Shared memory between two c program


 
Thread Tools Search this Thread
Top Forums Programming Shared memory between two c program
# 1  
Old 12-15-2011
Shared memory between two c program

i have to shared a variable between two different c programs with shared memory and i do these:
Code:
int main() {
    int a=5,b=7;
    int buffer[1];
    int *point;
    int shmid;
    shmid=shmget(IPC_PRIVATE , sizeof(buffer[1]),0666);
    point=(int *)shmat(shmid,NULL,0);
    point[0]=a;
    point[1]=b;
    printf("FIRST = %d\nSECOND = %d",point[0],point[1]);
    fflush(stdout);
    sleep(300);
    shmdt(point);
    exit(0);
}

the other is :
Code:
int main(){
    int buffer[1];
    int *point;
    int shmid;
    shmid=shmget(IPC_PRIVATE,sizeof(buffer[1]),0666);
    point=(int *)shmat(shmid,NULL,0);
    printf("first number =%d\nsecond number =%d\n",point[0],point[1]);
    fflush(stdout);
    shmdt(point);
    exit(0);
}

but if i execute the first and after the second, i don't have 5 on first number and 7 on second but both zero. why?
maybe i have to use a key to identify shared memory and not IPC_PRIVATE? ( i also must use semaphores but it's only begin of this program )
# 2  
Old 12-15-2011
Yes, you either need a key or to pass the segment's ID. IPC_PRIVATE is meant to be used between two programs that can pass the ID, either because one forked the other and the ID is resident, or because they passed it in some other way. So, in IPC_PRIVATE world, you'd shmget in one and then pass the shmid to the other to shmat.

If you use the command "ipcs -m" you'll see all the shared memory segments. The KEY for IPC_PRIVATE (usually) is 0xffffffff, but you'll see a bunch of them. Each time you run this program you'll be creating another one. The ID for each will be different. Ultimately, it's the ID that determines which segment you attach to.

Think of shmget as a way to both create and lookup that ID. If you don't pass the same ID to each shmat call, you won't be looking at the same segment. Make sense?

Also, you probably mean sizeof(buffer) not sizeof(buffer[1]). The first will insure the segment can store the whole array, your way provides the segment only enough space to hold one element. Of course, this will cause an overflow.

edit: P.S. since you're never asking the shared memory segment be removed, it'll linger forever. You'll want to use ipcrm to get rid of it. Of course, you could also shmctl with the IPC_RMID command to ask it be removed. It'll hang around until all applications currently attached to it have detached. The kernel will detach the application when it is killed as well, so while it's good practice to shmdt, if you application dies in the middle the kernel will do it. Of course, on some O/Ses I've seen that fail and segments get "stuck" and the dumb machine needs a reboot to clean them up, lol.

Last edited by DreamWarrior; 12-15-2011 at 02:02 PM..
This User Gave Thanks to DreamWarrior For This Post:
# 3  
Old 12-15-2011
You can avoid IPC issues by using mmap instead. It can map memory from a file, allowing you to access the same data into two processes' memory:

Code:
#include <sys/types.h>
#include <sys/mman.h>
#include <unistd.h>
#include <fcntl.h>

int main(void)
{
        int *mem;
        int fd=open("datafile", O_RDWR);
        if(fd < 0)
        {
                perror("couldn't open");
                return(1);
        }


        ftruncate(fd, getpagesize());  // make file long enough
        mem=mmap(NULL, getpagesize(), PROT_READ|PROT_WRITE, 
                MAP_SHARED, fd, 0);

        if(mem == MAP_FAILED)
        {
                 perror("Couldn't mmap");
                 return(1);
        }

        mem[0]=0xdeadbeef; // Should appear in file and other process
        munmap(mem, getpagesize());
        close(fd);
        return(0);
}

# 4  
Old 12-15-2011
Quote:
Originally Posted by DreamWarrior
Yes, you either need a key or to pass the segment's ID. IPC_PRIVATE is meant to be used between two programs that can pass the ID, either because one forked the other and the ID is resident, or because they passed it in some other way. So, in IPC_PRIVATE world, you'd shmget in one and then pass the shmid to the other to shmat.

If you use the command "ipcs -m" you'll see all the shared memory segments. The KEY for IPC_PRIVATE (usually) is 0xffffffff, but you'll see a bunch of them. Each time you run this program you'll be creating another one. The ID for each will be different. Ultimately, it's the ID that determines which segment you attach to.

Think of shmget as a way to both create and lookup that ID. If you don't pass the same ID to each shmat call, you won't be looking at the same segment. Make sense?

