Re: Can shared memory data be used as a signal like gtk-input-add does with a pipe?

[G Hasse <gh raditex se>]

On Thu, 27 Sep 2001, Marco Quezada wrote:

> I'm not sure if this is the best and or only way to do it but I just have a
> timeout that runs at a certain rate (10 Hz in my case) and looks at the value
> of the variable in memory, if the value changes then it goes in and does
> whatever processing it needs to do.

This is not the way to do it. The best way is in fact to use a
select or poll on a file-descriptor - but you could also use a semaphore
like this.

/*

Simple example
semaphore.c

*/

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


void main( int argc , char *argv[] )
{

int shmid, semid, i , j, size=256;
char *buf;

pid_t pid;
key_t key;

static struct sembuf raise[] = {
  {0,1,0}
};

static struct sembuf isdown[] = {
  {0,0,0}
};

static struct sembuf isup[] = {
  {0,-1,0},
  {0,1,0}
};

static struct sembuf lower[] = {
  {0,-1,0}
};

if( pid = fork() ) {  /* Parent */

   printf("Parent: pid=%d\n", getpid() );

   key = ftok( argv[0], 0 );
   shmid = shmget( key, size, IPC_CREAT | SHM_R | SHM_W );
   buf = shmat( shmid, 0 , 0);


   semid = semget( key , 1, IPC_CREAT | SEM_R | SEM_A );
   printf("Parent semid=%d\n", semid);

   semop( semid, isdown, 1);
   sprintf(buf, "%8.8d_", getpid() );
   printf("parent: msg=%s\n", buf);
   semop(semid, raise, 1);

   for ( i=0; i<8 ; i++) {

     semop(semid, isdown, 1);
     strcat(buf, "*");
     semop(semid, raise, 1);

   }

   sleep(1);


} else { /* Child */

   sleep(2);

   printf("Child: pid=%d\n", getpid() );

   key = ftok( argv[0], 0 );

   shmid = shmget( key, size,  SHM_R | SHM_W );
   buf = shmat( shmid, 0 , 0);

   semid = semget( key , 1, SEM_R | SEM_A );
   printf("Child semid=%d\n", semid);

   semop(semid, isup, 2);
  
   printf("child: msg=%s\n", buf);

   semop(semid, lower, 1);
   
   for( i = 0 ; i < 4; i++ ) {
     semop(semid, isup, 2);
     printf("child: msg=%s\n", buf );
     semop(semid, lower, 1);
   }

   semop(semid, isup, 2);
   sprintf(buf, "%8.8d_", getpid());
   semop(semid, lower, 1);
   
   for( i = 0 ; i < 4; i++ ) {
     semop(semid, isup, 2);
     printf("child: msg=%s\n", buf );
     semop(semid, lower, 1);
   }

   shmdt(buf);
   semctl(semid, 0, IPC_RMID, NULL); /* Remove semaphore */
   shmctl(shmid, IPC_RMID, NULL); /* Remove memory */

}

}

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Göran Hasse            email: gh raditex se     Tel: +46 8 694 92 70
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