a protocol buffer message, the variable length is defined as
n_variable_name
instead of
variable_name.len
eg.
variable_name = frequency
// the length have to be n_frequency
The reason I want to have a macro that do this is because I have a lot of protobuf messages repeating the functionality of the function above. For the sake of readability and modularity I think it is a good idea to define a new function that take care of this.
Any advice would be much appreciated
Thanks

Answer 1

The canonical way to write a multi-line macro is to wrap it inside a do ... while(0) statement. This ensures that the macro works as a single statement. Eg.:
#define macro(a,b,c) \
do { \
(a) = (b)*(c); \
} while (0) /* No trailing semi-colon */
...
macro (x, y, z);
for (i = 0; i < n; i++)
macro (x, y, z); /* This wouldn't work properly without do...while(0) */
You should put parenthesis around your macro parameters to avoid surprises when someone passes unexpected arguments, such as macro (t, 1+2, 3+4); You also need a helper macro to concatenate the parameters. Perhaps you're looking for something like this:
#define CONCAT(x,y) x ## y
#define SET_PROTOBUF_REPEATED(msg_ptr, field_name, src_data, src_data_length) \
do { \
(msg_ptr)->CONCAT(n_,field_name) = (src_data_length); \
\
(msg_ptr)->(field_name) = malloc((src_data_length)); \
memcpy((msg_ptr)->(field_name), (src_data), (src_data_length)); \
} while (0)