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Shared memory between two c program

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# 8  
Old 12-16-2011
the first:
Code:
#include <unistd.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <stdio.h>
#include <stdlib.h>
int main() {
    int a=5,b=7;
    key_t keyshm; // or key_t keyshm=0x200;
    keyshm=ftok("/tmp",32); //see up
    int buffer[1];
    int *point;
    int shmid;
    shmid=shmget(keyshm , sizeof(buffer),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 second:
Code:
#include <unistd.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <stdio.h>
#include <stdlib.h>

int main(){
    int buffer[1];
    int *point;
    key_t keyshm;
    keyshm=ftok("/tmp",32);
    int shmid;
    shmid=shmget(keyshm,sizeof(buffer),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);
}

# 9  
Old 12-16-2011
Modifying your program a bit:

Code:
#include <unistd.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <stdio.h>
#include <stdlib.h>

void die(const char *msg)
{
        perror(msg);
        exit(1);
}

int main() {
    int a=5,b=7;
    key_t keyshm; // or key_t keyshm=0x200;
    keyshm=ftok("/tmp",32); //see up
    int buffer[1];
    int *point;
    int shmid;
    shmid=shmget(keyshm , sizeof(buffer),0666);
    if(shmid < 0) die("Couldn't shmget");

    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);
}

Code:
$ ./shm1
Couldn't shmget: No such file or directory
$ man shmget
SHMGET(2)                  Linux Programmer's Manual                 SHMGET(2)

...

ERRORS
       On failure, errno is set to one of the following:

       EACCES The  user  does  not have permission to access the shared memory
              segment, and does not have the CAP_IPC_OWNER capability.

       EEXIST IPC_CREAT | IPC_EXCL was specified and the segment exists.

       EINVAL A new segment was to be created and size < SHMMIN or size > SHM-
              MAX,  or  no new segment was to be created, a segment with given
              key existed, but size is greater than the size of that segment.

       ENFILE The system limit on the total number  of  open  files  has  been
              reached.

       ENOENT No segment exists for the given key, and IPC_CREAT was not spec-
              ified.

       ENOMEM No memory could be allocated for segment overhead.

       ENOSPC All possible shared memory IDs  have  been  taken  (SHMMNI),  or
              allocating  a segment of the requested size would cause the sys-
              tem to exceed the system-wide limit on shared memory (SHMALL).

       EPERM  The SHM_HUGETLB flag was specified, but the caller was not priv-
              ileged (did not have the CAP_IPC_LOCK capability).

...

So you want 0666 | IPC_CREAT

And you should check the return values of everything in case anything you weren't expecting fails.
# 10  
Old 12-16-2011
Did you change the sizeof? You don't even use buffer in the first program, so you can get rid of that. Then when you shmget pass (sizeof(int) * 2) as the size argument.

Also, to the third argument, shmflg, you have to OR in IPC_CREAT. So the call should be:

Code:
key_t shmkey=0x200;
int id;
..,
id = shmget(shmkey, sizeof(int) * 2, 0666 | IPC_CREAT);

Without IPC_CREAT, it's likely that shmget is returning -1 which, since you're not doing error checking, is then passed to shmat which is returning NULL. When you attempt to reference through the NULL pointer you are seg faulting.

I suggest reading the man pages for shmget!

edit: I also suggest not using 0666 and using the proper mode flags; but that's your prerogative. For example:

Code:
#include <sys/stat.h>
...
mode_t mode = S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH;
...
id = shmget(shmkey, sizeof(int) * 2, mode | IPC_CREAT);

edit2: haha, beaten by hours, didn't even notice there was a second page.
Last edited by DreamWarrior; 12-16-2011 at 03:33 PM..
# 11  
Old 12-16-2011
Quote:
Originally Posted by DreamWarrior
Did you change the sizeof? You don't even use buffer in the first program, so you can get rid of that. Then when you shmget pass (sizeof(int) * 2) as the size argument.

Also, to the third argument, shmflg, you have to OR in IPC_CREAT. So the call should be:

Code:
key_t shmkey=0x200;
int id;
..,
id = shmget(shmkey, sizeof(int) * 2, 0666 | IPC_CREAT);

Without IPC_CREAT, it's likely that shmget is returning -1 which, since you're not doing error checking, is then passed to shmat which is returning NULL. When you attempt to reference through the NULL pointer you are seg faulting.

I suggest reading the man pages for shmget!

edit: I also suggest not using 0666 and using the proper mode flags; but that's your prerogative. For example:

Code:
#include <sys/stat.h>
...
mode_t mode = S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH;
...
id = shmget(shmkey, sizeof(int) * 2, mode | IPC_CREAT);

edit2: haha, beaten by hours, didn't even notice there was a second page.
thanks now it works very well, i've a very last question...Smilie where i find manual of mode flags?
# 12  
Old 12-16-2011
Quote:
Originally Posted by tafazzi87
thanks now it works very well, i've a very last question...Smilie where i find manual of mode flags?
Code:
man 2 chmod

