main.c

#include <stdio.h>

#include <errno.h>
#include <string.h>
#define err(s) fprintf(stderr, s "%s\n", strerror(errno))

#define F1_STR "1.txt"
#define F2_STR "2.txt"

int main(void) {
    int ret = 0;
    FILE *f1;
    if ((f1 = fopen(F1_STR, "r"))) {

        FILE *f2;
        if ((f2 = fopen(F2_STR, "r"))) {

            char s1[128];
            char s2[128];
            while (fscanf(f1, "%s", s1) != EOF &&
                   fscanf(f2, "%s", s2) != EOF) {
                if (strcmp(s1, s2) != 0) {
                    puts(s2);
                }
            }

            fclose(f2);
        } else {
            ret = 1;
            err(F2_STR ": ");
        }
        fclose(f1);
    } else {
        ret = 1;
        err(F1_STR ": ");
    }

    return ret;
}