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shmget returning 0

Posted on 2008-10-26
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Last Modified: 2008-10-28
I have written this code

The call to CreateSharedMem creates and returns the id of shared memory segment but when i call GetSharedMem it returns 0?

Any idea what i am doing wrong ?

Thank You
int CreateSharedMem(char* key_file, char id, int seg_size)

{

        int shm_id;

        key_t keyval;

        if ((keyval = ftok(key_file,id))==-1)

        {

        	perror("ftok");

        	return -1;

        } 

        // Can use IPC_EXCL which will fail and returns -1 if a segment with the same key exists

        if((shm_id = shmget( keyval, seg_size, IPC_CREAT | 0666 | IPC_EXCL )) == -1)

        {

        		perror("shmget");

                return(-1);

        }

   

        return(shm_id);

}
 

int GetSharedMem(char* key_file,char id, int seg_size)

{

        int shm_id;

        key_t key_val;

        if ((key_val = ftok(key_file,id))==-1)

        {

        	perror("ftok");

        	return -1;

        } 

        printf("Key is %d",key_val);

        // Can use IPC_EXCL which will fail and returns -1 if a segment with the same key exists

        if((shm_id = shmget( key_val , seg_size, IPC_CREAT | 0666 ) == -1))

        {

        		perror("shmget");

                return(-1);

        }

        return(shm_id);

	

}
 

main()

{

.

.
 

 /* Create a shared memory */

	int my_shm_id = CreateSharedMem("MySharedMem.c",'A',1000);

	printf("%d : Shared mem is created\n",my_shm_id);

	

	

/* Get the same shared memory */

       int my_shm_id = GetSharedMem("MySharedMem.c",'A',1000);	

       printf("%d is Shared mem that is created\n",my_shm_id);

.

.

}

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Question by:Novice_
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LVL 53

Accepted Solution

by:
Infinity08 earned 125 total points
ID: 22810811
Due to operator precedence, this :

        if ((shm_id = shmget( key_val , seg_size, IPC_CREAT | 0666 ) == -1))

is the same as this :

        if ((   shm_id = (shmget( key_val , seg_size, IPC_CREAT | 0666 ) == -1)   ))

So, you check whether the return value of shmget is == -1, and then you place the result of that comparison in shm_id (0 if false, non-zero if true).

shm_id being 0 means that the shmget call succeeded.


I would suspect that you intended this instead :

        if ((shm_id = shmget( key_val , seg_size, IPC_CREAT | 0666 )) == -1)
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