C - How to assign a value inside my macro?

I am trying to assign a value inside my x-macro, but I don't really understand why it is not working:

#include <stdio.h>

typedef struct
{
    int a;
    int b;
} struct_t;

#define MY_LIST \
    MY_ELEMENT(a) \
    MY_ELEMENT(b)

#define MY_ELEMENT(x) struct_t x; \ 
x.a=33;
MY_LIST 
#undef MY_ELEMENT

int main(void)
{
    fprintf(stdout, "a: %d\n", a.a);
    return 0;
}

When compiling this I get the following error:

test.c:14:2: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or   ‘__attribute__’ before ‘.’ token
 x.a=33;

Could somebody explain why I am getting this error and how to solve this?

You need to look into the preprocessed form of your code in your source file slethoprod.c. With GCC, you can get it with gcc -C -E slethoprod.c > slethoprod.i then inspect (with an editor or a pager) that slethoprod.i file.

It contains stuff like:

struct_t a; a.a = 33; struct_t b; b.a = 33;

which is obviously not valid C code (since it has some assignment outside of any function, at file scope; remember that an initialization in a declaration is not an assignment).

You might want to have some definition (with initialization) such as

struct_t a = {33};

or even (for readability purposes) a struct initialization like

struct_t b = {.a=33};

and you could play fancy preprocessor tricks to get that.

Look into some C reference site and/or study the C11 standard n1570 to learn more about C. Read also the documentation of your compiler (e.g. GCC) and of your preprocessor (e.g. cpp).

BTW, I personally feel that naming a global with the same name a as some field in it is poor taste (even if it is legal, since field names and global variables have different namespaces). For readability purposes, I recommend avoiding that.