An example of a condition variable implementation using mutex'es from FreeRTOS for a NXP chip; however, it's easily portable to other architectures.

conditionVariable.cpp

/*Copyright (C) 
 * 2021 - fjrg76 at hotmail dot com
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License
 * as published by the Free Software Foundation; either version 2
 * of the License, or (at your option) any later version.
 * 
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 * 
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA  02111-1307, USA.
 * 
 */

/* Some theory (in spanish) here: http://fjrg76.com/2021/06/08/variables_de_condicion_y_monitores_i/ */

#include <cstdint>
#include <cstdlib>
// for rand() and friends

#include "board.h"
#include <cr_section_macros.h>


#include "FreeRTOSConfig.h"
#include "FreeRTOS.h"
#include "task.h"
#include "semphr.h"


//----------------------------------------------------------------------
// Random number generator stuff: 
//----------------------------------------------------------------------

uint32_t xorshift32_state;

void xorshift_seed( uint32_t seed )
{
   xorshift32_state = seed;
}

uint32_t xorshift32()
{
   uint32_t y = xorshift32_state;

   y ^= y << 13;
   y ^= y >> 17;
   y ^= y << 5;

   return( xorshift32_state = y );
}

void ADC_Config()
{
   Chip_ADC_Init(LPC_ADC0, 0);
   Chip_ADC_SetClockRate(LPC_ADC0, ADC_MAX_SAMPLE_RATE);
   Chip_ADC_SetupSequencer( LPC_ADC0, ADC_SEQA_IDX, (ADC_SEQ_CTRL_CHANSEL(0) | ADC_SEQ_CTRL_MODE_EOS));
   Chip_SYSCTL_PowerUp(SYSCTL_POWERDOWN_TS_PD);
   Chip_ADC_SetADC0Input(LPC_ADC0, ADC_INSEL_TS);
   Chip_ADC_SetTrim(LPC_ADC0, ADC_TRIM_VRANGE_HIGHV);
   Chip_ADC_StartCalibration(LPC_ADC0);
   while (!(Chip_ADC_IsCalibrationDone(LPC_ADC0))) {}
	Chip_ADC_EnableSequencer(LPC_ADC0, ADC_SEQA_IDX);
}

uint32_t calculate_seed()
{
   uint32_t seed = 0;

   for( size_t i = 0; i < 16; ++i ){
      Chip_ADC_StartSequencer(LPC_ADC0, ADC_SEQA_IDX);

      while( ( Chip_ADC_GetDataReg( LPC_ADC0, 0 ) & ( ADC_SEQ_GDAT_DATAVALID ) ) == false ){ ; }

      seed += Chip_ADC_GetDataReg( LPC_ADC0, 0 ); //ADC_DR_RESULT( Chip_ADC_GetDataReg( LPC_ADC0, 0 ) );
   }

   return seed * seed;
}

#define srand( x ) xorshift_seed( ( x ) )
#define rand() xorshift32()



#define DEBUG_ENABLED 0

#if DEBUG_ENABLED > 0
#define DEBUG_PRINT0(...) printf(__VA_ARGS__)
#else
#define DEBUG_PRINT0(...) ;
#endif  

//----------------------------------------------------------------------
// Conditional variable stuff: 
//----------------------------------------------------------------------

template<size_t Len>
class ConditionV
{
private:
   TaskHandle_t queue[ Len ]; 
   size_t head{ 0 };
   size_t tail{ 0 };
   size_t max { Len };
   size_t len { 0 };

public:
#if 0 
   ConditionV( SemaphoreHandle_t& the_mutex ) : mutex{ the_mutex }
   {
      // nothing
   }
#endif  

   bool CWait( SemaphoreHandle_t& mutex, TickType_t ticks = portMAX_DELAY )
   {
      TaskHandle_t self;

      taskENTER_CRITICAL();
      {
         self = xTaskGetCurrentTaskHandle();

         configASSERT( this->len < this->max );
         this->queue[ this->tail++ ] = self;
         if( this->tail == this->max ) this->tail = 0;
         ++this->len;

