CODEWITHSUNDEEP
cipcshared-memory
John Demetriou
John Demetriou

Reputation: 4377

Second shmget always returns access denied

I am trying to create two sets of shared memory segments. One to store integers and the other one to store strings. I keep getting an error on the second shmget call when I am creating the shared memory segment for my integers. 

 keyS=5678;
 keyI=6789;
   //create shared memory for titles and categories
  if(stringsID=shmget(keyS,sizeof(char*)*lineCounter*2,IPC_CREAT|0666)<0)
    {
      perror("shmget");
      fprintf(stderr, "Titles Cannot create shared memory  errno %i: %s\n",errno,strerror(errno));

      exit(-1);
    }

  //attach to shared memory
  if ((titlesSH = (char**)shmat(stringsID, NULL, 0)) ==(char**) -1)
    {
      perror("shmat");
      exit(1);
    }

  //titles start at titlesSH[0] while categories start at titlesSH[lineCounter]
  categoriesSH=titlesSH+lineCounter;


  //create shared memory for stock and codes
  if(intsID=shmget(keyI,sizeof(int)*lineCounter*2,IPC_CREAT|0666)<0)
    {
      perror("shmget");
      fprintf(stderr, "Stock Cannot create shared memory  errno %i: %s\n",errno,strerror(errno));

      exit(-1);
    }
  //attach to shared memory
  if((stockSH=(int*)shmat(intsID,NULL,0))==(int*)-1)
    {
      perror("shmat");
      exit(-1);
    }

When I run ipcs in terminal I see that there are no ipcs with the keys that I am specifying in my code but I tried different keys and the result is always the same. Am I doing something wrong?

I tried removing 0666 from privileges when I am calling shmget and it does stop (shmget does not return -1) and let's my copy contents in but when I try to get the contents from it I get a segmentation fault.

If I remove the extra shared memory segment and keep only one everything is working fine. I know there is a workaround to this (store everything as a string in a single larger memory segment) but I also want to know why this occurs

**EDIT the workaround I noted above is currently working I am storing all my information in a single shared memory segment that stores strings and it's size is the sum of the two segments I was trying to create. So shared memory space is not an issue. ipcs only gives me 9 ipcs running so I suppose the limit of ipcs and the limit of space are not the issue in this case

Upvotes: 1

Views: 405

Answers (1)

joop
joop

Reputation: 4503

if(stringsID=shmget(keyS,sizeof(char*)*lineCounter*2,IPC_CREAT|0666)<0)
/* and */
if(intsID=shmget(keyI,sizeof(int)*lineCounter*2,IPC_CREAT|0666)<0)

Should most probably be:

if( (stringsID=shmget(keyS,sizeof(char*)*lineCounter*2,IPC_CREAT|0666)) < 0)
/* and */
if( (intsID=shmget(keyI,sizeof(int)*lineCounter*2,IPC_CREAT|0666)) <0)

To demonstrate,

  1. compile the following program (does your compiler issue any warnings? mine does!)
  2. Run it
  3. do ls -ltr
  4. run it again
  5. do ls -ltr again
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>

int main(void) {

int fd;

if (fd=open("omgwtf", O_RDWR|O_CREAT, 0644) < 0) {
        fprintf(stderr, "WTF!\n" ); }

fprintf(stderr, "fd=%d\n", fd );

return 0;
}

Note: for extra fun, you could add O_EXCL to the flags.

Upvotes: 1

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