Example: Using semaphore set and shared memory functions
This example illustrates programs that support the client/server model.
Note: By using the code examples, you agree to the terms of the Code license and disclaimer information.
Server program
This program acts as a server to the client program (see Client program). The buffer is a shared memory segment. The process synchronization is done using semaphores.
Use the Create C Module (CRTCMOD) and the Create Program (CRTPGM) commands to create this program.
Call this program with no parameters before calling the client program.
/*********************************************************************/
/*********************************************************************/
/* */
/* FUNCTION: This program acts as a server to the client program. */
/* */
/* LANGUAGE: ILE C */
/* */
/* APIs USED: semctl(), semget(), semop(), */
/* shmat(), shmctl(), shmdt(), shmget() */
/* ftok() */
/* */
/*********************************************************************/
/*********************************************************************/
#include <stdio.h>
#include <string.h>
#include <sys/ipc.h>
#include <sys/sem.h>
#include <sys/shm.h>
#define SEMKEYPATH "/dev/null" /* Path used on ftok for semget key */
#define SEMKEYID 1 /* Id used on ftok for semget key */
#define SHMKEYPATH "/dev/null" /* Path used on ftok for shmget key */
#define SHMKEYID 1 /* Id used on ftok for shmget key */
#define NUMSEMS 2 /* Num of sems in created sem set */
#define SIZEOFSHMSEG 50 /* Size of the shared mem segment */
#define NUMMSG 2 /* Server only doing two "receives"
on shm segment */
int main(int argc, char *argv[])
{
int rc, semid, shmid, i;
key_t semkey, shmkey;
void *shm_address;
struct sembuf operations[2];
struct shmid_ds shmid_struct;
short sarray[NUMSEMS];
/* Generate an IPC key for the semaphore set and the shared */
/* memory segment. Typically, an application specific path and */
/* id would be used to generate the IPC key. */
semkey = ftok(SEMKEYPATH,SEMKEYID);
if ( semkey == (key_t)-1 )
{
printf("main: ftok() for sem failed\n");
return -1;
}
shmkey = ftok(SHMKEYPATH,SHMKEYID);
if ( shmkey == (key_t)-1 )
{
printf("main: ftok() for shm failed\n");
return -1;
}
/* Create a semaphore set using the IPC key. The number of */
/* semaphores in the set is two. 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 | IPC_EXCL );
if ( semid == -1 )
{
printf("main: semget() failed\n");
return -1;
}
/* Initialize the first semaphore in the set to 0 and the */
/* second semaphore in the set to 0. */
/* */
/* The first semaphore in the sem set means: */
/* '1' -- The shared memory segment is being used. */
/* '0' -- The shared memory segment is freed. */
/* The second semaphore in the sem set means: */
/* '1' -- The shared memory segment has been changed by */
/* the client. */
/* '0' -- The shared memory segment has not been */
/* changed by the client. */
sarray[0] = 0;
sarray[1] = 0;
/* The '1' on this command is a no-op, because the SETALL command*/
/* is used. */
rc = semctl( semid, 1, SETALL, sarray);
if(rc == -1)
{
printf("main: semctl() initialization failed\n");
return -1;
}
/* Create a shared memory segment using the IPC key. The */
/* size of the segment is a constant. The specified permissions */
/* give everyone read/write access to the shared memory segment. */
/* If a shared memory segment already exists for this key, */
/* return an error. */
shmid = shmget(shmkey, SIZEOFSHMSEG, 0666 | IPC_CREAT | IPC_EXCL);
if (shmid == -1)
{
printf("main: shmget() failed\n");
return -1;
}
/* Attach the shared memory segment to the server process. */
shm_address = shmat(shmid, NULL, 0);
if ( shm_address==NULL )
{
printf("main: shmat() failed\n");
return -1;
}
printf("Ready for client jobs\n");
/* Loop only a specified number of times for this example. */
for (i=0; i < NUMMSG; i++)
{
/* Set the structure passed into the semop() to first wait */
/* for the second semval to equal 1, then decrement it to */
/* allow the next signal that the client writes to it. */
/* Next, set the first semaphore to equal 1, which means */
/* that the shared memory segment is busy. */
operations[0].sem_num = 1;
/* Operate on the second sem */
operations[0].sem_op = -1;
/* Decrement the semval by one */
operations[0].sem_flg = 0;
/* Allow a wait to occur */
operations[1].sem_num = 0;
/* Operate on the first sem */
operations[1].sem_op = 1;
/* Increment the semval by 1 */
operations[1].sem_flg = IPC_NOWAIT;
/* Do not allow to wait */
rc = semop( semid, operations, 2 );
if (rc == -1)
{
printf("main: semop() failed\n");
return -1;
}
/* Print the shared memory contents. */
printf("Server Received : \"%s\"\n", (char *) shm_address);
/* Signal the first semaphore to free the shared memory. */
operations[0].sem_num = 0;
operations[0].sem_op = -1;
operations[0].sem_flg = IPC_NOWAIT;
rc = semop( semid, operations, 1 );
if (rc == -1)
{
printf("main: semop() failed\n");
return -1;
}
} /* End of FOR LOOP */
/* Clean up the environment by removing the semid structure, */
/* detaching the shared memory segment, and then performing */
/* the delete on the shared memory segment ID. */
rc = semctl( semid, 1, IPC_RMID );
if (rc==-1)
{
printf("main: semctl() remove id failed\n");
return -1;
}
rc = shmdt(shm_address);
if (rc==-1)
{
printf("main: shmdt() failed\n");
return -1;
}
rc = shmctl(shmid, IPC_RMID, &shmid_struct);
if (rc==-1)
{
printf("main: shmctl() failed\n");
return -1;
}
return 0;
}
Client program
This program acts as a client to the server program (see Server program). The program is run after the message Ready for client jobs appears from the server program.
