Help with shared memory and semaphores

strider1974 · 08-10-2009, 05:48 AM

I have to write a program that solves the producer/consumer problem using shared memory, cicular buffer, mutex's and semaphores.

My problem is that while the course teaches us the theory behind these structures I have been given nothing on how to implement these in C.

I have been looking at a number of different places on the web trying to wrap my head around how to use these in C but I am still having major issues.

I am asking if anyone could point me to some reference materials that clearly and simply outline how to create shared memory, semaphores, mutex's, circular buffers etc. preferably using pthreads

thank you

Michael

Zlatko · 08-10-2009, 10:31 AM

Hello strider.

I assume you're using linux and that the system V interprocess communication (IPC) mechanism are OK to use. You could also use posix shared memory and posix semaphores.

You don't really need shared memory or semaphores if the producer and consumer are threads in the same process (as you imply), but I understand that it may be a requirement for your assignment.

The functions you're looking for are
shmget, shmat, and shmdt for shared memory. You can read the man pages for them.
Here is a simple example of creating shared memory.

Code:

#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <sys/stat.h>
#include <fcntl.h>

typedef struct 
{
    int x;
} WorkUnit;

WorkUnit* pShm;

int main(void)
{
    int shmid;
    pid_t child;

    key_t key = 0x1234;
    // Or use ftok function
    
    if ((shmid = shmget(key, sizeof(WorkUnit)*10, IPC_CREAT | 0666)) < 0)
    {
	perror("shmget");
	return 1;
    }
    

    if ((pShm = (WorkUnit*)shmat(shmid, NULL, 0)) == (void*)-1)
    {
	perror("shmat");
	return -1;
    }

    /* Do some work then detach from shared memory */

    shmdt((void*)pShm);

    return 0;

For semaphores, you need the semget, and semop functions. I guess you'll be using a binary semaphore to get exclusive access to your shared memory. If you're using producer and consumer threads, then you should use pthread mutex objects instead of semaphores but you could use semaphores as an exercise.

I hope that starts you off. You really need to specify more information about your problem.

strider1974 · 08-14-2009, 05:34 AM

Zlato

cheers, this got headed in the right direction even though my post was lacking in details

Sorry for the late reply it has been a hell of week

08-10-2009, 05:48 AM	#1
strider1974 Registered User Join Date: Apr 2009 Posts: 13	Help with shared memory and semaphores I have to write a program that solves the producer/consumer problem using shared memory, cicular buffer, mutex's and semaphores. My problem is that while the course teaches us the theory behind these structures I have been given nothing on how to implement these in C. I have been looking at a number of different places on the web trying to wrap my head around how to use these in C but I am still having major issues. I am asking if anyone could point me to some reference materials that clearly and simply outline how to create shared memory, semaphores, mutex's, circular buffers etc. preferably using pthreads thank you Michael
strider1974 is offline

08-10-2009, 10:31 AM	#2
Zlatko pwning noobs Join Date: Jun 2009 Location: The Great White North Posts: 125	Hello strider. I assume you're using linux and that the system V interprocess communication (IPC) mechanism are OK to use. You could also use posix shared memory and posix semaphores. You don't really need shared memory or semaphores if the producer and consumer are threads in the same process (as you imply), but I understand that it may be a requirement for your assignment. The functions you're looking for are shmget, shmat, and shmdt for shared memory. You can read the man pages for them. Here is a simple example of creating shared memory. Code: #include <stdlib.h> #include <stdio.h> #include <errno.h> #include <sys/types.h> #include <sys/ipc.h> #include <sys/shm.h> #include <sys/stat.h> #include <fcntl.h> typedef struct { int x; } WorkUnit; WorkUnit* pShm; int main(void) { int shmid; pid_t child; key_t key = 0x1234; // Or use ftok function if ((shmid = shmget(key, sizeof(WorkUnit)10, IPC_CREAT \| 0666)) < 0) { perror("shmget"); return 1; } if ((pShm = (WorkUnit)shmat(shmid, NULL, 0)) == (void)-1) { perror("shmat"); return -1; } / Do some work then detach from shared memory / shmdt((void)pShm); return 0; For semaphores, you need the semget, and semop functions. I guess you'll be using a binary semaphore to get exclusive access to your shared memory. If you're using producer and consumer threads, then you should use pthread mutex objects instead of semaphores but you could use semaphores as an exercise. I hope that starts you off. You really need to specify more information about your problem. __________________ Sun Certified Java Programmer / Developer IEEE CSDP
Zlatko is offline

08-14-2009, 05:34 AM	#3
strider1974 Registered User Join Date: Apr 2009 Posts: 13	Zlato cheers, this got headed in the right direction even though my post was lacking in details Sorry for the late reply it has been a hell of week
strider1974 is offline

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