How to convert 64 bit hex value to double in c?

3F947AE147AE147B₁₆ is the encoding for an IEEE-754 binary64 (a.k.a. “double precision”) datum with value 0.0200000000000000004163336342344337026588618755340576171875. Supposing your C implementation uses that format for double and has 64-bit integers with the same endianness, you can decode it (not convert it) by copying its bytes into a double and printing them:

#include <errno.h>
#include <limits.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>


int main(void)
{
    char *string = "0x3f947ae147ae147b";

    //  Set errno to zero before using strtoull.
    errno = 0;
    char *end;
    unsigned long long t = strtoull(string, &end, 16);

    //  Test whether stroull did not accept all characters.
    if (*end)
    {
        fprintf(stderr,
            "Error, string \"%s\", is not a proper hexadecimal numeral.\n",
            string);
        exit(EXIT_FAILURE);
    }

    //  Move the value to a 64-bit unsigned integer.
    uint64_t encoding = t;

    /*  Test whether the number is too large, either because strtoull reported
        an error or because it does not fit in a uint64_t.
    */
    if ((t == ULLONG_MAX && errno) || t != encoding)
    {
        fprintf(stderr, "Error, string \"%s\", is bigger than expected.\n",
            string);
        exit(EXIT_FAILURE);
    }

    //  Copy the bytes into a double.
    double x;
    memcpy(&x, &encoding, sizeof x);
    printf("%.9999g\n", x);
}

This should output “0.0200000000000000004163336342344337026588618755340576171875”.

If your C implementation does not support this format, you can decode it:

• Separate the 64 bits into s, e, f, where s is the leading bit, e is the next 11 bits, and f is the remaining 52 bits.
• If e is 2047 and f is zero, report the value is +∞ or −∞, according to whether s is 0 or 1, and stop.
• If e is 2047 and f is not zero, report the value is a NaN (Not a Number) and stop.
• If e is not zero, add 2⁵² to f. If e is zero, change it to one.
• The magnitude of the represented value is f•2⁻⁵²•2^e−1023, and its sign is + or − according to whether s is 0 or 1.