Use the CRTCMOD and CRTPGM commands to create this program.
Call this program with no parameters after calling the server program.
/*********************************************************************/
/*********************************************************************/
/* */
/* FUNCTION: This program acts as a client to the server program. */
/* */
/* LANGUAGE: ILE C */
/* */
/* APIs USED: semget(), semop(), */
/* shmget(), shmat(), shmdt() */
/* ftok() */
/* */
/*********************************************************************/
/*********************************************************************/
#include <stdio.h>
#include <string.h>
#include <sys/ipc.h>
#include <sys/sem.h>
#include <sys/shm.h>
#define SEMKEYPATH "/dev/null" /* Path used on ftok for semget key */
#define SEMKEYID 1 /* Id used on ftok for semget key */
#define SHMKEYPATH "/dev/null" /* Path used on ftok for shmget key */
#define SHMKEYID 1 /* Id used on ftok for shmget key */
#define NUMSEMS 2
#define SIZEOFSHMSEG 50
int main(int argc, char *argv[])
{
struct sembuf operations[2];
void *shm_address;
int semid, shmid, rc;
key_t semkey, shmkey;
/* Generate an IPC key for the semaphore set and the shared */
/* memory segment. Typically, an application specific path and */
/* id would be used to generate the IPC key. */
semkey = ftok(SEMKEYPATH,SEMKEYID);
if ( semkey == (key_t)-1 )
{
printf("main: ftok() for sem failed\n");
return -1;
}
shmkey = ftok(SHMKEYPATH,SHMKEYID);
if ( shmkey == (key_t)-1 )
{
printf("main: ftok() for shm failed\n");
return -1;
}
/* Get the already created semaphore ID associated with key. */
/* If the semaphore set does not exist, then it will not be */
/* created, and an error will occur. */
semid = semget( semkey, NUMSEMS, 0666);
if ( semid == -1 )
{
printf("main: semget() failed\n");
return -1;
}
/* Get the already created shared memory ID associated with key. */
/* If the shared memory ID does not exist, then it will not be */
/* created, and an error will occur. */
shmid = shmget(shmkey, SIZEOFSHMSEG, 0666);
if (shmid == -1)
{
printf("main: shmget() failed\n");
return -1;
}
/* Attach the shared memory segment to the client process. */
shm_address = shmat(shmid, NULL, 0);
if ( shm_address==NULL )
{
printf("main: shmat() failed\n");
return -1;
}
/* First, check to see if the first semaphore is a zero. If it */
/* is not, it is busy right now. The semop() command will wait */
/* for the semaphore to reach zero before running the semop(). */
/* When it is zero, increment the first semaphore to show that */
/* the shared memory segment is busy. */
operations[0].sem_num = 0;
/* Operate on the first sem */
operations[0].sem_op = 0;
/* Wait for the value to be=0 */
operations[0].sem_flg = 0;
/* Allow a wait to occur */
operations[1].sem_num = 0;
/* Operate on the first sem */
operations[1].sem_op = 1;
/* Increment the semval by one */
operations[1].sem_flg = 0;
/* Allow a wait to occur */
rc = semop( semid, operations, 2 );
if (rc == -1)
{
printf("main: semop() failed\n");
return -1;
}
strcpy((char *) shm_address, "Hello from Client");
/* Release the shared memory segment by decrementing the in-use */
/* semaphore (the first one). Increment the second semaphore to */
/* show that the client is finished with it. */
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 */
operations[1].sem_num = 1;
/* Operate on the second sem */
operations[1].sem_op = 1;
/* Increment the semval by one */
operations[1].sem_flg = 0;
/* Allow a wait to occur */
rc = semop( semid, operations, 2 );
if (rc == -1)
{
printf("main: semop() failed\n");
return -1;
}
/* Detach the shared memory segment from the current process. */
rc = shmdt(shm_address);
if (rc==-1)
{
printf("main: shmdt() failed\n");
return -1;
}
return 0;
}