Decided to see if I could still write C from memory.

nostalgia.c

// Compile: `gcc -Wall -o hello nostalgia.c`
// Run: `./hello`

#include <stdlib.h>
#include <stdio.h>

int main(int argc, char** argv) {
	printf("Hello world.\n");
	return(0);
}

Technically I overdid it. This is the bare minimum:

#include <stdio.h>
int main() {
    printf("Hello world.\n");
}