I'll try to keep this short, but basically I need to figure out a way to load data in shared memory (this file will be called load.c) I will later access the data with a print.c program.
The "data" is in the form of a student database that looks like this
I store it in a struct that i defined in a header file that looks like this(the ssn's are fake obviously, they are later used for a "change.c" portion)
Basically Im trying to figure out the best way to load an undefined number of "entires" of students into shared memory, so that I can access them later. I think I have the beginning part done...
However Im unsure of how to go about reading them in, I would assume I could store them in a vector/array.....but I wouldn't know how to do that when It came around to accessing them, especially when I dont know how many there are.
I do need to provide mutual exclusion (no more than one writer, but as many readers as I want)...altho Im a bit unsure of how many sephamores to use, I was using a default of 5.....but Im not sure if that is correct...Also I know i need to synchronize concurrent access since multiple writers/readers will be accessing change.c. The whole sephamores thing is a tad confusing tho...
Any hints/ideas/tips would be SUPER helpful. im not asking for an answer, but maybe a point in the right direction. Shared memory is a new thing for me.
HI,
I am dynamically loading shared libraries using shl_load(). There are multiple processes (50 or more) which loads the same shared library. Will Unix internally load only one copy of the shared library or it will load multiple copies.
Can I have memory issues if this is done.
Thanks,... (1 Reply)
I am debugging in gdb a program that dynamically loads libodbcinst.so. I want to debug the code from libodbcinst - I can break in it, see the source and step through it, but whenever I try printing a variable, I get (e.g. for the variable ret):
No symbol "ret" in current context.
In my program... (0 Replies)
Hello,
I wonder if I can write my userdefined data structures(ex: a list) to a shared memory segment?
I know, the shm functions get (void*) parameter so I should be able to read and write a list into the shared memory.
may someone inform and clarify me about that, please? (1 Reply)
Hi All
I have been given by someone else header file and a shared library to be used by my C++ application. Compilation is fine but when I try to executes the application I receive the following error.
./first: error while loading shared libraries: libMyLib.so.9: cannot open shared object file:... (2 Replies)
Hi,
I have an application running on AIX. The app is deployed on Webspshere server. Due to some reason, i have to make use of a third party library (Sigar API's) from my application. This library requires an .so file as well.
Is there any location where i can put this *.so file and it will... (1 Reply)
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)
Hello,
I am trying to run a program from my local account and receive the following error:
/local/app: error while loading shared libraries: libtiff.so.3: cannot open shared object file: No such file or directory
On the root account (which I DO NOT have access to), I see that libtiff.so.4... (3 Replies)
I am trying to run a C++ program which uses a static library libprun.a. During compilation, I am loading this library file using a environment variable as below.
LIBDIR = ${CUSTOM_PATH}/lib
LOADLIBS = $(LIBDIR)/libgqlcomm.a \
$(LIBDIR)/libgsml.a \
... (7 Replies)
Discussion started by: vdivb
7 Replies
LEARN ABOUT X11R4
sem_overview
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 supported only 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 filesystem
On Linux, named semaphores are created in a virtual filesystem, 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.
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), shm_over-
view(7)COLOPHON
This page is part of release 4.15 of the Linux man-pages project. A description of the project, information about reporting bugs, and the
latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.
Linux 2017-05-03 SEM_OVERVIEW(7)