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 REDHAT
shmget
SHMGET(2) Linux Programmer's Manual SHMGET(2)NAME
shmget - allocates a shared memory segment
SYNOPSIS
#include <sys/ipc.h>
#include <sys/shm.h>
int shmget(key_t key, int size, int shmflg);
DESCRIPTION
shmget() returns the identifier of the shared memory segment associated to the value of the argument key. A new shared memory segment,
with size equal to the round up of size to a multiple of PAGE_SIZE, is created if key has value IPC_PRIVATE or key isn't IPC_PRIVATE, no
shared memory segment is associated to key, and IPC_CREAT is asserted in shmflg (i.e. shmflg&IPC_CREAT isn't zero).
The value shmflg is composed of:
IPC_CREAT to create a new segment. If this flag is not used, then shmget() will find the segment associated with key, check to see if the
user has permission to receive the shmid associated with the segment, and ensure the segment is not marked for destruction.
IPC_EXCL used with IPC_CREAT to ensure failure if the segment exists.
mode_flags (lowest 9 bits)
specifying the permissions granted to the owner, group, and world. Presently, the execute permissions are not used by the sys-
tem.
If a new segment is created, the access permissions from shmflg are copied into the shm_perm member of the shmid_ds structure that defines
the segment. The shmid_ds structure:
struct shmid_ds {
struct ipc_perm shm_perm; /* operation perms */
int shm_segsz; /* size of segment (bytes) */
time_t shm_atime; /* last attach time */
time_t shm_dtime; /* last detach time */
time_t shm_ctime; /* last change time */
unsigned short shm_cpid; /* pid of creator */
unsigned short shm_lpid; /* pid of last operator */
short shm_nattch; /* no. of current attaches */
};
struct ipc_perm {
key_t key;
ushort uid; /* owner euid and egid */
ushort gid;
ushort cuid; /* creator euid and egid */
ushort cgid;
ushort mode; /* lower 9 bits of shmflg */
ushort seq; /* sequence number */
};
Furthermore, while creating, the system call initializes the system shared memory segment data structure shmid_ds as follows:
shm_perm.cuid and shm_perm.uid are set to the effective user-ID of the calling process.
shm_perm.cgid and shm_perm.gid are set to the effective group-ID of the calling process.
The lowest order 9 bits of shm_perm.mode are set to the lowest order 9 bit of shmflg.
shm_segsz is set to the value of size.
shm_lpid, shm_nattch, shm_atime and shm_dtime are set to 0.
shm_ctime is set to the current time.
If the shared memory segment already exists, the access permissions are verified, and a check is made to see if it is marked for destruc-
tion.
SYSTEM CALLS
fork() After a fork() the child inherits the attached shared memory segments.
exec() After an exec() all attached shared memory segments are detached (not destroyed).
exit() Upon exit() all attached shared memory segments are detached (not destroyed).
RETURN VALUE
A valid segment identifier, shmid, is returned on success, -1 on error.
ERRORS
On failure, errno is set to one of the following:
EINVAL is returned if a new segment was to be created and size < SHMMIN or size > SHMMAX, or no new segment was to be created, a seg-
ment with given key existed, but size is greater than the size of that segment.
EEXIST is returned if IPC_CREAT | IPC_EXCL was specified and the segment exists.
EIDRM is returned if the segment is marked as destroyed, or was removed.
ENOSPC is returned if all possible shared memory id's have been taken (SHMMNI) or if allocating a segment of the requested size would
cause the system to exceed the system-wide limit on shared memory (SHMALL).
ENOENT is returned if no segment exists for the given key, and IPC_CREAT was not specified.
EACCES is returned if the user does not have permission to access the shared memory segment.
ENOMEM is returned if no memory could be allocated for segment overhead.
NOTES
IPC_PRIVATE isn't a flag field but a key_t type. If this special value is used for key, the system call ignores everything but the lowest
order 9 bits of shmflg and creates a new shared memory segment (on success).
The followings are limits on shared memory segment resources affecting a shmget call:
SHMALL System wide maximum of shared memory pages: policy dependent.
SHMMAX Maximum size in bytes for a shared memory segment: implementation dependent (currently 4M).
SHMMIN Minimum size in bytes for a shared memory segment: implementation dependent (currently 1 byte, though PAGE_SIZE is the effective
minimum size).
SHMMNI System wide maximum number of shared memory segments: implementation dependent (currently 4096, was 128 before Linux 2.3.99).
The implementation has no specific limits for the per process maximum number of shared memory segments (SHMSEG).
BUGS
Use of IPC_PRIVATE doesn't inhibit to other processes the access to the allocated shared memory segment.
There is currently no intrinsic way for a process to ensure exclusive access to a shared memory segment. Asserting both IPC_CREAT and
IPC_EXCL in shmflg only ensures (on success) that a new shared memory segment will be created, it doesn't imply exclusive access to the
segment.
CONFORMING TO
SVr4, SVID. SVr4 documents an additional error condition EEXIST. Neither SVr4 nor SVID documents an EIDRM condition.
SEE ALSO ftok(3), ipc(5), shmctl(2), shmat(2), shmdt(2)Linux 0.99.11 1993-11-28 SHMGET(2)