IPC - Shared Programming Program in Linux

Question

I am trying to create a program which can communicate between two processes naming, processor.c and receiver.c, acting as client and server, with the help of shared memory.

The first program receiver.c runs in an infinite loop receiving alpha numeric strings as input from the user one line at a time. After reading one line from the standard input, this program sends this information to the other program. The sharing of data between the two processes should take place via shared memory. The second program processor.c creates an output file vowels.out and waits for user input to be sent by the receiver program. As soon as one line is received from the receiver, it counts the number of vowels in that line and dumps the vowel count along with the original line in the vowels.out file. This program also runs in an infinite loop.

Here are the programs -

processor.c

#include <sys/shm.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <stdio.h>
#define SHM_SIZE 10240  /* make it a 10K shared memory segment */
void main()
{
    int v=0;
    char ch;
    int id_shm;
    key_t random_key;   /* Keys are used for requesting resources*/
    char *shmem, *seg;

    /* Segment named "6400", should be created by the server.  */
    random_key = 6400;

    /* Locate the segment.  Used - shmget() */
    if ((id_shm = shmget(random_key, SHM_SIZE, 0660)) < 0) {
        perror("shmget");
        exit(1);
    }
    /* Attaching segment to the data spaces.  Used - shmat() */
    if ((shmem = shmat(id_shm, NULL, 0)) == (char *) -1) {
        perror("shmat");
        exit(1);
    }

    /* Creation of File */
    FILE *op_file;
    op_file = fopen("vowels.out", "a");

    /* Now read what the server put in the memory.  */
    seg = shmem;

    while (shmem != NULL)  /* 'while' - use of entry controlled loop */
    {   
    while ((ch=fgetc(op_file))!=EOF)
    {
        if (ch=='A'||ch=='a'||ch=='E'||ch=='e'||ch=='I'||ch=='i'||ch=='O'||ch=='o'||ch=='U'||ch=='u')
        {
            v++;
        }
    }

        fprintf(op_file, "%s, %d \n", shmem, v);

          *shmem = "*";
    while(*shmem == "*")
    {
        sleep(6);
    } 
    } 
    fclose(op_file);
}

receiver.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#define SHM_SIZE 10240  /* make it a 10K shared memory segment */
void main()
{
    key_t random_key;  /* Keys are used for requesting resources*/
    int id_shm;
    char *data, *shmem;


    random_key = 6400;

    /* Creation of the segment. Used - shmget()*/
    if ((id_shm = shmget(random_key, SHM_SIZE, 0660 | IPC_CREAT)) == -1) 
    {
        perror("shmget");
        exit(1);
    }

    /* Addition of segment to the data spaces.  Used - shmat() */
    if ((shmem = shmat(id_shm, NULL, 0)) == (char *) -1) 
    {
        perror("shmat");
        exit(1);
    }

    while (1)    /* 'while' - use of entry controlled loop to check on access */
    {
        scanf("%s", data);
        data = shmem;   /* Mapping of data */

    while(*shmem != "*")
    { 
        sleep(6);
    }
    }
 }

But I am receiving error specifying Segmentation Fault

Can anyone please help me with this? Is there something I am doing wrong in this program?

Thank you in advance.

Can you isolate the line where the segmentation fault happens? Does it also fails if you compile it with debug information (e.g., with -g in gcc)? As you are using Linux, you can also try Valgrind to collect more information about the error. — Philipp Claßen
– Philipp Claßen, Commented Jul 15, 2014 at 1:58
I am a beginner in linux and am using virtual box on a windows 8.1 machine. During the compilation process it shows warning such as -- receiver.c:36:15: comparison between pointer and integer[enabled by default] — Pranav Bhoraskar
– Pranav Bhoraskar, Commented Jul 15, 2014 at 2:09

TonyB · Accepted Answer · 2014-07-15 02:44:14Z

5

Other than several compiler warnings (*shmem = "*"; and while(*shmem == "*") ) and a couple of programming errors... You have a design issue.

Shared memory access/updates should be controlled with a semaphore to serialize access to the data and protect against partial updates.

As to your segmentation fault... It appears to be in receiver.c at scanf("%s", data);, because you are receiving input from stdin into data, which is simply a pointer to char... ... i.e. no space for the input has been allocated.

Hopefully, that should get you going...

edited Jul 15, 2014 at 2:44

answered Jul 15, 2014 at 2:11

TonyB

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