Can't add an array with data to shared memory

Question

I am trying to write a file with data into my shared memory segment. However everything I have tried seems just to give the error Segmentation fault. I have been searching the internet for help for more then one day.

int main(int argc, char *argv[]).
{
    int sm;
    char *data;
    int pid=atoi(argv[1]);
    int key=atoi(argv[2]);
    char (*d)[1025];
    data=(char*) malloc(1025);


    //put the data in the shared memory segment
    FILE *file=fopen(argv[3], "r"); //r for read
    if (file==0)
    {printf("Could not open file");}
    else
    {
        while(fgets(data, 1025, file)!=NULL)
        {
            fputs(data, file);
        //  puts(d);
        }       
        fclose(file);
    }

    //access shared memory 
    //S_IWUSR gives owner the write permession
    sm = shmget(key, 1024, S_IWUSR); 
    //create a pointer to the shared memory segment
    d =  shmat(sm, (void *)0, 0); //shared memory id, shmaddr, shmflg
    //for (int j=0; j<100; j++)
        strcpy(d[0], data);

    shmdt(d); //detach the shared memory segment
    //remove the shared memory segment
    shmctl(sm, IPC_RMID, NULL);
}

Any help would be greatly appreciated Thanks in advance

EDIT: added malloc

EDIT2: maybe I should rephrase my question, my problem is to get the data into my shared memory

I think it is the strcpy line that fails, at least every thing I try with adding my data to the shared memory fails — gudnylara7, Dec 17 '12 at 11:33
Have you noticed that char (*d)[1025] is a pointer to an array of 1025 chars (did you meant char d[1025] which is an array of 1025 chars which is already allocated). As @benjarobin mentioned your data is not allocated — giorashc, Dec 17 '12 at 11:38
Well if I change it to just char d[1025] I can't use it to attach to my shared memory — gudnylara7, Dec 17 '12 at 11:46
I fail to comprehend, why you read a line from the file and then write it back to the same file?! — Hristo Iliev, Dec 17 '12 at 16:41
was just a mistake, didn't realize i was doing it. But if you check the answer by benjarobin it shows what i was trying to accomplish — gudnylara7, Dec 17 '12 at 18:03

Jeyaram · Answer 1 · 2012-12-17T11:39:03.140

0

File is opened in read mode only.

change it to rw+ which will open a file in read/write mode. If the file is not available, it will be created.

fputs(data, file); here file is opened in read-only mode but write is happening.

But Im not sure, why you trying to write the read data to same file. You should consider your design first.

2. char (*d)[1025]; is a pointer to an char array of size 1025. You are using it in strcpy().

3. memory should be allocated for data either statically or dynamically.

edited Dec 17 '12 at 11:39

answered Dec 17 '12 at 11:22

Jeyaram

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I am trying to add the file from argv[3] into my shared memory and thought it would be good to use fputs. what other way is better? – gudnylara7 Dec 17 '12 at 11:32
Problem in `while(fgets(data, 1025, file)!=NULL)`. Bcos memory for data is not allocated as @benjarobin stated. – Jeyaram Dec 17 '12 at 11:34
But is it possible to put this somehow in the shared memory? If I just use puts(data) then my data gets printed out so I thought it worked – gudnylara7 Dec 17 '12 at 11:38
For number 1. How do I change it to write mode as well? 2. In strcpy I'm trying to move my array to the shared memory. d is the pointer that points to my shared memory and data is supposed to be the data to be added to the shared memory – gudnylara7 Dec 17 '12 at 11:43
For number 1, I was searching for another way and saw this here http://stackoverflow.com/questions/8147389/how-to-read-multiple-lines-of-input-from-user-using-fgets-and-write-it-into-a-fi don't understand why my way is wrong as it is almost the same Ooops just realized the problem now :) but still get segmentation fault – gudnylara7 Dec 17 '12 at 12:03

benjarobin · Accepted Answer · 2012-12-17T13:17:54.460

+1 about rjayavrp answer

And I can add that data is not allocated... This is just a pointer and not a array of chars..

Moreover what are you trying to do with

char (*d)[1025];

A quick and dirty example :

#include <stdio.h>
#include <stdlib.h>
#include <sys/ipc.h>
#include <sys/shm.h>

#define SIZE_SHARED_MEMORY  1024

int main(int argc, char *argv[])
{
    int sm = -1;
    char *d = NULL;
    FILE *file = NULL;
    key_t key = 0;
    int ret = 0;

    if (argc != 4)
    {
        return 1;
    }

    key = atoi(argv[2]);

    //access shared memory
    sm = shmget(key, SIZE_SHARED_MEMORY, IPC_CREAT | 0600);
    if (sm == -1)
    {
        perror("shmget : Failed");
        return 2;
    }

    //create a pointer to the shared memory segment
    d =  shmat(sm, (char *)0, 0);
    if (d == (void *)-1)
    {
        perror("shmat : Failed");
        return 3;
    }

    // Open the file
    file = fopen(argv[3], "r");
    if (file == NULL)
    {
        perror("fopen : Failed");
        ret  = 4;
    }
    else
    {
        if(fgets(d, SIZE_SHARED_MEMORY, file) == NULL)
        {
            perror("fgets : Failed");
            ret = 5;
        }
        fclose(file);
    }

    shmdt(d); //detach the shared memory segment

    // remove the shared memory segment ???
    // Don't understand why you are doing this
    shmctl(sm, IPC_RMID, NULL);

   return ret;
}

My shared memory should hold 1024 letters, so I thought my data could have 1024 letters as well. Like I said I have tried a lot of ways to get my data into the shared memory without luck — gudnylara7, Dec 17 '12 at 11:35
Thanks! But how can you check if the data is in the memory segment? — gudnylara7, Dec 17 '12 at 13:28
Well I already did and also figured it out, no reason for answering like this. But your example helped me get rid of the things I didn't need and figure everything out so thanks. — gudnylara7, Dec 17 '12 at 17:02

Can't add an array with data to shared memory

2 Answers2