Thread: Shared Memroy Indroduction Program

Hello All,

I am having troubles creating a program that uses shared memory. As far as I can tell I am doing everything correctly but I keep getting a seg fault when I run the code (see code below for explanation). The basic thought is for a program to create a shared memory place and then fork a process that will use the shared space to sum up numbers passed in through the command line.

Code:

        printf("in main");
        /** the identifier for the shared memory segment */
        int shared_mem_id;

        /** pointer to the address space */
        int *shared_mem;

        /** the size (in bytes) of the shared memory segment */
        int segment_size = sizeof(int);
        printf("declariation of stuff");

        /** generate key used in shmget() */
        key_t key = ftok(argv[2], 1);
        printf("got key");

        /** allocate  a shared memory segment */
        shared_mem_id = shmget(key, segment_size, S_IRUSR | S_IWUSR);
    printf("got mem_id");

        /** attach the shared memory segment */
        shared_mem =  shmat(shared_mem_id, NULL, 0);
        printf("created mem stuff");

        /** initalize shared_mem to 0 */
        /** this is where i get to before it seg faults */
        *shared_mem = 0;
        printf("created mem stuff and inialize");

Any help would be greatly appriciated.

Thanks

It would help immensely if you could provide compilable code, without that, I can only give you a visual eval, not recreate your problem and pinpoint it. Also, you should find some GDB tutorials (like the one we have here: Cprogramming.com - Tutorials - An Introduction to GDB), so you can learn to debug these things yourself. Here are 2 problems I see off the bat:

1. You never check the return values of any of the shared memory/IPC functions: ftok, shmget, shmat, or presumably anything else that you may call in the rest of your code. You should check the return values, and if they fail, print a useful error message using the perror function.
2. You use argv[2], but I don't see you use argv[1]. Remember, arrays start at 0 in C. If you call your program as "foo ftok_key", then argv[0] is "foo", argv[1] is "ftok_key" and argv[2] is NULL.
3. Use fprintf(stderr, ...) for debug output, since stderr is unbuffered. stdin, which printf prints to, is line-buffered, so your debug statements may not show up even though you passed that point in the code before the crash. Debugging with print statements becomes a non-issue when you learn to use GDB.

Thank you Codeplug for the tip about IPC_CREAT. That turned out to fix my whole program. Also thanks for the tips about error checking the values I was getting back.

anduril462:

Thanks to you too. Next time I will give compilable code with my problem and also thanks for the pointer to the GDB tutorial.