Why share memory segment retrieve last input data only.

Question

I have a sample code for server and client.But when i run client with fist input data then sever is not detecting that when i run the client second then server prints first input data.please some body explain me this behavior of my code.

1.Sever.c

#include"share.h"
main()
{
    int ret=0;
    int shmid=0;
    void *shmaddr=0;
    int running=1;
    shm_buf *shmstruct=NULL;
    shmid=shminit();
    if(ret== EXIT_FAILURE)
        return;
    if((shmaddr=shmattach(shmid)) == NULL)
        return;
    shmstruct=(shm_buf*)shmaddr;
    while(running)
    {
        if( shmstruct->flag ==0)
        {
            printf("waiting for client......\n");
        }
        else
        {
            printf("%s\n", shmstruct->buffer);
            if(strncmp(shmstruct->buffer,"end",3)==0)
            {
                break;
            }
            shmstruct->flag=0;
        }
        sleep(2);
    }
    ret=shmdetach(shmaddr);

    if(ret== EXIT_FAILURE)
        return;

    ret=shmdel(shmid);
    if(ret== EXIT_FAILURE)
        return;

}

2.client.c

#include"share.h"
main()
{
    int ret=0;
    int shmid=0;
    void *shmaddr=0;
    shm_buf *shmstruct=NULL;
    shmid=shminit();
    if(ret== EXIT_FAILURE)
        return;
    if((shmaddr=shmattach(shmid)) == NULL)
        return;
    shmstruct=(shm_buf*)shmaddr;
    if( shmstruct->flag ==0)
    {
        shmstruct->flag=1;
        printf("Enter meeeage:");
        fgets( shmstruct->buffer,sizeof(shm_buf),stdin);
    }
}

3.share.h

#include<stdio.h>
#include<string.h>
#include<sys/shm.h>
#include<sys/ipc.h>
#include<error.h>
#include<errno.h>
#define EXIT_FAILURE -1
#define EXIT_SUCCESS 0
#define TRUE 1
#define FALSE 0
struct share
{
    int flag;
    char buffer[BUFSIZ];
};
typedef struct share shm_buf;
int shminit()
{
    int shmid=0;
    key_t key=ftok("/home/rajdhar/sample/shm/share.h",'A');
    if(key<0)
    {
        printf("ftok failed:%s\n",strerror(errno));
        return EXIT_FAILURE;
    }
    printf("key has been created successfuuly\n");

    shmid=shmget(key,sizeof(shm_buf),IPC_CREAT|0666);
    if(shmid<0)
    {
        printf("shmget failed:%s\n",strerror(errno));
        return EXIT_FAILURE;
    }
    printf("%d share memory has been created successfully\n",shmid);
    return shmid;
}

void *shmattach(int shmid)
{
    void *shmaddr=NULL;
    shmaddr=shmat(shmid,(void *)0,0);
    if(shmaddr == NULL)
    {
        printf("shmat failed:%s\n",strerror(errno));
        return NULL;
    }
    printf("memory has been attached with process successfulyy\n");
    return shmaddr;
}

int shmdetach(void *shmaddr)
{
    int ret = FALSE;
    ret=shmdt(shmaddr);
    if(ret<0)
    {   
        printf("shmdt failed:%s\n",strerror(errno));
        return EXIT_FAILURE;
    }
    printf("share memory has been detached successfully\n");
    return EXIT_SUCCESS;
}

int shmdel(int shmid)
{
    int ret = FALSE;
    ret=shmctl(shmid,IPC_RMID,0);
    if(ret<0)
    {
        printf("shmdt failed:%s\n",strerror(errno));
        return EXIT_FAILURE;
    }
    printf("Share memory has been deleted properly\n");
    return TRUE;
}

Example:- 1->./server 2->./client enter message:hello world server will not print any thing. 3-> ./client enter message:i am in the world. server prints "hello world"

Answer 1

In the client you have this:

shmstruct->flag=1;
printf("Enter meeeage:");
fgets( shmstruct->buffer,sizeof(shm_buf),stdin);

Which notifies the server before the data has been read into shmstruct->buffer.

You should probably set shmstruct->flag to 1 only when it has read the data into shmstruct->buffer:

printf("Enter meeeage:");
fgets( shmstruct->buffer,sizeof(shm_buf),stdin);
shmstruct->flag=1;

In production code you may like to use process shared mutex and condition variable to protect your shared state from corruption and notify the waiting process that the state has changed.