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 FREEBSD
shmctl
SHMCTL(2) BSD System Calls Manual SHMCTL(2)NAME
shmctl -- shared memory control
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
int
shmctl(int shmid, int cmd, struct shmid_ds *buf);
DESCRIPTION
Performs the action specified by cmd on the shared memory segment identified by shmid:
IPC_STAT Fetch the segment's struct shmid_ds, storing it in the memory pointed to by buf.
IPC_SET Changes the shm_perm.uid, shm_perm.gid, and shm_perm.mode members of the segment's struct shmid_ds to match those of the struct
pointed to by buf. The calling process's effective uid must match either shm_perm.uid or shm_perm.cuid, or it must have supe-
ruser privileges.
IPC_RMID Removes the segment from the system. The removal will not take effect until all processes having attached the segment have
exited; however, once the IPC_RMID operation has taken place, no further processes will be allowed to attach the segment. For
the operation to succeed, the calling process's effective uid must match shm_perm.uid or shm_perm.cuid, or the process must have
superuser privileges.
The shmid_ds structure is defined as follows:
struct shmid_ds {
struct ipc_perm shm_perm; /* operation permission structure */
size_t shm_segsz; /* size of segment in bytes */
pid_t shm_lpid; /* process ID of last shared memory op */
pid_t shm_cpid; /* process ID of creator */
int shm_nattch; /* number of current attaches */
time_t shm_atime; /* time of last shmat() */
time_t shm_dtime; /* time of last shmdt() */
time_t shm_ctime; /* time of last change by shmctl() */
};
RETURN VALUES
The shmctl() function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate
the error.
ERRORS
The shmctl() system call will fail if:
[EINVAL] Invalid operation, or no shared memory segment was found corresponding to shmid.
[EPERM] The calling process's effective uid does not match the uid of the shared memory segment's owner or creator.
[EACCES] Permission denied due to mismatch between operation and mode of shared memory segment.
SEE ALSO shmat(2), shmdt(2), shmget(2), ftok(3)BSD July 17, 1995 BSD