Login or Register to Ask a Question and Join Our Community

Sponsored Content
Full Discussion: Shared memory between two c program
Top Forums Programming Shared memory between two c program Post 302582583 by Corona688 on Friday 16th of December 2011 11:38:46 AM
Old 12-16-2011
Modifying your program a bit:

Code:
#include <unistd.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <stdio.h>
#include <stdlib.h>

void die(const char *msg)
{
        perror(msg);
        exit(1);
}

int main() {
    int a=5,b=7;
    key_t keyshm; // or key_t keyshm=0x200;
    keyshm=ftok("/tmp",32); //see up
    int buffer[1];
    int *point;
    int shmid;
    shmid=shmget(keyshm , sizeof(buffer),0666);
    if(shmid < 0) die("Couldn't shmget");

    point=(int *)shmat(shmid,NULL,0);
    point[0]=a;
    point[1]=b;
    printf("FIRST = %d\nSECOND = %d",point[0],point[1]);
    fflush(stdout);
    sleep(300);
    shmdt(point);
    exit(0);
}

Code:
$ ./shm1
Couldn't shmget: No such file or directory
$ man shmget
SHMGET(2)                  Linux Programmer's Manual                 SHMGET(2)

...

ERRORS
       On failure, errno is set to one of the following:

       EACCES The  user  does  not have permission to access the shared memory
              segment, and does not have the CAP_IPC_OWNER capability.

       EEXIST IPC_CREAT | IPC_EXCL was specified and the segment exists.

       EINVAL A new segment was to be created and size < SHMMIN or size > SHM-
              MAX,  or  no new segment was to be created, a segment with given
              key existed, but size is greater than the size of that segment.

       ENFILE The system limit on the total number  of  open  files  has  been
              reached.

       ENOENT No segment exists for the given key, and IPC_CREAT was not spec-
              ified.

       ENOMEM No memory could be allocated for segment overhead.

       ENOSPC All possible shared memory IDs  have  been  taken  (SHMMNI),  or
              allocating  a segment of the requested size would cause the sys-
              tem to exceed the system-wide limit on shared memory (SHMALL).

       EPERM  The SHM_HUGETLB flag was specified, but the caller was not priv-
              ileged (did not have the CAP_IPC_LOCK capability).

...

So you want 0666 | IPC_CREAT

And you should check the return values of everything in case anything you weren't expecting fails.
 

10 More Discussions You Might Find Interesting

1. Programming

Shared memory

Dear Reader, Is is necessary to attach / dettach the shared memory segments for write operations , if more than one program is accessing same shared memory segments.. I have used semaphore mutex and still I'm getting segmentation fault when I write to the segment when other program is already... (1 Reply)
Discussion started by: joseph_shibu
1 Replies

2. UNIX for Advanced & Expert Users

Shared memory shortage but lots of unused memory

I am running HP-UX B.11.11. I'm increasing a parameter for a database engine so that it uses more memory to buffer the disk drive (to speed up performance). I have over 5GB of memory not being used. But when I try to start the DB with the increased buffer parameter I get told. "Not... (1 Reply)
Discussion started by: cjcamaro
1 Replies

3. Linux

all about shared memory

Hi all :confused: , I am new to unix.I have been asked to implement shared memory in user's mode.What does this mean?What is the difference it makes in kernel mode and in users mode?What are the advantages of this impemenation(user's mode)? And also i would like to know why exactly shared... (0 Replies)
Discussion started by: vijaya2006
0 Replies

4. Programming

help with shared memory

what i want to do is have an int that can been written into by 2 processes but my code doesn't seem to work. #include <sys/types.h> #include <sys/ipc.h> #include <sys/sem.h> #include <sys/shm.h> #include<stdio.h> #define KEY1 (1492) int main() { int shmid; volatile int * addr;... (6 Replies)
Discussion started by: ddx08
6 Replies

5. Programming

memory sharing - not shared memory -

hi, this is the problem: i want to swap a linked list between 4 processes (unrelated), is there any way i can do that just by sending a pointer to a structure? //example typedef struct node { int x; char c; struct node *next; } node; or i should send the items ( x,c ) by... (9 Replies)
Discussion started by: elzalem
9 Replies

