How to combine two 32-bit integers into one 64-bit integer?

Question

I have a count register, which is made up of two 32-bit unsigned integers, one for the higher 32 bits of the value (most significant word), and other for the lower 32 bits of the value (least significant word).

What is the best way in C to combine these two 32-bit unsigned integers and then display as a large number?

In specific:

leastSignificantWord = 4294967295; //2^32-1

printf("Counter: %u%u", mostSignificantWord,leastSignificantWord);

This would print fine.

When the number is incremented to 4294967296, I have it so the leastSignificantWord wipes to 0, and mostSignificantWord (0 initially) is now 1. The whole counter should now read 4294967296, but right now it just reads 10, because I'm just concatenating 1 from mostSignificantWord and 0 from leastSignificantWord.

How should I make it display 4294967296 instead of 10?

Answer 1

It might be advantageous to use unsigned integers with explicit sizes in this case:

#include <stdio.h>
#include <inttypes.h>

int main(void) {
  uint32_t leastSignificantWord = 0;
  uint32_t mostSignificantWord = 1;
  uint64_t i = (uint64_t) mostSignificantWord << 32 | leastSignificantWord;
  printf("%" PRIu64 "\n", i);

  return 0;
}

Output

4294967296

Break down of (uint64_t) mostSignificantWord << 32 | leastSignificantWord

• (typename) does typecasting in C. It changes value data type to typename.

  (uint64_t) 0x00000001 -> 0x0000000000000001

• << does left shift. In C left shift on unsigned integers performs logical shift.

  0x0000000000000001 << 32 -> 0x0000000100000000

• | does 'bitwise or' (logical OR on bits of the operands).

  0b0101 | 0b1001 -> 0b1101

Answer 2

long long val = (long long) mostSignificantWord << 32 | leastSignificantWord;
printf( "%lli", val );

Answer 3

my take:

unsigned int low = <SOME-32-BIT-CONSTRANT>
unsigned int high = <SOME-32-BIT-CONSTANT>

unsigned long long data64;

data64 = (unsigned long long) high << 32 | low;

printf ("%llx\n", data64); /* hexadecimal output */
printf ("%lld\n", data64); /* decimal output */

Another approach:

unsigned int low = <SOME-32-BIT-CONSTRANT>
unsigned int high = <SOME-32-BIT-CONSTANT>

unsigned long long data64;
unsigned char * ptr = (unsigned char *) &data;

memcpy (ptr+0, &low, 4);
memcpy (ptr+4, &high, 4);

printf ("%llx\n", data64); /* hexadecimal output */
printf ("%lld\n", data64); /* decimal output */

Both versions work, and they will have similar performance (the compiler will optimize the memcpy away).

The second version does not work with big-endian targets but otoh it takes the guess-work away if the constant 32 should be 32 or 32ull. Something I'm never sure when I see shifts with constants greater than 31.

Answer 4

There's another way using arrays and pointers:

#include <stdio.h>
#include <inttypes.h>

int main(void) {
    // Two uint32_t to one uint64_t
    uint32_t val1[2] = {1000, 90000};
    uint64_t *val1_u64_ptr = (uint64_t*)val1; //intermediate pointer cast to avoid Wstrict-aliasing warnings
    uint64_t val2 = *val1_u64_ptr;
    printf("val2: %" PRIu64 "\n", val2);
    // val2: 386547056641000


    // back to uint32_t array from uint64_t
    uint64_t val3 = 386547056641000ull;
    uint32_t *val4 = (uint32_t*)&val3;
    printf("val4: %" PRIu32 ", %" PRIu32 "\n", val4[0], val4[1]);
    // val4: 1000, 90000
    
    return 0;
}

This code for me is much easier to understand and read. You are just creating a contiguous space in memory with two 32-bit unsigned int and then this same memory space is read as a single 64-bit unsigned int value and vice-versa. There are no operations involved only memory being read as different types.

EDIT

Forgot to mention that this is great if you already have a 64-bit array read from somewhere then you could easily read everything as 32-bit array pairs:

#include <stdio.h>
#include <inttypes.h>

int main() {
    uint64_t array64[] = {
        386547056641000ull,
        93929935171414ull,
        186655006591110ull,
        73141496240875ull,
        161460097995400ull,
        351282298325439ull,
        97310615654411ull,
        104561732955680ull,
        383587691986172ull,
        386547056641000ull
    };

    int n_items = sizeof(array64) / sizeof(array64[0]);
    
    uint32_t* array32 = (uint32_t*)&array64;
    for (int ii = 0; ii < n_items * 2; ii += 2) {
        printf("[%" PRIu32 ", %" PRIu32 "]\n", array32[ii], array32[ii + 1]);
    }

    return 0;
}

Output:

