Thread: Problem with shared memory

  1. #1
    Registered User
    Join Date
    Apr 2008

    Problem with shared memory

    I'm trying to use shared memory. So far I have
    #include <sys/ipc.h>
    #include <sys/shm.h>
    #include <sys/types.h>
    #include <unistd.h>
    #include <semaphore.h>
    #include <stdio.h>
    #include <stdlib.h>
    #include <string.h>
    #define MAX_PATH_LEN 1024
    #define ARRAY_SIZE 50
    typedef struct _task{
    	char path [MAX_PATH_LEN];
    	char thread_id[MAX_PATH_LEN];
    	char op;
    	int index;
    typedef struct _sharedMem{
    	int lastIn;
    	int numTasks;
    	sem_t mutex;
    	int size;
    	task array [ARRAY_SIZE];
    int main(void){
    	int i;
    	int id = shmget(1234, sizeof(sharedMem), IPC_CREAT|0777);
    	sharedMem* shMem = shmat(id, NULL, 0);
    	printf("init 1\n");
    	shMem->lastIn = 0;
    	printf("init 2\n");
    	shMem->numTasks = 0;
    	printf("init 3\n");
    	sem_init(&(shMem->mutex), 0, 1);
    	printf("init 4\n");
    	shMem->size = ARRAY_SIZE;
    	for(i = 0; i < shMem->size; i++){
    		strcpy(shMem->array[i].path, " ");
    	return 1;
    The output for this is
    init 1
    Segmentation fault
    I have no idea why. Do we have to do something to the structure before start using it?
    If so what?

  2. #2
    Registered User
    Join Date
    Apr 2008
    You should check all the system calls status, are you sure your shm object is correctly created and attached? no.

    PS: you should use ftok() to create a key_t typed object instead of '1234', because some values are reserved
    Last edited by root4; 01-06-2009 at 12:47 PM.

  3. #3
    Registered User Codeplug's Avatar
    Join Date
    Mar 2003
    For reference:
    ftok() gets you a key_t value - to then pass to semget() for a valid shmid - which can then be passed to shmat().

    The other Posix API for shared memory is shm_open(), mmap(), and shm_unlink().

    Do yourself a big favor and get into the habit of always checking return values and error codes.


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