      } taskEXIT_CRITICAL();

      xSemaphoreGive( mutex );
      vTaskSuspend( self );
      
      return xSemaphoreTake( mutex, ticks );
   }

   bool CWaitFor( SemaphoreHandle_t& mutex, TickType_t ticks )
   {
      return CWait( mutex, ticks );
   }

   bool CWaitUntil( SemaphoreHandle_t mutex, TickType_t& last, TickType_t ticks ) 
   {
      volatile TickType_t now = xTaskGetTickCount();
      // don't optimize

      TickType_t until = last + ticks - now;
      // overflows are handled automatically

      last = until;

      return CWait( mutex, until );
   }

   /**
    * @brief Wakes up the task that has been waiting the longer.
    */
   void CNotify()
   {
      TaskHandle_t task = nullptr;

      taskENTER_CRITICAL();
      {
         if( this->len > 0 ){
            task = this->queue[ this->head++ ];

            if( this->head == this->max ) this->head = 0;
            --this->len;
         }
      } taskEXIT_CRITICAL();

      if( task != nullptr ) vTaskResume( task );
   }

   /**
    * @brief Wakes up all tasks.
    */
   void CNotifyAll()
   {
      taskENTER_CRITICAL();
      {
         while( this->len ) CNotify();
      } taskEXIT_CRITICAL();
   }
};


//----------------------------------------------------------------------
// Example: 
//----------------------------------------------------------------------

ConditionV<4> data_avail;

ConditionV<4> space_avail;

/* Queue stuff. Should be an abstract type (e.g. a C++ class): */
char buf[ 8 ];
size_t head{ 0 };
size_t tail{ 0 };
size_t max { 8 };
size_t len { 0 };

SemaphoreHandle_t mutex;


void put( char c )
{
   xSemaphoreTake( mutex, portMAX_DELAY );

   while( len >= max ){
      DEBUG_PRINT0( "*\n" );
      space_avail.CWait( mutex );
   }

   buf[ tail++ ] = c;
   if( tail >= max ) tail = 0;
   ++len;

   data_avail.CNotify();
   DEBUG_PRINT0( "/\n" );

   xSemaphoreGive( mutex );
}

char get()
{
   xSemaphoreTake( mutex, portMAX_DELAY );

   while( len == 0 ){
      DEBUG_PRINT0( "+\n" );
      data_avail.CWait( mutex );
   }

   char c = buf[ head++ ];
   if( head >= max ) head = 0;
   --len;

   space_avail.CNotify();
   DEBUG_PRINT0( "&\n" );

   xSemaphoreGive( mutex );

   return c;
}

void producer( void* pvParameters )
{
   uint32_t offset = (uint32_t)pvParameters;
   char cont = 0;

   char* task_name = pcTaskGetName( NULL );

   char letter = offset;

   while( 1 )
   {
      DEBUGOUT( "%s->%c\n", task_name, letter );
      put( letter++ );

      ++cont;
      if( cont == 10 ){
         cont = 0;
         letter = offset;
      }

      vTaskDelay( pdMS_TO_TICKS( ( rand() % 50 ) + 20 ) );
   }
}

void consumer( void* pvParameters )
{
   char* task_name = pcTaskGetName( NULL );

   while( 1 )
   {
      char c = get();

      DEBUGOUT( "%s<-%c\n", task_name, c );

      vTaskDelay( pdMS_TO_TICKS( ( rand() % 150 ) + 25 ) );
   }
}



//----------------------------------------------------------------------
// Driver program: 
//----------------------------------------------------------------------
int main(void) {

   SystemCoreClockUpdate();
   Board_Init();
   Board_LED_Set(0, false);

   ADC_Config();
   srand( calculate_seed() );


   xTaskCreate( producer, "P0", 128,  (void*) 'A', tskIDLE_PRIORITY + 1, NULL ); // prints out: ABC...J
   xTaskCreate( producer, "P1", 128,  (void*) 'a', tskIDLE_PRIORITY + 1, NULL ); // prints out: abc...j
   xTaskCreate( producer, "P2", 128,  (void*) '0', tskIDLE_PRIORITY + 1, NULL ); // prints out: 012...9

   xTaskCreate( consumer, "C0", 128, NULL, tskIDLE_PRIORITY, NULL );
   xTaskCreate( consumer, "C1", 128, NULL, tskIDLE_PRIORITY, NULL );

   mutex = xSemaphoreCreateMutex();

   vTaskStartScheduler();

   while( 1 )
   {

   }

}