#include <stdio.h>
#include <string.h>

static void printDoubleAsBigEndianHexBytes(double x) {
    unsigned char buf[sizeof(double)];
    size_t i;
    memcpy(buf, &x, sizeof(double));
    printf("0x");
    for (i = 0; i < sizeof(double); ++i) {
        printf("%02x", buf[i]);
    }
    puts("");
}

int main(void) {
    const char* const fileName = "sample.dat";
    FILE* fp;
    double v;
    size_t len;

    puts("Input a real number:");
    if (scanf("%lf", &v) != 1) {
        fprintf(stderr, "Failed to read a number from stdin.\n");
        return -1;
    }
    printDoubleAsBigEndianHexBytes(v);

    /* ファイルへの書き込み */
    fp = fopen(fileName, "wb");
    if (fp == NULL) {
        fprintf(stderr, "Cannot open \"%s\" in write mode.\n", fileName);
        return -1;
    }
    len = fwrite(&v, sizeof(double), 1, fp); 
    if (len != 1) {
        fclose(fp);
        fprintf(stderr, "Failed to write to \"%s\".\n", fileName);
        return -1;
    }
    fclose(fp);

    /* ファイルからの読み込み */
    fp = fopen(fileName, "rb");
    if (fp == NULL) {
        fprintf(stderr, "Cannot open \"%s\" in read mode.\n", fileName);
        return -1;
    }
    len = fread(&v, sizeof(double), 1, fp);
    if (len != 1) {
        fclose(fp);
        fprintf(stderr, "Failed to read from \"%s\".\n", fileName);
        return -1;
    }
    fclose(fp);
    printf("Read value: %f\n", v);
    printDoubleAsBigEndianHexBytes(v);

    return 0;
}