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)
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)
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)
hi all
I have been asked to implement shared memory in user mode?But :rolleyes: im not understanding how to do this.And what does it mean in user mode?Is it that we need to simulate what kernal does...like attaching ,detaching memory..etc...?
And would also like to know why shared memory is... (3 Replies)
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)
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)
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)
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
shmctl
shmctl(2) System Calls Manual shmctl(2)Name
shmctl - shared memory control operations
Syntax
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
int shmctl (shmid, cmd, buf)
int shmid, cmd;
struct shmid_ds *buf;
Description
The system call provides a variety of shared memory control operations, as specified by cmd. The following cmds are available:
IPC_STAT Place the current value of each member of the data structure associated with shmid into the structure pointed to by buf.
The contents of this structure are defined in
IPC_SET Set the value of the following members of the data structure associated with shmid to the corresponding value found in the
structure pointed to by buf:
shm_perm.uid
shm_perm.gid
shm_perm.mode /* only low 9 bits */
This cmd can only be executed by a process that has an effective user ID equal to either that of the superuser or to the
value of in the data structure associated with shmid.
IPC_RMID Remove the shared memory identifier specified by shmid from the system and destroy the shared memory segment and data struc-
ture associated with it. This cmd can only be executed by a process that has an effective user ID equal to either that of
the superuser or to the value of in the data structure associated with shmid.
SHM_LOCK Lock the shared memory segment specified by shmid in memory. Lock prevents the shared memory segment from being swapped or
paged. This cmd can only be executed by a process that has an effective user ID equal to the superuser.
SHM_UNLOCK Unlock the shared memory segment specified by shmid. This cmd can only be executed by a process that has an effective user
ID equal to the superuser.
Return Values
Upon successful completion, a value of zero (0) is returned. Otherwise, a value of -1 is returned, and errno is set to indicated the
error.
Diagnostics
The system call fails if any of the following is true:
[EINVAL] The shmid is not a valid shared memory identifier.
[EINVAL] The cmd is not a valid command.
[EACCES] The cmd is equal to IPC_STAT, and read permission is denied to the calling process. For further information, see
[EPERM] The cmd is equal to IPC_RMID or IPC_SET, and the effective user ID of the calling process is not equal to that of the supe-
ruser or to the value of in the data structure associated with shmid.
[EPERM] The cmd is equal to SHM_LOCK or SHM_UNLOCK and the effective user ID of the calling process is not equal to that of the
superuser.
[EINVAL] The cmd is equal to SHM_LOCK, and the shared memory segment is currently locked by this process.
[EINVAL] The cmd is equal to SHM_UNLOCK, and the shared memory segment specified by shmid is not currently locked in memory by this
process.
[EFAULT] The buf points to an illegal address.
See Alsoshmget(2), shmop(2)shmctl(2)