Reverse bytes for 64-bit value

You cannot use char x for a pointer!!!! A char is only a single byte long.

You need at the very least

unsigned long int swapPtr(unsigned long int x) {

Or better, use the type of the pointer

void* swapPtr(void* x) {

Quite likely your compiler will complain when you start bit shifting pointers; in that case you're better off explicitly casting your argument to an unsigned 64 bit integer:

#include <stdint.h>
uint64_t x;

Note also that you have to call with the address of a variable, so you call with

result = swapLong(&loc);

not *loc (which looks at the place where loc is pointing - the value, not the address).

Complete program:

#include <stdio.h>
#include <stdint.h>

uint64_t swapLong(void *X) {
  uint64_t x = (uint64_t) X;
x = (x & 0x00000000FFFFFFFF) << 32 | (x & 0xFFFFFFFF00000000) >> 32;
x = (x & 0x0000FFFF0000FFFF) << 16 | (x & 0xFFFF0000FFFF0000) >> 16;
x = (x & 0x00FF00FF00FF00FF) << 8  | (x & 0xFF00FF00FF00FF00) >> 8;
return x;
}

int main(void) {
  char a;
  printf("the address of a is 0x%016llx\n", (uint64_t)(&a));
  printf("swapping all the bytes gives 0x%016llx\n",(uint64_t)swapLong(&a));
}

Output:

the address of a is 0x00007fff6b133b1b
swapping all the bytes gives 0x1b3b136bff7f0000

EDIT you could use something like

#include <inttypes.h>

printf("the address of a is 0x%016" PRIx64 "\n", (uint64_t)(&a));

where the macro PRIx64 expands into "the format string you need to print a 64 bit number in hex". It is a little cleaner than the above.

Here is an alternative way for converting a 64-bit value from LE to BE or vice-versa.

You can basically apply this method any type, by defining var_type:

typedef long long var_type;

Reverse by pointer:

void swapPtr(var_type* x)
{
    char* px = (char*)x;
    for (int i=0; i<sizeof(var_type)/2; i++)
    {
        char temp = px[i];
        px[i] = px[sizeof(var_type)-1-i];
        px[sizeof(var_type)-1-i] = temp;
    }
}

Reverse by value:

var_type swapVal(var_type x)
{
    var_type y;
    char* px = (char*)&x;
    char* py = (char*)&y;
    for (int i=0; i<sizeof(var_type); i++)
        py[i] = px[sizeof(var_type)-1-i];
    return y;
}

You may also use _bswap64 intrinsic (which has latency of 2 and a throughput of 0.5 on Skylake Architecture). It is a wrapper for the assembly instruction bswap r64 so probably the most efficient :

Reverse the byte order of 64-bit integer a, and store the result in dst. This intrinsic is provided for conversion between little and big endian values.

#include <immintrin.h>

uint64_t swapLongIntrinsic(void *X) {
    return __bswap_64((uint64_t) X);
}

NB: Don't forget the header