# How to generate a random int in C?

Is there a function to generate a random int number in C? Or will I have to use a third party library?

## Solutions

Note: Don't use `rand()` for security. If you need a cryptographically secure number, see this answer instead.

``````#include <time.h>
#include <stdlib.h>

srand(time(NULL));   // Initialization, should only be called once.
int r = rand();      // Returns a pseudo-random integer between 0 and RAND_MAX.
``````

On Linux, you might prefer to use random and srandom.

The `rand()` function in `<stdlib.h>` returns a pseudo-random integer between 0 and `RAND_MAX`. You can use `srand(unsigned int seed)` to set a seed.

It's common practice to use the `%` operator in conjunction with `rand()` to get a different range (though bear in mind that this throws off the uniformity somewhat). For example:

``````/* random int between 0 and 19 */
int r = rand() % 20;
``````

If you really care about uniformity you can do something like this:

``````/* Returns an integer in the range [0, n).
*
* Uses rand(), and so is affected-by/affects the same seed.
*/
int randint(int n) {
if ((n - 1) == RAND_MAX) {
return rand();
} else {
// Supporting larger values for n would requires an even more
// elaborate implementation that combines multiple calls to rand()
assert (n <= RAND_MAX)

// Chop off all of the values that would cause skew...
int end = RAND_MAX / n; // truncate skew
assert (end > 0);
end *= n;

// ... and ignore results from rand() that fall above that limit.
// (Worst case the loop condition should succeed 50% of the time,
// so we can expect to bail out of this loop pretty quickly.)
int r;
while ((r = rand()) >= end);

return r % n;
}
}
``````

### If you need secure random characters or integers:

As addressed in how to safely generate random numbers in various programming languages, you'll want to do one of the following:

• Use libsodium's `randombytes` API
• Re-implement what you need from libsodium's sysrandom implementation yourself, very carefully
• More broadly, use `/dev/urandom`, not `/dev/random`. Not OpenSSL (or other userspace PRNGs).

For example:

``````#include "sodium.h"

int foo()
{
char myString;
uint32_t myInt;

if (sodium_init() < 0) {
/* panic! the library couldn't be initialized, it is not safe to use */
return 1;
}

/* myString will be an array of 32 random bytes, not null-terminated */
randombytes_buf(myString, 32);

/* myInt will be a random number between 0 and 9 */
myInt = randombytes_uniform(10);
}
``````

`randombytes_uniform()` is cryptographically secure and unbiased.

Tags: C / Random

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