Question

I am trying to attach an array of strings to the shared memory in C. I have tried my best to attach the array of strings ( array1 and array 2 to the shared memory).

Here, array1 and array2 are arrays of strings of width 20 characters and size 5 ( How do I specify that in the attachment is also not very clear to me).
Also, a and b are 1-D integer and float arrays respectively, of size 5.

I want to change the state of the array of strings by updating their value at runtime, as I am doing.

#include <stdio.h>
#include <stdlib.h>
#include<sys/shm.h>
#define NUMBER_OF_DATA  5
int main()
{
   int size=(NUMBER_OF_DATA*(sizeof(int)+sizeof(float))) + (2*(20*NUMBER_OF_DATA));
   key_t key;
   key=ftok("/home/android/Desktop/newww.c",4);
   int shmid=shmget(key,size,0777|IPC_CREAT);

   int *a=(int *)shmat(shmid,0,0);
   float *b=(float *)(a+NUMBER_OF_DATA);
   char **array1=(char **)(b+NUMBER_OF_DATA);
   char **array2=(char **)(array1+(20*NUMBER_OF_DATA));
   int i;
   for(i=0;i<5;i++)
   {
       printf("enter value\n");
       scanf("%s",array1[i]);
   }
   shmdt(&shmid);
   shmctl(shmid,IPC_RMID,0);
   return 0;
}

My other process does the following

int shmid=shmget(key,size,0777|IPC_CREAT);


 int *a1=(int *)shmat(shmid,0,0);
  float *b1=(float *)(a1+NUMBER_OF_DATA);
  char **array11=(char **)(b1+NUMBER_OF_DATA);
  char **array22=(char **)((char *)array11+(20*NUMBER_OF_DATA));
  for(i=0;i<NUMBER_OF_DATA;i++)
    {
      a1[i]=aaa[i];
      b1[i]=bbb[i];
      array11[i]=array111[i];
      array22[i]=array2222[i];
    }

where aaa,bbb,array111 and array222 are other arrays from which the values are loaded into the shared memory by this process.
These 2 processes are together not helping me achieve what i wanted.

It would be great if someone could point out the reason and tell me the correct way to attach the array of strings to memory. Thanks.

Answer 1

Let’s use a debugger to find where the error is happening. First compile with debugging turned on, then run it:

$ gcc -g foo.c -o foo
$ gdb foo

GNU gdb 6.3.50-20050815 (Apple version gdb-1822) (Sun Aug  5 03:00:42 UTC 2012)
Copyright 2004 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB.  Type "show warranty" for details.
This GDB was configured as "x86_64-apple-darwin"...Reading symbols for shared libraries .. done

(gdb) run
Starting program: foo 
Reading symbols for shared libraries +............................. done
enter value
12

Program received signal EXC_BAD_ACCESS, Could not access memory.
Reason: KERN_INVALID_ADDRESS at address: 0x0000000000000000
0x00007fff928d3143 in __svfscanf_l ()

The bt command, short for backtrace, will show where the error occurred:

(gdb) bt
#0  0x00007fff928d3143 in __svfscanf_l ()
#1  0x00007fff928d0f6f in scanf ()
#2  0x0000000100000e6b in main () at foo.c:20

Here it’s line 20, calling scanf(). Let’s move up the stack to get into the right frame:

(gdb) up
#1  0x00007fff928d0f6f in scanf ()
(gdb) up
#2  0x0000000100000e6b in main () at foo.c:20
20         scanf("%s",array1[i]);

And now the p command, short for print, to examine values.

(gdb) p array1
$1 = (char **) 0x100034028
(gdb) p i
$2 = 0
(gdb) p array1[i]
$3 = 0x0

Aha! The line scanf("%s", array1[i]) is trying to store a string to the value of array1[i]—0—rather than to its address.

Let’s fix that by changing the line to:

   scanf("%s", &array1[i]);

Now, recompile, and it works:

$ ./foo
enter value
12
enter value
14
enter value
15
enter value
17
enter value
19

However, there’s now a compiler warning on my machine:

foo.c: In function ‘main’:
foo.c:20: warning: format ‘%s’ expects type ‘char *’, but argument 2 has type ‘char **’
foo.c:20: warning: format ‘%s’ expects type ‘char *’, but argument 2 has type ‘char **’

But that’s another question for you to figure out 🙂