Byteswap

static int byteswap(
    const char* filename,
    uint8_t* buf,
    size_t buffersize,
    size_t recordsize,
    int quiet
) {
    int returncode = 0;
    FILE* f = NULL;

    f = fopen(filename, "r+b");
    if(!f) { goto error_f; }

    for(;;) {
        size_t rec;
        size_t size;
        //
        // Read
        //
        clearerr(f);
        size = fread(buf, 1, buffersize, f);
        if(size == 0) {
            // Check for error
            if(ferror(f)) {
                goto error_f;
            }
            // Otherwise, done
            break;
        }
        //
        // Seek backwards
        //
        if(fseeko(f, -((off_t)size), SEEK_CUR) != 0) {
            goto error_f;
        }

        if(size > buffersize) { size = buffersize; }
        if(size < buffersize) {
            //
            // Round up to the next record, filling with zeroes as we go
            //
            size_t extra = size % recordsize;
            if(extra) {
                for(; extra < recordsize; extra++) {
                    buf[size++] = 0;
                }
            }
        }
        //
        // Reverse each record in memory
        //
        for(rec = 0; rec < size; rec += recordsize) {
            size_t a = rec;
            size_t b = rec + recordsize - 1;
            for(; a < b; a++, b--) {
                uint8_t t = buf[a];
                buf[a] = buf[b];
                buf[b] =  t;
            }
        }
        //
        // Write
        //
        if(fwrite(buf, 1, size, f) != size) {
            goto error_f;
        }
        fflush(f);
    }

    if(!quiet) {
        printf("%s: Done\n", filename);
    }
    goto done;

error_f:
    printfileerror(f, filename);
    goto error;

error:
    returncode = 1;

done:
    if(f != NULL) { fclose(f); }
    return returncode;
}