Accessing array out of bounds in C/C++

Perquisite : Arrays in C/C++

In high level languages such as Java, there are functions which prevent you from accessing array out of bound by generating a exception such as java.lang.ArrayIndexOutOfBoundsException. But in case of C, there is no such functionality, so programmer need to take care of this situation.

What if programmer accidentally accesses any index of array which is out of bound ?

C don’t provide any specification which deal with problem of accessing invalid index. As per ISO C standard it is called Undefined Behavior.
An undefined behavior (UB) is a result of executing computer code whose behavior is not prescribed by the language specification to which the code can adhere to, for the current state of the program (e.g. memory). This generally happens when the translator of the source code makes certain assumptions, but these assumptions are not satisfied during execution.

Examples of Undefined Behavior while accessing array out of bounds

1. Access non allocated location of memory: The program can access some piece of memory which is owned by it.

   // Program to demonstrate
   // accessing array out of bounds
   #include <stdio.h>
   int main()
   {
       int arr[] = {1,2,3,4,5};
       printf("arr [0] is %d\n", arr[0]);
   
       // arr[10] is out of bound
       printf("arr[10] is %d\n", arr[10]);
       return 0;
   }

   Output :

   arr [0] is 1
   arr[10] is -1786647872
   

   It can be observed here, that arr[10] is accessing a memory location containing a garbage value.

2. Segmentation fault: The program can access some piece of memory which is not owned by it, which can cause crashing of program such as segmentation fault.

   // Program to demonstrate
   // accessing array out of bounds
   #include <stdio.h>
   int main()
   {
       int arr[] = {1,2,3,4,5};
       printf("arr [0] is %d\n",arr[0]);
       printf("arr[10] is %d\n",arr[10]);
   
       // allocation memory to out of bound
       // element
       arr[10] = 11;
       printf("arr[10] is %d\n",arr[10]);
       return 0;
   }

   Output :
   Runtime Error : Segmentation Fault (SIGSEGV)

Important Points:

• Stay inside the bounds of the array in C programming while using arrays to avoid any such errors.
• C++ however offers the std::vector class template, which does not require to perform bounds checking. A vector also has the std::at() member function which can perform bounds-checking.

If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to review-team@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.