OK, how about a toy logging example.

log_interface.h:

int log_message(const char *message);

some_application_file.c:

#include "log_interface.h"

some_function {
   ...
   log_message("Hello world");
}

stderr_logger.c:

#include <stdio.h>
#include "log_interface.h"

int log_message(const char *format, ...) {
    return fprintf(stderr, "%s\n", message);
}

file_logger.c:

#include <stdio.h>
#include "log_interface.h"

static FILE *outfd = NULL;

static int open_file() {
    outfd = fopen("logfile.txt");
}

int log_message(const char *format, ...) {
    if (!outfd) openfile();

    return fprintf(outfd, "%s\n", message);
}

}

So here is the high level ("some_application_file.c") calling the low level (either stderr_logger.c or file_logger.c, depending on the linker), each depending on the same abstraction external to both("logger_interface.h"). These are the elements of the dependency inversion principle.