Pseudo-random numbers

Names specified here
Name	Description			Source	Availability
`RAND_MAX`	Maximum of random range	M		`<stdlib.h>`	C89	C90	C95	C99	C11
`rand()`	Generate pseudo-random number		(·)	`<stdlib.h>`	C89	C90	C95	C99	C11
`srand()`	Seed random-number generator		(·)	`<stdlib.h>`	C89	C90	C95	C99	C11

<stdlib.h> declares functions for generating sequences of pseudo-random numbers.

#include <stdlib.h>
int rand(void);

rand returns the next random number in the sequence, in the range zero to RAND_MAX, which is at least 32767. If you want an integer in a specific range [a, b), you should scale it to the correct range:

double random_unif_r01(void)
{
  return rand() / (RAND_MAX + 1.0);
}

int random_unif_iab(int a, int b)
{
  return random_unif_r01() * (b - a) + a;
}

rand may have reasonably uniform distribution, but there may be platform-specific alternatives with better statistical properties.

POSIX, BSD and SVID define random to yield 32-bit pseudo-random numbers.

#include <stdlib.h>
void srand(unsigned seed);

srand re-seeds the sequence using the provided argument. The initial seed is 1.

An effective and portable way to randomize the initial seed is to use the current time:

#include <stdlib.h>
#include <time.h>
srand(time(NULL));

However, this technique and the use of rand in general are not suitable for cryptography.