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    FILE *ifp, *evofp, *odofp;
 
    if ((ifp = fopen("infile.txt", "r")) == NULL) {
        fprintf(stderr, "error: can't open input file" "\n");
        return EXIT_FAILURE;
    }
 
    if ((evofp = fopen("even.txt", "w")) == NULL) {
        fprintf(stderr, "error: can't open or create output file" "\n");
        return EXIT_FAILURE;
    }
 
    if ((odofp = fopen("odd.txt", "w")) == NULL) {
        fprintf(stderr, "error: can't open or create output file" "\n");
        return EXIT_FAILURE;
    }
 
...
 
    fclose(ifp);
    fclose(evofp);
    fclose(odofp);

    char line[1000];
    long lineno = 0;
 
    while (fgets(line, sizeof line, ifp) != NULL) {
        lineno++;
        if (lineno % 2 == 0)
            fputs(line, evofp);
        else
            fputs(line, odofp);
    }

    char line[1000];
    long lineno = 0;
 
    while (fgets(line, sizeof line, ifp) != NULL)
        fputs(line, (++lineno % 2 == 0) ? evofp : odofp);