C unknown access violation at first index in array

Question

Below I have a method that constructs a permutated string of a given string (str). I don't really know why but sometimes while debugging I receive the following exception:

Unhandled exception at 0x01282665 in test.exe: 0xC0000005: Access violation 
writing     location 0x00000000.

when trying to assign ('u') at index 0 in ret_str (ret_str[l]=elem[0])

unsigned char* getPermStr(long length,unsigned char* strt,unsigned char* elem){
    unsigned char* ret_str;
    long l = 0;
    ret_str = (unsigned char*) calloc(length,sizeof(unsigned char));
    while(l < length){
        if(elem < (strt+length-1)){
            ret_str[l]=elem[0];  // ACCESS VIOLATION HERE
            elem+=1;
        }else{
            ret_str[l]=elem[0];
            elem = strt;
        }
        l+=1; 
    }
    return ret_str;
}

I don't see why the access violation occurs... I'm within the bounds of my ret_str so what is wrong? BTW: The string ret_str is free'd after the function call.

UPDATE: There was no problem with elem. The reason was that I allocated memory while there was no memory left on the heap for dynamic allocation (due of lots of memory leaks) so calloc returned a NULL pointer. That's why the error occured.

If I were you, I'd check if ret_str and elem are not NULL ;) — Anthony Teisseire, Oct 19 '12 at 12:07
It's telling you that ret_str is null. Think about why that might be. — Paul Tomblin, Oct 19 '12 at 12:07
So that means that calloc didn't allocate memory. Elem is not NULL in this case... — user1745184, Oct 19 '12 at 12:11
The root cuase for the error most probably lies in the memory referred by `elem`. — alk, Oct 19 '12 at 16:57
There was no problem with elem. The reason was that I allocated memory while there was no memory left on the heap for dynamic allocation (due of lots of memory leaks) so calloc returned a NULL pointer. That's why the error occured. — user1745184, Oct 20 '12 at 15:26

score 1 · Answer 1 · answered Oct 19 '12 at 12:25

1

You need to check whether elem is null. If it is null your function should return an error code.

answered Oct 19 '12 at 12:25

Lajos Arpad

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If not `NULL` `elem` most certainly points to memory not belonging to the process. – alk Oct 19 '12 at 16:58
There was no problem with elem. The reason was that I allocated memory while there was no memory left on the heap for dynamic allocation (due of lots of memory leaks) so calloc returned a NULL pointer. That's why the error occured. – user1745184 Oct 20 '12 at 15:25

score 0 · Answer 2 · answered Oct 19 '12 at 12:06

0

ret_str = (unsigned char*) calloc(length,sizeof(unsigned char)); Change this line to

ret_str = malloc(length * sizeof(unsigned char));
if(ret_str == NULL){ return "" ;}
//--whatever
while(l < length){
        if(elem < (strt+length-1)){
            ret_str[l]=elem[0];  // ACCESS VIOLATION HERE
            elem+=1;
        }else{
            ret_str[l]=elem[0];
            elem = strt;
        }
        l+=1; 
    }

Also make sure, elem is accessible. Chances are, elem isn't initialised.

answered Oct 19 '12 at 12:06

Aniket Inge

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It's giving the error on write, so it's gotta be ret_str not elem. – Paul Tomblin Oct 19 '12 at 12:08
In C, it is preferable not to use explicit typecasting when using malloc/calloc. – askmish Oct 19 '12 at 12:08
no its not preferable nor necessary. Also there are certain problems using "malloc" – Aniket Inge Oct 19 '12 at 12:09
1

@askmish http://stackoverflow.com/questions/4993327/is-typecast-required-in-malloc read this to know why its not preferred to typecast malloc. – Aniket Inge Oct 19 '12 at 12:15
I wanted to say that there's no problem in calloc or malloc in OP's code. – askmish Oct 19 '12 at 12:18
@askmish its obvious he isn't checking for NULL. Other than that, there seems to be NO problem with the code. You think he is hiding something? – Aniket Inge Oct 19 '12 at 12:20

C unknown access violation at first index in array

2 Answers2