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Error retrieving value from IPC shared memory

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# 1  
Old 07-29-2013
Error retrieving value from IPC shared memory
Assume in 1.txt file ,i have saved 32
writer.c
-----------
Code:
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <stdio.h>
#include<stdlib.h>
#include<fcntl.h>
int main(){
int shm_id;
key_t mem_key;
int *shm_ptr;
int fd=open("1.txt",O_CREAT|O_RDWR,0777);
mem_key = ftok("file.txt", '6');printf("Key=%d\n",mem_key);
shm_id = shmget(mem_key, 4*sizeof(int), IPC_CREAT | 0666);
printf("shm_id=%d\n",shm_id);
if (shm_id < 0) {
printf("*** shmget error (server) ***\n");
exit(1);
}
shm_ptr = (int *) shmat(shm_id, NULL, 0); /* attach */
printf("Printing value in the shared memory%x--&shm_ptr=%p\n",shm_ptr,&shm_ptr);
if (shm_ptr == (int *)-1) {
printf("*** shmat error (server) ***\n");
exit(1);
}
read(fd,&shm_ptr,sizeof(shm_ptr));
write(1,&shm_ptr,sizeof(shm_ptr));------------(1)>here its printing the correct value 32
return 0;
}

　

reader.c
---------
Code:
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <stdio.h>
#include<stdlib.h>
#include<fcntl.h>
int main(){
int shm_id;
key_t mem_key;
int *shm_ptr;
mem_key = ftok("file.txt", '6');printf("Key=%d\n",mem_key);
shm_id = shmget(mem_key, 4*sizeof(int), IPC_CREAT | 0666);
printf("shm_id=%d\n",shm_id);
if (shm_id < 0) {
printf("*** shmget error (server) ***\n");
exit(1);
}
shm_ptr = (int *) shmat(shm_id, NULL, 0); /* attach */
printf("Attached to %x\n",shm_ptr);
if (shm_ptr == (int *)-1) {
printf("*** shmat error (server) ***\n");
exit(1);
}
printf("Printing value in the shared memory :%x &shm_ptr=%p\n",shm_ptr,&shm_ptr);
write(1,&shm_ptr,sizeof(shm_ptr));------>(2)//Here its printing some junk value
　
return 0;
}

Can any body pls tell me why am i not able to retrieve the correct value in reader.c?shmid & key values are same in both the programs.Where i went wrong?Any help is very appreciable?

Moderator's Comments:
Mod Comment Use code tags please, see PM.
Last edited by zaxxon; 07-29-2013 at 08:36 AM.. Reason: code tags
# 2  
Old 07-30-2013
There are quite a few things wrong with both the reader and writer programs...however the one thing that jumps out is that both the read and write calls expect a pointer as their 2nd argument...while you are providing an int** pointer...so once the 2nd argument to read and write is made an int* type then it should work...
# 3  
Old 07-31-2013
As shamrock said, there were several things wrong with both reader.c and writer.c. You might try the following replacements for them:
reader.c:
Code:
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/ipc.h>
#include <sys/shm.h>

int
main() {
        key_t   mem_key;
        int     shm_id;
        char    *shm_ptr;

        if((mem_key = ftok("writer", '6')) == (key_t)-1) {
                perror("reader ftok() failed");
                exit(10);
        }
        printf("Key = 0x%llx\n", (long long)mem_key);
        if((shm_id = shmget(mem_key, 80, 0444)) == -1) {
                perror("reader shmget error");
                exit(20);
        }
        printf("shm_id = %d\n", shm_id);
        if((shm_ptr = (char *)shmat(shm_id, (void *)0, 0)) == (char *)-1) {
                perror("reader shmat error");
                exit(30);
        }
        printf("reader attached @ %p\n", shm_ptr);
        printf("String found in shared memory: '%s'\n", shm_ptr);
        return 0;
}

writer.c:
Code:
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <unistd.h>

int main() {
        key_t   mem_key;
        int     shm_id;
        char    *shm_ptr;

        if((mem_key = ftok("writer", '6')) == (key_t)-1) {
                perror("writer ftok() failed");
                exit(1);
        }
        printf("Key = 0x%llx\n", (long long)mem_key);
        if((shm_id = shmget(mem_key, 80, IPC_CREAT | 0644)) == -1) {
                perror("writer shmget() failed");
                exit(2);
        }
        printf("shm_id=%d\n", shm_id);
        if((shm_ptr = (char *)shmat(shm_id, (void *)0, 0)) == (char *)-1) {
                perror("writer shmat() failed");
                exit(3);
        }
        printf("writer attached @ %p\n", shm_ptr);
        snprintf(shm_ptr, 80, "Loaded by pid %d", getpid());
        printf("Data loaded into shared memory: '%s'\n", shm_ptr);
        return 0;
}

Run:
Code:
make reader writer

to build them. A file named writer must exist for reader to be able to access the shared memory segment created by writer. If you run reader before you run writer, reader will fail because the shared memory segment hasn't been created yet. (You may want to try this to see what happens.)

When writer runs, it creates the shared memory segment (if it doesn't already exist); then it writes a string into the segment and prints the string it wrote to stdout before exiting.

You can use the command ipcs -ma to see all of the active shared memory segments on your system. Each time you rebuild writer the inode number of the file named writer may change; if it does, the next run of writer will create a new shared memory segment instead of using the previously created segment. (This is a "feature" of the way ftok() works.)

The reader program can be run any number of times you want and it will show you the current contents of the shared memory segment. The writer program can also be run any number of times and each run will update the string stored in the shared memory segment.

You can remove a shared memory segment using the command:
Code:
ipcrm -m ID
       or
ipcrm -M Key

You can find a shared memory segment's ID and Key from the output of the ipcs command given above and from the output of the reader and writer commands given in this message.

This was tested on Mac OS X running on a MacBook Pro laptop, but should work on any system that provides the shared memory features first developed for UNIX System III and now required on all UNIX branded systems. It is also required on POSIX conforming systems that claim to support the X/Open System Interfaces extension option.
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