Why does random() return a 32 bit number?


The documentation says that random() returns a long. When I ask for sizeof(long), I get 8 bytes. So a long should be 64 bits. But when I ask for the hex value of random(), I always get 8 numbers in hex, which is a 32 bit number. And the authors seem to have expected this (left shifting 32 and then “bir Orring” another 32 bit number from random() to get a random 64 bit number). Is there something I am not understanding about long? Or does our seed from srandom((unsigned int) time(NULL)) play a role? Any explanations for this behavior?


Try printing the long value in binary form, and see if that sheds any light. Maybe, random() is returning a 32-bit value in a 64-bit long.


How do you print a number in binary form. Do you create your own function for that, or is there a format specifier you can use in printf of NSLog?

#include <stdio.h>
#include <stdlib.h>

void printLongBinary (long);

int main (int argc, const char * argv[]) {
    printLongBinary (0);
    printLongBinary (1);
    printLongBinary (2);
    printLongBinary (3);
    printLongBinary (4);
    printLongBinary (5);
    printLongBinary (-1);
    printLongBinary (random ());
    return 0;

void printLongBinary (long value)
    const unsigned long NUMBITS = 8 * sizeof (value);
    printf ("%ld: ", value);
    for (unsigned long bit = 0; bit < NUMBITS; ++bit) {
        const char digit = value & ((long)1 << (long)bit) ? '1' : '0';
        printf ("%c", digit);
    printf ("\n");
0: 0000000000000000000000000000000000000000000000000000000000000000
1: 1000000000000000000000000000000000000000000000000000000000000000
2: 0100000000000000000000000000000000000000000000000000000000000000
3: 1100000000000000000000000000000000000000000000000000000000000000
4: 0010000000000000000000000000000000000000000000000000000000000000
5: 1010000000000000000000000000000000000000000000000000000000000000
-1: 1111111111111111111111111111111111111111111111111111111111111111
1804289383: 1110011010100010110100011101011000000000000000000000000000000000

Thanks for the function. It looks like random() only returns a 32 bit number.


random() does actually return a long and, at least in modern macOS, a long is 64-bits. However, there is a catch: random() only generates 32 bits of random data and fits those 32 bits inside a 64-bit long.

So, if we were to print the output of random() in binary format, we would get something like this:

That is the reason why the author needs to execute this code…
int64_t randomBytes = (random() << 32) | random();
To generate 8 bytes (64 bits) of random data, like this: