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enum {len = 12};
char id[len];  /* id will hold the ID, starting with the characters "ID" */
               /* followed by a random integer */
int r;
int num;
/* ... */
r = rand();  /* generate a random integer */
num = snprintf(id, len, "ID%-d", r);  /* generate the ID */
/* ... */

...

Compliant Solution (POSIX)

A better pseudorandom number generator is the random() function.

enum {len = 12};
char id[len];  /* id will hold the ID, starting with the characters "ID" */
               /* followed by a random integer */
int r;
int num;
/* ... */
srandom(time(0));  /* seed the PRNG with the current time */
/* ... */
r = random();  /* generate a random integer */
num = snprintf(id, len, "ID%-d", r);  /* generate the ID */
/* ... */

However, the instance of the random() function in this example uses time(0) as a seed. With a trivial seed like time(0), the results from random() are also predictable.

Compliant Solution (POSIX)

This compliant solution improves the previous non-compliant code example by adding some additional sources of information to the seed.

While the low dozen bits generated by rand() go through a cyclic pattern, all the bits generated by random() are usable.

enum {len = 12};
char id[len];  /* id will hold the ID, starting with the characters "ID" */
               /* followed by a random integer */
int r;
int num;
/* ... */
srandom(time(0)*getpid());  /* seed the PRNG with the current time */
/* ... */
r = random();  /* generate a random integer */
num = snprintf(id, len, "ID%-d", r);  /* generate the ID */
/* ... */

...

To achieve the best random numbers possible, an implementation-specific function must be used. When unpredictability really matters and speed is not an issue, such as in the creation of strong cryptographic keys, use a true entropy source such as /dev/random or a hardware device capable of generating random numbers. Note that the /dev/random device may block for a long time if there are not enough events going on to generate sufficient entropy.In many cases, however, it will be acceptable to simply use a pseudorandom number generator from a cryptographic library (such as the Mersenne Twister) and seed it with data read from /dev/random.

Compliant Solution (Windows)

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