Also, you probably mean sizeof(buffer) not sizeof(buffer[1]). The first will insure the segment can store the whole array, your way provides the segment only enough space to hold one element. Of course, this will cause an overflow.

edit: P.S. since you're never asking the shared memory segment be removed, it'll linger forever. You'll want to use ipcrm to get rid of it. Of course, you could also shmctl with the IPC_RMID command to ask it be removed. It'll hang around until all applications currently attached to it have detached. The kernel will detach the application when it is killed as well, so while it's good practice to shmdt, if you application dies in the middle the kernel will do it. Of course, on some O/Ses I've seen that fail and segments get "stuck" and the dumb machine needs a reboot to clean them up, lol.
thanks a lot for your exhaustive answer, i will try soon

---------- Post updated at 05:06 PM ---------- Previous update was at 04:26 PM ----------

so, for example i have to declare :
Code:
key_t shmkey=0x200;

and then use that for shmget in both programs?
# 5  
Old 12-16-2011
Quote:
Originally Posted by tafazzi87
thanks a lot for your exhaustive answer, i will try soon
You're welcome.
Quote:
Originally Posted by tafazzi87
so, for example i have to declare :
Code:
key_t shmkey=0x200;

and then use that for shmget in both programs?
That will work, there is also the ftok function. I've seen hard-coded keys and ftok used, more often the former (don't know why, really). Another method I've seen is a hard-coded base with a xor'd in integer from the environment. I suppose for a toy application, it doesn't much matter. For something more production quality, you should probably at the very least insure that the key is configurable so that it can't conflict with any other potentially running application.
# 6  
Old 12-16-2011
if i use
Code:
key_t shmkey=0x200;

it doesn't work, it gives segmentation fault and also if i use ftok function...
maybe i'm wrong to use ftok(), which type of file i use in path argument?
# 7  
Old 12-16-2011
It shouldn't segmentation fault even if you give it a wrong token. It sounds like you forgot to do any error checking.

Please post your program. We can't see your computer from here.
Login or Register to Ask a Question

Previous Thread | Next Thread

10 More Discussions You Might Find Interesting

1. Programming

Shared library with acces to shared memory.

Hello. I am new to this forum and I would like to ask for advice about low level POSIX programming. I have to implement a POSIX compliant C shared library. A file will have some variables and the shared library will have some functions which need those variables. There is one special... (5 Replies)
Discussion started by: iamjag
5 Replies

2. AIX

shared memory

1.How to know wich process is using the shared memory? 2.How to flush (release) the process from the shared memory? (1 Reply)
Discussion started by: pchangba
1 Replies

3. UNIX for Advanced & Expert Users

Shared Memory

Hi, Using ipcs we can see shared memory, etc.. details. How can I add/remove shared memory(command name)? Thanks, Naga:cool: (2 Replies)
Discussion started by: Nagapandi
2 Replies

4. Programming

Shared memory for shared library

I am writing a shared library in Linux (but compatible with other UNIXes) and I want to allow multiple instances to share a piece of memory -- 1 byte is enough. What's the "best" way to do this? I want to optimize for speed and portability. Obviously, I'll have to worry about mutual exclusion. (0 Replies)
Discussion started by: otheus
0 Replies

5. Programming

Shared memory in shared library

I need to create a shared library to access an in memory DB. The DB is not huge, but big enough to make it cumbersome to carry around in every single process using the shared library. Luckily, it is pretty static information, so I don't need to worry much about synchronizing the data between... (12 Replies)
Discussion started by: DreamWarrior
12 Replies

6. Programming

memory sharing - not shared memory -

hi, this is the problem: i want to swap a linked list between 4 processes (unrelated), is there any way i can do that just by sending a pointer to a structure? //example typedef struct node { int x; char c; struct node *next; } node; or i should send the items ( x,c ) by... (9 Replies)
Discussion started by: elzalem
9 Replies

7. Programming

help with shared memory

what i want to do is have an int that can been written into by 2 processes but my code doesn't seem to work. #include <sys/types.h> #include <sys/ipc.h> #include <sys/sem.h> #include <sys/shm.h> #include<stdio.h> #define KEY1 (1492) int main() { int shmid; volatile int * addr;... (6 Replies)
Discussion started by: ddx08
6 Replies

8. Linux

all about shared memory

Hi all :confused: , I am new to unix.I have been asked to implement shared memory in user's mode.What does this mean?What is the difference it makes in kernel mode and in users mode?What are the advantages of this impemenation(user's mode)? And also i would like to know why exactly shared... (0 Replies)
Discussion started by: vijaya2006
0 Replies

9. UNIX for Advanced & Expert Users

Shared memory shortage but lots of unused memory

I am running HP-UX B.11.11. I'm increasing a parameter for a database engine so that it uses more memory to buffer the disk drive (to speed up performance). I have over 5GB of memory not being used. But when I try to start the DB with the increased buffer parameter I get told. "Not... (1 Reply)
Discussion started by: cjcamaro
1 Replies

10. Programming

Shared memory

Dear Reader, Is is necessary to attach / dettach the shared memory segments for write operations , if more than one program is accessing same shared memory segments.. I have used semaphore mutex and still I'm getting segmentation fault when I write to the segment when other program is already... (1 Reply)
Discussion started by: joseph_shibu
1 Replies
Login or Register to Ask a Question