6. Programming

Shared memory in shared library

I need to create a shared library to access an in memory DB. The DB is not huge, but big enough to make it cumbersome to carry around in every single process using the shared library. Luckily, it is pretty static information, so I don't need to worry much about synchronizing the data between... (12 Replies)
Discussion started by: DreamWarrior
12 Replies

7. Programming

Shared memory for shared library

I am writing a shared library in Linux (but compatible with other UNIXes) and I want to allow multiple instances to share a piece of memory -- 1 byte is enough. What's the "best" way to do this? I want to optimize for speed and portability. Obviously, I'll have to worry about mutual exclusion. (0 Replies)
Discussion started by: otheus
0 Replies

8. UNIX for Advanced & Expert Users

Shared Memory

Hi, Using ipcs we can see shared memory, etc.. details. How can I add/remove shared memory(command name)? Thanks, Naga:cool: (2 Replies)
Discussion started by: Nagapandi
2 Replies

9. AIX

shared memory

1.How to know wich process is using the shared memory? 2.How to flush (release) the process from the shared memory? (1 Reply)
Discussion started by: pchangba
1 Replies

10. Programming

Shared library with acces to shared memory.

Hello. I am new to this forum and I would like to ask for advice about low level POSIX programming. I have to implement a POSIX compliant C shared library. A file will have some variables and the shared library will have some functions which need those variables. There is one special... (5 Replies)
Discussion started by: iamjag
5 Replies

LEARN ABOUT ULTRIX

shmop

shmop(2)							System Calls Manual							  shmop(2)

Name
       shmop, shmat, shmdt - shared memory operations

Syntax
       #include <sys/types.h>
       #include <sys/ipc.h>
       #include <sys/shm.h>

       char *shmat (shmid, shmaddr, shmflg)
       int shmid;
       char *shmaddr;
       int shmflg;

       int shmdt (shmaddr)
       char *shmaddr;

Description
       The  system  call attaches the shared memory segment associated with the shared memory identifier specified by shmid to the data segment of
       the calling process.  The segment is attached at the address specified by one of the following criteria:

       If shmaddr is equal to zero, the segment is attached at the first available address as selected by the system.

       If shmaddr is not equal to zero and (shmflg & SHM_RND ) is true, the segment is attached at the address given by (shmaddr- (shmaddr modulus
       SHMLBA )).

       If shmaddr is not equal to zero and (shmflg & SHM_RND ) is false, the segment is attached at the address given by shmaddr.

       The segment is attached for reading if (shmflg & SHM_RDONLY ) is true. Otherwise, it is attached for reading and writing.

       The system call detaches from the calling process's data segment the shared memory segment located at the address specified by shmaddr.

Return Values
       Upon successful completion, the return values are as follows:

       o   The system call returns the data segment start address of the attached shared memory segment.

       o   The system call returns a value of zero (0).

       Otherwise, a value of -1 is returned, and errno is set to indicate the error.

Diagnostics
       The system call fails and not attach the shared memory segment, if any of the following is true:

       [EINVAL]       The shmid is not a valid shared memory identifier.

       [EACCES]       Operation permission is denied to the calling process.  For further information, see

       [ENOMEM]       The available data space is not large enough to accommodate the shared memory segment.

       [EINVAL]       The shmaddr is not equal to zero, and the value of (shmaddr- (shmaddr modulus SHMLBA )) is an illegal address.

       [EINVAL]       The shmaddr is not equal to zero, (shmflg & SHM_RND ) is false, and the value of shmaddr is an illegal address.

       [EMFILE]       The number of shared memory segments attached to the calling process would exceed the system imposed limit.

       The fails and does not detach the shared memory segment if:

       [EINVAL]       The shmaddr is not the data segment start address of a shared memory segment.

See Also
       execve(2), exit(2), fork(2), shmctl(2), shmget(2)

																	  shmop(2)
All times are GMT -4. The time now is 06:01 PM.
Unix & Linux Forums Content Copyright 1993-2022. All Rights Reserved.
Privacy Policy