It describes the modes there.
# 13  
Old 12-22-2011
i've another problem, now i want to add two numbers and print result, two number are written in the first and second elements of buffer of shared memory and after call an adder program and he make sum and put it into third element of buffer so the main program prints result.
well if i have 3+5 program give 114678723
why?
this is the code for main:
Code:
#include <sys/types.h>
#include <sys/sem.h>
#include <errno.h>
#include <unistd.h>
#include <stdio.h>
#include <sys/shm.h>
#include <sys/ipc.h>
#include <stdlib.h>
#define SEMPERM 0600

typedef union _semun {
    int val;
    struct semid_ds *buf;
    unsigned short *array;
}semun;
int initsem (key_t semkey){
    int status=0, semid;
    semid=semget(semkey, 1 , SEMPERM | IPC_CREAT | IPC_EXCL);
    if (semid==-1){
        if (errno==EEXIST){semid=semget(semkey,1,0);}
    } else {
        semun arg;
        arg.val=0;
        status=semctl(semid,0,SETVAL,arg);
    }
    if ((semid==-1) || (status==-1)){
        perror("initsem fallita");
        return (-1);
    }
    return (semid);
}
int waitSem(int semid){
    struct sembuf wait_buf;
    wait_buf.sem_num=0;
    wait_buf.sem_op=1;
    wait_buf.sem_flg=SEM_UNDO;
    if (semop(semid,&wait_buf, 1)==-1){
        perror("waitSem Fallita");
        exit(-1);
    }
    return 0;
}

int signalSem (int semid){
    struct sembuf signal_buf;
    signal_buf.sem_num=0;
    signal_buf.sem_op=1;
    signal_buf.sem_flg=SEM_UNDO;
    if (semop(semid,&signal_buf,1)==-1){
        perror("signalSem fallita");
        exit(1);
    }
    return 0;
}
int main (){
    key_t chiavemem=0x100,keysem=0x050;
    int sem,a,id,*point;
    pid_t figlio;
    int buffer[2];
    sem=initsem(keysem);
    if(sem<0){
        perror ("creazione semaforo fallita");
        exit (-1);
    }
    id=shmget(chiavemem,sizeof(buffer[2]),0777 |IPC_CREAT);
    if (id<0){
        perror("id main errato");
        exit(-1);
    }
    point=(int *)shmat(id,NULL,0);
    if (point<0){
        perror ("Errato attacco main");
        exit (-1);
    }
    waitSem(sem);
    point[0]=3;
    point[1]=5;
    signalSem(sem);
    figlio=fork();
    if (figlio<0){
        perror("fork fallita");
        exit (-1);
    }
    if (figlio==0){
        execvp("/home/francesco/NetBeansProjects/Esercizio1/addizione",NULL);
        a=point[2];
        fflush(stdout);
        exit(0);
    }
    printf("il risultato è %d",a);
    shmdt(point);
    shmctl(id,IPC_RMID,0);
    exit(0);
}

instead adder is:
Code:
#include <sys/types.h>
#include <sys/sem.h>
#include <errno.h>
#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <sys/shm.h>
#include <sys/ipc.h>
#define SEMPERM 0600

typedef union _semun {
    int val;
    struct semid_ds *buf;
    unsigned short *array;
}semun;
int initsem (key_t semkey){
    int status=0, semid;
    semid=semget(semkey, 1 , SEMPERM | IPC_CREAT | IPC_EXCL);
    if (semid==-1){
        if (errno==EEXIST){semid=semget(semkey,1,0);}
    } else {
        semun arg;
        arg.val=0;
        status=semctl(semid,0,SETVAL,arg);
    }
    if ((semid==-1) || (status==-1)){
        perror("initsem fallita");
        return (-1);
    }
    return (semid);
}
int waitSem(int semid){
    struct sembuf wait_buf;
    wait_buf.sem_num=0;
    wait_buf.sem_op=1;
    wait_buf.sem_flg=SEM_UNDO;
    if (semop(semid,&wait_buf, 1)==-1){
        perror("waitSem Fallita");
        exit(-1);
    }
    return 0;
}

int signalSem (int semid){
    struct sembuf signal_buf;
    signal_buf.sem_num=0;
    signal_buf.sem_op=1;
    signal_buf.sem_flg=SEM_UNDO;
    if (semop(semid,&signal_buf,1)==-1){
        perror("signalSem fallita");
        exit(1);
    }
    return 0;
}
int main(){
    int id,*point,a,b,c;
    key_t keysem=0x050,chiavemem=0x100;
    int sem,buffer[2];
    sem=initsem(keysem);
    if (sem<0){
        perror("errore creazione semaforo");
        exit(-1);
    }
    id=shmget(chiavemem,sizeof(buffer[2]),0777|IPC_CREAT|IPC_EXCL);
    if(id<0){
        perror("errore creazione memoria condivisa");
        exit(-1);
    }
    point=(int *)shmat(id,NULL,0); 
    if (point<0){
        perror("Attacco additore errato");
        exit(-1);
    }
    waitSem(sem);
    point[0]=a;
    point[1]=b;
    c=a+b;
    point[2]=c;
    signalSem(sem);
    shmdt(point);
    exit(0);
}

P.S. Merry Christmas
# 14  
Old 12-22-2011
I didn't even run the first one, at all, and your second program didn't bother waiting for it.

Next time I run it, it complains that the shared memory segment already exists. Why are you using IPC_EXCL all the time?

Only one of your programs should be allowed to create the semaphore, in any case.
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