Idiots guide to shared memory
Georg Feil
ghfeil at white.toronto.edu
Fri Dec 22 05:51:59 AEST 1989
In article <3906 at brazos.Rice.edu> C.Elvin at ee.surrey.ac.uk writes:
>Does anybody out there have a few small, simple programs that demonstrate
>the use of the shared memory facilities. I have tried to follow the
>manual pages for shmget and shmat etc etc but am unable to attach a shared
>segment.
Yes, it is a bitch the first time you try it. The hardest part is figuring
out where to put the segment so it doesn't interfere with sbrk() and
malloc(). Here is a code fragment that I use to do things in a
semi-reasonable way.
Note that if you put pointers into shared memory (e.g. as part of a linked
list), you must map in the shared memory at the same place in all programs
involved. The code below will probably not do this properly. The solution
I use is to run the program once using the method below, and print out the
shared memory address obtained. This can then be hard-coded into all
programs involved.
Georg.
---------------------------------------------------------
#include <stdio.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#define SHM_KEY 12345678 /* shared mem segment id, some big number */
#define SHM_SIZE 0x8000 /* shared mem segment size, 32K */
#define MALLOC_SIZE 0x32000 /* amount of room to leave for malloc, 200K */
extern int errno;
int main()
{
int err;
int shmid; /* shared memory ID */
char* shared_mem; /* address of start of shared memory */
struct shmid_ds dummy_m; /* dummy return value from shmctl() */
caddr_t break_addr; /* original position of break */
caddr_t sbrk();
shmid=shmget(SHM_KEY,SHM_SIZE, SHM_R | SHM_W | IPC_CREAT);
if (shmid<0) {
fprintf(stderr,"shmtest: Error %d from shmget\n",errno);
exit(1);
}
break_addr=sbrk(0); /* remember original position of break */
err=(int) sbrk(0x32000); /* add to data area to make room for */
/* malloc free store */
/* **Note: The gap left between the break and the start of shared memory */
/* counts under the "missing" category of the 'pstat -s' memory display, */
/* i.e. it is not available for other processes! */
if (err<0) {
fprintf(stderr,"shmtest: Error %d from sbrk()\n",errno);
exit(1);
}
shared_mem = (char*) shmat(shmid, 0, 0); /* let system choose position */
if ((int) shared_mem==-1) {
fprintf(stderr,"shmtest: Error %d from shmat\n",errno);
exit(1);
}
err= brk(break_addr); /* put break back to original position */
if (err<0) {
fprintf(stderr,"shmtest: Error %d from brk()\n",errno);
exit(1);
}
/*** work with shared mem here, segment starts at shared_mem ***/
shmdt((char*) shared_mem); /* detach memory segment */
/* Remove shared memory segment: this should be omitted if you want the
segment to stick around between runs. */
err=shmctl(shmid,IPC_RMID,&dummy_m);
if (err==-1)
fprintf(stderr,"shmtest: error %d from shmctl()!\n",errno);
}
--
Georg Feil Internet: ghfeil at white.toronto.edu
-or- : georg at sgl.ists.ca
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