Using the Library
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#include <libexplain/libexplain.h>
#include <stdlib.h>
#include <unistd.h>

static void
process(FILE *fp)
{
    for (;;)
    {
        char buffer[4096];
        size_t n = explain_fread_or_die(buffer, 1, sizeof(buffer), fp);
        if (!n)
            break;
        explain_fwrite_or_die(buffer, 1, n, stdout);
    }
}

int
main(int argc, char **argv)
{
    for (;;)
    {
        int c = getopt(argc, argv, "o:");
        if (c == EOF)
            break;
        switch (c)
        {
        case 'o':
            explain_freopen_or_die(optarg, "w", stdout);
            break;
        default:
            fprintf(stderr, "Usage: %ss [ -o <filename> ] "
                "<filename>...\n", argv[0]);
            return EXIT_FAILURE;
        }
    }
    if (optind == argc)
        process(stdin);
    else
    {
        while (optind < argc)
        {
            FILE *fp = explain_fopen_or_die(argv[optind]++, "r");
            process(fp);
            explain_fclose_or_die(fp);
        }
    }
    explain_fflush_or_die(stdout);
    return EXIT_SUCCESS;
}