How to Handle Run-time memory allocation bad pointer error in C Programs (not C++)?

Question

if is it possible run-time bad pointer exception in C language?.

I am using below compiler.

Note : Microsoft Visual C++ Compiler

Sample Programs Below.

#include <stdio.h>
#include <stdlib.h>
#include <Windows.h>
#include <Windef.h>

typedef struct tagTest{
int i;
char *str;
}   Test, 
    FAR *LPTest, 
    FAR **LLPTEST;

 extern LLPTEST m;

 int main()
 {

  int i;

  LLPTEST m = NULL;

  m = (LLPTEST)malloc(sizeof(LLPTEST));

  // Here We Check m[0] memory allocated or not ?
  // example: if(m[0]), or if(m[0]->i) any other idea. here m[0] is CXX0030 error expression cannot be evaluated.

  /* allocate memory */
  for(i=0; i<10; i++)
  {
     m[i] = (LPTest) malloc(sizeof(Test));
     m[i]->i = i;
     m[i]->str = (char*) malloc(10*sizeof(char));
     m[i]->str = "Test";
 }

 return 0;
}

C doesn't have exceptions. So you'll need to make it clear what you are referring to. — David Heffernan, Aug 12 '14 at 07:16
You need to check if `malloc` returns NULL, if that's what you mean. — Juri Robl, Aug 12 '14 at 07:20
Whether your `malloc` worked is the least of the problems with this code. You're allocating space for a (single) pointer-to-pointer, storing that in a pointer-to-pointer, then trying to iterate through 10 times that memory... — etheranger, Aug 12 '14 at 07:21
I do not used 10*sizeof(LPTEST). becuase it is my sample example. my source code i do not know how many number of malloc used? — Vijay Kumbhani, Aug 12 '14 at 07:24
@VijayKumbhani That comment does not make any sense. Write in intelligible English or do not write at all. — The Paramagnetic Croissant, Aug 12 '14 at 07:38
Also, [you must not cast the return value of `malloc()`](http://stackoverflow.com/questions/605845/do-i-cast-the-result-of-malloc/605858#605858). — The Paramagnetic Croissant, Aug 12 '14 at 07:38
MSVC has SEH. http://msdn.microsoft.com/en-us/library/swezty51.aspx — BLUEPIXY, Aug 12 '14 at 11:28

score 3 · Accepted Answer · answered Aug 12 '14 at 07:16

3

No. C doesn't support exceptions, so there's nothing to catch. What you're seeing isn't a "bad pointer exception," it's a memory access error -- there is no way to recover from it.

answered Aug 12 '14 at 07:16

Patrick Collins

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score 3 · Answer 2 · edited May 23 '17 at 10:26

You have several problems in your code. Here's a list of some of them:

Don't cast the return of malloc
For m you allocate sizeof(LLPTEST) bytes, but you should really allocate sizeof(LPTest)
Continuing the previous point, you only allocate one pointer, so only m[0] is valid, all other indexes will cause you to write out of bounds. You should do e.g.
```
m = malloc(sizeof(LPTest) * 10);
```
This point is the cause of your problems, as it causes undefined behavior
You allocate memory for m[i]->str, but then you overwrite that pointer with a pointer to a string literal, thereby loosing the pointer to the allocated memory (i.e. you have a memory leak)
Continuing the previous point, because m[i]->str now points to a string literal, and not something you allocated yourself, you can not free this pointer
No error checking, remember that malloc can fail

If you don't know how many items you need to allocate for m beforehand, you can use realloc to reallocate with a larger size.

score 0 · Answer 3 · answered Aug 12 '14 at 12:37

Some exceptions can catch MSVC is to extend the syntax.

#include <windows.h>
#include <stdio.h>

typedef struct tagTest{
    int i;
    char *str;
} Test;

int main(){
    Test *m;
    //m = malloc(sizeof(Test));//it can be avoided by examining whether NULL simply.
    m = NULL;//malloc is unable to allocate memory
    __try{
        m->i = 0;//exception occurs
        m->str = "Test";
    }
    __except(EXCEPTION_EXECUTE_HANDLER){
        if(EXCEPTION_ACCESS_VIOLATION==GetExceptionCode())
            puts("ACCESS VIOLATION");
        puts("EXCEPTION OCCURRED");
    }
}

How to Handle Run-time memory allocation bad pointer error in C Programs (not C++)?

3 Answers3