#include <signal.h>
// #include all other things

void my_signal_handler(int signal_number) {
  //It's ok to do nothing here
  //you can also add code to handle the signal if you want
}


int main() {
  //do stuff

  signal(SIGUSR1, my_signal_handler);

  //fork() etc

  if (i_am_the_child_process) {
      pause();
  } else {
      //do parent process stuff
      kill(childpid, SIGUSR1);  //this will cause the child to wake up
  }
  //do more stuff :)
}

int wakeupflag; //global variable

void my_signal_handler(int signum) {
    wakeupflag = 1;
}

int main() {

  signal(SIGUSR1, my_signal_handler);
  //fork etc

  if (i_am_the_child) {
    wakeupflag = 0;
    while (!wakeupflag)
      pause();
  }
  //blah blah blah
}

#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>

#define SEMKEYPATH  "/dev/null" /* Path used on ftok for semget key */
#define SEMKEYID     1          /* Id used on ftok for semget key   */
#define NUMSEMS      1          /* Num of sems in created sem set   */

union semun {
   int val;              /* used for SETVAL only */
   struct semid_ds *buf; /* for IPC_STAT and IPC_SET */
   ushort *array;        /* used for GETALL and SETALL */
};

int main(void)
{
   key_t semkey;
   int rc, semid;
   int status;
   union semun arg;
   struct sembuf operations[1];

   /* Generate an IPC key for the semaphore set                      */
   semkey = ftok(SEMKEYPATH, SEMKEYID);
   if ( semkey == (key_t)-1 ){
      printf("ftok() for sem failed\n");
      return -1;
   }

   /* Create a semaphore set using the IPC key.  The number of       */
   /* semaphores in the set is one.  If a semaphore set already      */
   /* exists for the key, return an error. The specified permissions */
   /* give everyone read/write access to the semaphore set.          */
   semid = semget( semkey, NUMSEMS, 0666 | IPC_CREAT );
   if ( semid == -1 ){
      perror("semget");
      return -1;
   }

   /* initialize semaphore #0 to 1: */
   arg.val = 1;
   if (semctl(semid, 0, SETVAL, arg) == -1) {
      perror("semctl");
      return -1;
   }
   printf("Parent sem initialized to 1\n");
   /* Set the structure passed into the semop()                      */
   operations[0].sem_num = 0;     /* Operate on the first sem        */
   operations[0].sem_op = -1;     /* Decrement the semval by one     */
   operations[0].sem_flg = 0;     /* Allow a wait to occur           */
   rc = semop( semid, operations, 1 );
   if (rc == -1){
      perror("semop");
      return -1;
   }
   printf("Parent decremented sem to 0\n");
   pid_t pid;
   pid = fork();
   if (pid < 0) {
      perror("fork");
      exit(EXIT_FAILURE);
   }

   if (pid > 0) {                   /* Parent process                */
      printf("Parent Locked.\n");
      printf("Parent does some stuff with pid %d.\n", pid);

      operations[0].sem_op = 1;     /* Increment the semval by 1     */
      rc = semop( semid, operations, 1 );
      if (rc == -1) {
         perror("semop");
         return -1;
      }
      printf("Parent Unlocked\n");
      /* parent does other stuff */
      waitpid(-1, &status, 0);
   }

   if (pid == 0) {                  /* Child process                 */
      printf("Child Trying to lock...\n");

      /* Set the structure passed into the semop()                   */
      operations[0].sem_num = 0;     /* Operate on the first sem     */
      operations[0].sem_op = -1;     /* Decrement the semval by one  */
      operations[0].sem_flg = 0;     /* Allow a wait to occur        */
      rc = semop( semid, operations, 1 );
      if (rc == -1) {
         perror("semop");
         return -1;
      }
      printf("Child Locked.\n");

      operations[0].sem_op = 1;     /* Increment the semval by 1     */
      rc = semop( semid, operations, 1 );
      if (rc == -1) {
         perror("semop");
         return -1;
      }
      printf("Child Unlocked\n");

      /* Clean up the environment by removing the semid structure    */
      rc = semctl( semid, 0, IPC_RMID, arg );
      if (rc==-1)
      {
         printf("main: semctl() remove id failed\n");
         return -1;
      }
      printf("Child can continue\n");
   }

}