Saving array of structs to shared memory

Question

I'm trying to create a piece of shared memory that holds an array of structs. In my current code when i run it i get a segmentation fault. I think I may need to use memcpy but am severely stuck at the moment. Any help would be mch appreciated...

#include <stdio.h>
#include <stdlib.h>
#include <sys/wait.h>
#include <assert.h>
#include <stdio.h>
#include <string.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/shm.h>
#include <unistd.h>
#include "header.h"


int main()
{
    key_t key = 1234;
    int shmid;
    int i = 1;

    struct companyInfo * pdata[5];

    strcpy(pdata[0]->companyName,"AIB");
    pdata[0]->sharePrice = 11.02;
    strcpy(pdata[1]->companyName,"Bank Of Ireland");
    pdata[1]->sharePrice = 10.02;
    strcpy(pdata[2]->companyName,"Permanent TSB");
    pdata[2]->sharePrice = 9.02;
    strcpy(pdata[3]->companyName,"Bank Od Scotland");
    pdata[3]->sharePrice = 8.02;
    strcpy(pdata[4]->companyName,"Ulster Bank");
    pdata[4]->sharePrice = 7.02;



    int sizeOfCompanyInfo = sizeof(struct companyInfo);

    int sizeMem = sizeOfCompanyInfo*5;

    printf("Memory Size: %d\n", sizeMem);

    shmid = shmget(key, sizeMem, 0644 | IPC_CREAT);
    if(shmid == -1)
    {
        perror("shmget");       
        exit(1);
    }

    *pdata = (struct companyInfo*) shmat(shmid, (void*) 0, 0);
    if(*pdata == (struct companyInfo*) -1)
    {
        perror("schmat error");
        exit(1);
    }

    printf("name is %s and %f . \n",pdata[0]->companyName,pdata[0]->sharePrice);

    exit(0);

}

the header.h file is as follows...

struct companyInfo
{
    double sharePrice;
    char companyName[100];
};

Where in your code does the seg fault occur? Can you step through with a debugger or even just put printf() statements in to see at least where the crash occurs? — chrisaycock
– chrisaycock, Commented Mar 11, 2012 at 22:49
It doesnt seem to like me assigning values to the array, when i strcpy into pdata[1] the segmentation fault occurs. i found it by throwing in printfs — kev670
– kev670, Commented Mar 11, 2012 at 22:55
Then it sounds like pdata[1] wasn't initialized properly. Fortunately, there are a couple answers below that explain how to fix that. — chrisaycock
– chrisaycock, Commented Mar 11, 2012 at 22:56
You have two answers that point out the problem. I would accept hmjd's answer because it is more complete and provides code. — Carey Gregory
– Carey Gregory, Commented Mar 11, 2012 at 22:58

hmjd · Accepted Answer · 2012-03-11 22:54:07Z

3

struct companyInfo * pdata[5];

contains an array of 5 uninitialized pointers. You need too allocate memory for each element in the array before using them:

for (int i = 0; i < 5; i++)
{
    pdata[i] = malloc(sizeof(companyInfo));
}

or just declare an array of struct companyInfo as there does not appear to be any need for dynamic allocation:

struct companyInfo pdata[5];

answered Mar 11, 2012 at 22:54

hmjd

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2 Comments

kev670 Over a year ago

thanks. using malloc helped a lot and got rid of the segmentation fault.

blueshift Over a year ago

Unfortunately this doesn't make the shared memory work.. stackoverflow.com/questions/9660503/…

UmNyobe · Accepted Answer · 2012-03-11 22:54:38Z

3

pdata is a table of pointers so you need to create each struct companyInfo using malloc before being able to access them.

answered Mar 11, 2012 at 22:54

UmNyobe

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