[1000, 90000]
[3295375190, 21869]
[22874246, 43459]
[2498157291, 17029]
[3687404168, 37592]
[1218152895, 81789]
[3836596235, 22656]
[754134560, 24345]
[4162780412, 89310]
[1000, 90000]

Using union struct

Still better and more readable would be to use a struct union as from https://stackoverflow.com/a/2810339/2548351:

#include <stdio.h>
#include <inttypes.h>

typedef union {
  int64_t big;
  struct {
    int32_t x;
    int32_t y;
  };
} xy_t;

int main() {
    // initialize from 64-bit
    xy_t value = {386547056641000ull};
    printf("[%" PRIu32 ",%" PRIu32 "]\n", value.x, value.y);
    // [1000, 90000]
    
    // initialize as two 32-bit
    xy_t value2 = {.x = 1000, .y = 90000};
    printf("%" PRIu64, value.big);
    // 386547056641000

    return 0;
}

Answer 5

This code works when both upper32 and lower32 is negative:

data64 = ((LONGLONG)upper32<< 32) | ((LONGLONG)lower32& 0xffffffff);

Answer 6

Instead of attempting to print decimal, I often print in hex.

Thus ...

printf ("0x%x%08x\n", upper32, lower32);

Alternatively, depending upon the architecture, platform and compiler, sometimes you can get away with something like ...

printf ("%lld\n", lower32, upper32);

or

printf ("%lld\n", upper32, lower32);

However, this alternative method is very machine dependent (endian-ness, as well as 64 vs 32 bit, ...) and in general is not recommended.

Hope this helps.

Answer 7

You could do it by writing the 32-bit values to the right locations in memory:

unsigned long int data64;
data64=lowerword
*(&((unsigned int)data64)+1)=upperword;

This is machine-dependent however, for example it won't work correctly on big-endian processors.

Answer 8

Late at the game, but I needed such a thing similar to represent a numerical base10 string of a 64bit integer on a 32bit embedded env..

So, inspired by Link I wrote this code that can do what asked in the question, but not limiting on base10: can convert to any base from 2 to 10, and can be easly extended to base N.

void stringBaseAdd(char *buf, unsigned long add, int base){

    char tmp[65], *p, *q;
    int l=strlen(buf);
    int da1, da2, dar;
    int r;

    tmp[64]=0;
    q=&tmp[64];
    p=&buf[l-1];
    r=0;
    while(add && p>=buf){

        da1=add%base;
        add/=base;
        da2=*p-'0';
        dar=da1+da2+r;

        r=(dar>=base)? dar/base: 0;
        *p='0'+(dar%base);
        --p;
    }

    while(add){

        da1=add%base;
        add/=base;
        dar=da1+r;

        r=(dar>=base)? dar/base: 0;
        --q;
        *q='0'+(dar%base);
    }

    while(p>=buf && r){

        da2=*p-'0';
        dar=da2+r;

        r=(dar>=base)? 1: 0;
        *p='0'+(dar%base);
        --p;
    }

    if(r){

        --q;
        *q='0'+r;
    }

    l=strlen(q);
    if(l){

        memmove(&buf[l], buf, strlen(buf)+1);
        memcpy(buf, q, l);
    }
}
void stringBaseDouble(char *buf, int base){

    char *p;
    int l=strlen(buf);
    int da1, dar;
    int r;

    p=&buf[l-1];
    r=0;
    while(p>=buf){

        da1=*p-'0';
        dar=(da1<<1)+r;

        r=(dar>=base)? 1: 0;
        *p='0'+(dar%base);
        --p;
    }

    if(r){

        memmove(&buf[1], buf, strlen(buf)+1);
        *buf='1';
    }
}
void stringBaseInc(char *buf, int base){

    char *p;
    int l=strlen(buf);
    int da1, dar;
    int r;

    p=&buf[l-1];
    r=1;
    while(p>=buf && r){

        da1=*p-'0';
        dar=da1+r;

        r=(dar>=base)? 1: 0;
        *p='0'+(dar%base);
        --p;
    }

    if(r){

        memmove(&buf[1], buf, strlen(buf)+1);
        *buf='1';
    }
}
void stringLongLongInt(char *buf, unsigned long h, unsigned long l, int base){

    unsigned long init=l;
    int s=0, comb=0;

    if(h){

        comb=1;
        init=h;
        while(!(init&0x80000000L)){

            init<<=1;
            init|=(l&0x80000000L)? 1: 0;
            l<<=1;
            s++;
        }
    }

    buf[0]='0';
    buf[1]=0;
    stringBaseAdd(buf, init, base);

    if(comb){

        l>>=s;
        h=0x80000000L>>s;
        s=sizeof(l)*8-s;
        while(s--){

            stringBaseDouble(buf, base);
            if(l&h)
                stringBaseInc(buf, base);

            h>>=1;
        }
    }
}

If you ask for

char buff[20];
stringLongLongInt(buff, 1, 0, 10);

your buff will contain 4294967296