Solved

shmget returning 0

Posted on 2008-10-26
1
983 Views
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);
.
.
}

Open in new window

0
Comment
Question by:Novice_
1 Comment
 
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)
0

Featured Post

Master Your Team's Linux and Cloud Stack!

The average business loses $13.5M per year to ineffective training (per 1,000 employees). Keep ahead of the competition and combine in-person quality with online cost and flexibility by training with Linux Academy.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

Have you ever been frustrated by having to click seven times in order to retrieve a small bit of information from the web, always the same seven clicks, scrolling down and down until you reach your target? When you know the benefits of the command l…
The purpose of this article is to demonstrate how we can upgrade Python from version 2.7.6 to Python 2.7.10 on the Linux Mint operating system. I am using an Oracle Virtual Box where I have installed Linux Mint operating system version 17.2. Once yo…
Video by: Grant
The goal of this video is to provide viewers with basic examples to understand and use while-loops in the C programming language.
The goal of this video is to provide viewers with basic examples to understand opening and reading files in the C programming language.

856 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question