I created a shared memory and attached it. I also created the appropriate semaphores to sync. my read/write operations. However I do not know how am I supposed to do so. Anyone has got any ideas? I want to write int the form of 1234:23:444:... where each number between : means something to the user. I also want to be able to read whole parts between : and not the whole string. Please guys i really need some help here. Thnaks in advance
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)
Hi everyone. I asked once here and it went well, so I'm very happy with this community.
I have a new question to ask. I would like to read&write specify memory of a process which is not the process I'm doing it from, nor a child nor a parent.
So, I want to do a program similar to Cheat... (3 Replies)
Hi,
We have smb client running on two of the linux boxes and smb server on another linux system. During a backup operation which uses smb, read of a file was allowed while write to the same file was going on.Also simultaneous writes to the same file were allowed.Following are the settings in the... (1 Reply)
I have created and attached a shared memory segment. However I do not know how am I supposed to write and read data from it. I want to save various different data on this segment such as process IDs and other. Do you know how am I supposed to write these and be able to read such data in the correct... (1 Reply)
I have created a shared memory segment (which size is 64 bytes) using shmget, shmat e.t.c and i want to divide it into 2 areas. One area for input data and one area for output? How can i do that?
Furthermore, When i have to write my input data into the shared memory segment i want to write... (3 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)
I am looking for C program source code. Could you please help me in finding the source code required mentioned below.
program to create multiple threads (one master thread and rest worker threads) and using the threads write into and read from shared memory
Restrictions:
Only one thread... (2 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)
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 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)
SEM_OVERVIEW(7) Linux Programmer's Manual SEM_OVERVIEW(7)NAME
sem_overview - overview of POSIX semaphores
DESCRIPTION
POSIX semaphores allow processes and threads to synchronize their actions.
A semaphore is an integer whose value is never allowed to fall below zero. Two operations can be performed on semaphores: increment the
semaphore value by one (sem_post(3)); and decrement the semaphore value by one (sem_wait(3)). If the value of a semaphore is currently
zero, then a sem_wait(3) operation will block until the value becomes greater than zero.
POSIX semaphores come in two forms: named semaphores and unnamed semaphores.
Named semaphores
A named semaphore is identified by a name of the form /somename; that is, a null-terminated string of up to NAME_MAX-4 (i.e., 251)
characters consisting of an initial slash, followed by one or more characters, none of which are slashes. Two processes can operate
on the same named semaphore by passing the same name to sem_open(3).
The sem_open(3) function creates a new named semaphore or opens an existing named semaphore. After the semaphore has been opened,
it can be operated on using sem_post(3) and sem_wait(3). When a process has finished using the semaphore, it can use sem_close(3)
to close the semaphore. When all processes have finished using the semaphore, it can be removed from the system using
sem_unlink(3).
Unnamed semaphores (memory-based semaphores)
An unnamed semaphore does not have a name. Instead the semaphore is placed in a region of memory that is shared between multiple
threads (a thread-shared semaphore) or processes (a process-shared semaphore). A thread-shared semaphore is placed in an area of
memory shared between the threads of a process, for example, a global variable. A process-shared semaphore must be placed in a
shared memory region (e.g., a System V shared memory segment created using shmget(2), or a POSIX shared memory object built created
using shm_open(3)).
Before being used, an unnamed semaphore must be initialized using sem_init(3). It can then be operated on using sem_post(3) and
sem_wait(3). When the semaphore is no longer required, and before the memory in which it is located is deallocated, the semaphore
should be destroyed using sem_destroy(3).
The remainder of this section describes some specific details of the Linux implementation of POSIX semaphores.
Versions
Prior to kernel 2.6, Linux only supported unnamed, thread-shared semaphores. On a system with Linux 2.6 and a glibc that provides the NPTL
threading implementation, a complete implementation of POSIX semaphores is provided.
Persistence
POSIX named semaphores have kernel persistence: if not removed by sem_unlink(3), a semaphore will exist until the system is shut down.
Linking
Programs using the POSIX semaphores API must be compiled with cc -pthread to link against the real-time library, librt.
Accessing named semaphores via the file system
On Linux, named semaphores are created in a virtual file system, normally mounted under /dev/shm, with names of the form sem.somename.
(This is the reason that semaphore names are limited to NAME_MAX-4 rather than NAME_MAX characters.)
Since Linux 2.6.19, ACLs can be placed on files under this directory, to control object permissions on a per-user and per-group basis.
CONFORMING TO
POSIX.1-2001.
NOTES
System V semaphores (semget(2), semop(2), etc.) are an older semaphore API. POSIX semaphores provide a simpler, and better designed inter-
face than System V semaphores; on the other hand POSIX semaphores are less widely available (especially on older systems) than System V
semaphores.
EXAMPLE
An example of the use of various POSIX semaphore functions is shown in sem_wait(3).
SEE ALSO sem_close(3), sem_destroy(3), sem_getvalue(3), sem_init(3), sem_open(3), sem_post(3), sem_unlink(3), sem_wait(3), pthreads(7)COLOPHON
This page is part of release 3.44 of the Linux man-pages project. A description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
Linux 2012-05-13 SEM_OVERVIEW(7)