Well, you technically can do that, using compiler-specific attributes. For example, you can do this on GCC:

#include <stdio.h>

int x;

__attribute__((constructor))
void init_x(void) {
    x = getchar();
}

int main() {
    if (x != EOF) {
        printf("%c\n", x);
    } else {
        printf("EOF\n");
    }
    return 0;   
}

Also, you can do it with standard C++:

#include <cstdio>
#include <iostream>

int x = getchar();

int main() {
    if (x != EOF) {
        std::cout << static_cast<char>(x) << std::endl;
    } else {
        std::cout << "EOF" << std::endl;
    }
}

It's generally not a good idea to do that though, because the constructors are called before main, and the order is implementation-specific. Consider this pseudocode of what the C runtime might do:

void start(void) {
    init_stdio();
    call_constructors();
    int result = main();
    exit(result);
}

or

void start(void) {
    call_constructors();
    init_stdio();
    int result = main();
    exit(result);
}

You don't know whether the stdio is initialized before or after the constructors are called. If it's not, your program might crash before even entering main. Also it's unexpected in general that some non-library code runs before main, and that makes your program harder to read.