typedef struct
{
    void (*on_event)(my_struct * a_struct);
} my_struct;

void event_handler(my_struct * a_struct);

struct my_struct_t;

struct my_struct_t
{
    void (*on_event)(struct my_struct_t * a_struct);
};

typedef struct my_struct_t my_struct;

void event_handler(my_struct * a_struct);

cat 1.c
#include <stdio.h>

typedef struct _my_struct{
    void (*on_event)(struct _my_struct * a_struct);
    char *eventarg;
} my_struct;

void parcearg(my_struct *s){
        printf("arg: %s\n", s->eventarg);
}

int main(int argc, char **argv){
        my_struct s = {parcearg, NULL};
        int i;
        for(i = 1; i < argc; ++i){
                s.eventarg = argv[i];
                s.on_event(&s);
        }
        return 0;
}

gcc 1.c -Wall -Werror -Wextra && ./a.out hello world
arg: hello
arg: world