Generate a random byte stream

Question

I'm testing a network program that sends packet to a remote server using UDP/TCP, and for that, I want to generate some random byte streams.

Here's the function:

unsigned char *gen_rdm_bytestream(int num_bytes)
{
    unsigned char *stream = malloc(num_bytes);
    /*
     * here how to generate?
     */
    return stream;
}

Answer 1

Yes, you have int rand (void); function in C,

Returns a pseudo-random integral number in the range between 0 and RAND_MAX. RAND_MAX is a constant defined in <cstdlib>.

This number is generated by an algorithm that returns a sequence of apparently non-related numbers each time it is called. This algorithm uses a seed to generate the series, which should be initialized to some distinctive value using function srand().

EDIT:

As you are commenting, I have written a code for you that may help you to demonstrate, How to use rand(), the program and its output are:

#include <stdio.h>     
#include <stdlib.h>  /* srand, rand */
#include <time.h>    
int main (){
 int i=0;
 srand (time(NULL));
 printf("Five rand numbers: \n");
 for(i = 1; i <= 5; i++){
   printf("\n %d", rand());
 }
 printf("Five rand numbersb between 2 to 5: \n");
 for(i = 1; i <= 5; i++){
   printf("\n %d", (2 + rand()%4));
 }    
 return 1;
} 

Output:

 Five rand numbers: 

 1482376850
 1746468296
 1429725746
 595545676
 1544987577

 Five rand numbers, between 2 to 5: 

 2
 5
 3
 4
 3

Codepad Link

Answer 2

For each byte, you can call a random number generator function. The C standard provides the function rand. Before using it, you should initialize the random sequence with a call to srand.

gen_rdm_bytestream may then look something like that:

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

unsigned char *gen_rdm_bytestream (size_t num_bytes)
{
  unsigned char *stream = malloc (num_bytes);
  size_t i;

  for (i = 0; i < num_bytes; i++)
  {
    stream[i] = rand ();
  }

  return stream;
}

srand ((unsigned int) time (NULL));

Since stream is unsigned, if the value returned by rand is greater than UCHAR_MAX, she will be reduced (modulo UCHAR_MAX). Therefore you will get pseudo-random numbers between 0 and 255.

Answer 3

Here's a general function for producing random integer values in a given range:

#include <stdlib>

/**
 * Assumes srand has already been called
 */
int randInRange( int min, int max )
{
  double scale = 1.0 / (RAND_MAX + 1);
  double range = max - min + 1;
  return min + (int) ( rand() * scale * range );
}

Leveraging this to create unsigned char values:

u_char randomByte()
{
  return (u_char) randInRange( 0, 255 );
}

So,

for ( i = 0; i < numBytes; i++ )
  stream[i] = randomByte();

Answer 4

On UNIX, there is the getrandom system call.

#include <sys/random.h>
#include <stdlib.h>

unsigned char * gen_rdm_bytestream(size_t const num_bytes) {
    unsigned char * const stream = malloc(num_bytes);
    getrandom(stream, num_bytes, 0);
    return stream;
}