The reliable but not very sexy way to seed random

Development Libraries
53

The latest version...

#include <avr/eeprom.h>

/*==============================================================================
  Call reseedRandom once in setup to start random on a new sequence.  Uses 
  four bytes of EEPROM.
==============================================================================*/

void reseedRandom( uint32_t* address )
{
  static const uint32_t HappyPrime = 127807 /*937*/;
  uint32_t raw;
  unsigned long seed;

  // Read the previous raw value from EEPROM
  raw = eeprom_read_dword( address );

  // Loop until a seed within the valid range is found
  do
  {
    // Incrementing by a prime (except 2) every possible raw value is visited
    raw += HappyPrime;

    // Park-Miller is only 31 bits so ignore the most significant bit
    seed = raw & 0x7FFFFFFF;
  }
  while ( (seed < 1) || (seed > 2147483646) );

  // Seed the random number generator with the next value in the sequence
  srandom( seed );  

  // Save the new raw value for next time
  eeprom_write_dword( address, raw );
}

inline void reseedRandom( unsigned short address )
{
  reseedRandom( (uint32_t*)(address) );
}


/*==============================================================================
  So the reseedRandom raw value can be initialized allowing different 
  applications or instances to have different random sequences.

  Generate initial raw values...

  https://www.random.org/cgi-bin/randbyte?nbytes=4&format=h
  https://www.fourmilab.ch/cgi-bin/Hotbits?nbytes=4&fmt=c&npass=1&lpass=8&pwtype=3

==============================================================================*/

void reseedRandomInit( uint32_t* address, uint32_t value )
{
  eeprom_write_dword( address, value );
}

inline void reseedRandomInit( unsigned short address, uint32_t value )
{
  reseedRandomInit( (uint32_t*)(address), value );
}


uint32_t reseedRandomSeed EEMEM = 0xFFFFFFFF;

void setup( void )
{
/*
  // Example that sets the seed to a specific value
  // Typically only done during debugging
  // Uses EEMEM to determine the EEPROM address
  reseedRandomInit( &reseedRandomSeed, 42 );
*/

  // Example that reseeds the random number generator each time the application starts
  // Uses EEMEM to determine the EEPROM address
  // Most common use
  reseedRandom( &reseedRandomSeed );

/*
  // Example that sets the seed to a specific value
  // Typically only done during debugging
  // EEPROM address 0 (through 3) is used to store the seed
  reseedRandomInit( (unsigned short) 0, 42 );
*/

/*
  // Example that reseeds the random number generator each time the application starts
  // EEPROM address 0 (through 3) is used to store the seed
  reseedRandom( (unsigned short) 0 );
*/
}


void loop( void )
{
}

Edit: 937 changed to 127807 to avoid the "dead zone".