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
– Anthony Teisseire, Commented Oct 19, 2012 at 12:07
It's telling you that ret_str is null. Think about why that might be. — Paul Tomblin
– Paul Tomblin, Commented Oct 19, 2012 at 12:07
So that means that calloc didn't allocate memory. Elem is not NULL in this case... — user1745184
– user1745184, Commented Oct 19, 2012 at 12:11
The root cuase for the error most probably lies in the memory referred by elem. — alk
– alk, Commented Oct 19, 2012 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
– user1745184, Commented Oct 20, 2012 at 15:26

Lajos Arpad · Accepted Answer · 2012-10-19 12:25:09Z

1

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

answered Oct 19, 2012 at 12:25

Lajos Arpad

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2 Comments

alk Over a year ago

If not NULL elem most certainly points to memory not belonging to the process.

user1745184 Over a year ago

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.

Aniket Inge · Accepted Answer · 2012-10-19 12:06:51Z

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, 2012 at 12:06

Aniket Inge

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6 Comments

Paul Tomblin Over a year ago

It's giving the error on write, so it's gotta be ret_str not elem.

askmish Over a year ago

In C, it is preferable not to use explicit typecasting when using malloc/calloc.

Aniket Inge Over a year ago

no its not preferable nor necessary. Also there are certain problems using "malloc"

Aniket Inge Over a year ago

@askmish stackoverflow.com/questions/4993327/… read this to know why its not preferred to typecast malloc.

askmish Over a year ago

I wanted to say that there's no problem in calloc or malloc in OP's code.

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