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Full Discussion: Loading Data in shared memory (C++)
Top Forums Programming Loading Data in shared memory (C++) Post 302467561 by Mercfh on Friday 29th of October 2010 09:43:45 PM
Old 10-29-2010
Loading Data in shared memory (C++)
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
Code:
John Blakeman

111223333

560 Southbrook Dr. Lexington,  KY 40522

8591112222

Paul S Blair

111223344

3197 Trinity Rd. Lexington,  KY 40533

etc....

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)
Code:
#define KEY ((key_t)(10101)) /* Change to Last 5 digits of SSN */
#define SEGSIZE sizeof(struct StudentInfo)

#define NUM_SEMAPHS 5
#define SEMA_KEY ((key_t) (1010)) /* Last 4 digits of SSN */

struct StudentInfo{
char fName[20];
char lName[20];
char telNumber[15];
char whoModified[10];
};

void Wait(int semaph, int n);
void Signal(int semaph, int n);
int GetSemaphs(key_t k, int n);

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...

Code:
#include <stdio.h>
#include <string.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <sys/sem.h>
#include "header.h"

main()
{   
    int i,id;
    struct StudentInfo *infoptr;
    int sema_set;

    id = shmget(KEY, SEGSIZE,IPC_CREAT|0666); /* get shared memory to store data */
    if (id < 0){
        perror("create: shmget failed");
        exit(1);
}

    infoptr=(struct StudentInfo *)shmat(id,0,0); /* attach the shared memory segment to the process's address space */

    if(infoptr <= (struct StudentInfo *) (0)) {
        perror("create: semget failed");
        exit(2);
    }

/* store data in shared memory segment */
//here's where I need help

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.
Last edited by Mercfh; 10-29-2010 at 10:50 PM..
 

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LEARN ABOUT LINUX

shm.h

<sys/shm.h>(P)						     POSIX Programmer's Manual						    <sys/shm.h>(P)

NAME

       sys/shm.h - XSI shared memory facility

SYNOPSIS

       #include <sys/shm.h>

DESCRIPTION

       The <sys/shm.h> header shall define the following symbolic constants:

       SHM_RDONLY
	      Attach read-only (else read-write).

       SHM_RND
	      Round attach address to SHMLBA.

       The <sys/shm.h> header shall define the following symbolic value:

       SHMLBA Segment low boundary address multiple.

       The following data types shall be defined through typedef:

       shmatt_t
	      Unsigned	integer  used  for  the  number of current attaches that must be able to store values at least as large as a type unsigned
	      short.

       The shmid_ds structure shall contain the following members:

	      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 operation.
	      pid_t	      shm_cpid	 Process ID of creator.
	      shmatt_t	      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
	       ().

       The pid_t, time_t, key_t, and size_t types shall be defined as described in <sys/types.h> .

       The following shall be declared as functions and may also be defined as macros. Function prototypes shall be provided.

	      void *shmat(int, const void *, int);
	      int   shmctl(int, int, struct shmid_ds *);
	      int   shmdt(const void *);
	      int   shmget(key_t, size_t, int);

       In addition, all of the symbols from <sys/ipc.h> shall be defined when this header is included.

       The following sections are informative.

APPLICATION USAGE

       None.

RATIONALE

       None.

FUTURE DIRECTIONS

       None.

SEE ALSO

       <sys/ipc.h> , <sys/types.h> , the System Interfaces volume of IEEE Std 1003.1-2001, shmat(), shmctl(), shmdt(), shmget()

COPYRIGHT

       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for Information Technol-
       ogy -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
       Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and  the  original  IEEE
       and  The  Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained
       online at http://www.opengroup.org/unix/online.html .

IEEE
/The Open Group						       2003							    <sys/shm.h>(P)
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