Unknown error in the memory in C
I have a 2D dynamic array. I enter a line of 0's after line which has a biggest number:
void InsertZero(int **a, int pos){
int i, j;
a = (int**)realloc(a, n * sizeof(*a));
a[n-1] = (int*)calloc(n, sizeof(**a));
d = 0;
for(i = n-1; i > pos; i--){
for(j =开发者_StackOverflow社区 0; j < n; j++){
a[i][j] = a[i-1][j];
printf("%d ", a[i][j]);
}
}
for(i = 0; i < n; i++){
a[pos][i] = 0;
}
}
If i make a size of array 3, 5, 7, 9, ... it works correctly. But if a number of lines is 2, 4, 6, ... , it is an access violation error, when i try to print my array:
void Print(void){
int i, j;
for(i = 0; i < (n-d); i++){
for(j = 0; j < n; j++){
printf("%d\t", arr[i][j]);
}
printf("\n");
}
}
code: http://codepad.org/JcUis6W4
Looking at this I cannot make sense of.... Look at comment 1, you have n
set somewhere to realloc
a block of memory which a
is of type int **
- a double pointer, how are you calling this function? Secondly, comment 2, Why did you call calloc
when the realloc
on a double pointer was called previously...? Assume n
has value of 5, then, realloc
is called on double pointer a
, meaning a[0][1]..a[4][1]
, now calloc
is called thus a[4]
has a new block of memory...
void InsertZero(int **a, int pos){ int i, j; /* 1. */ a = (int**)realloc(a, n * sizeof(*a)); /* Bzzzzt....if realloc failed, a gets overwritten! */ /* 2. */ a[n-1] = (int*)calloc(n, sizeof(**a)); /* 3. */ d = 0; /* 4. */ for(i = n-1; i > pos; i--){ for(j = 0; j < n; j++){ a[i][j] = a[i-1][j]; printf("%d ", a[i][j]); } } for(i = 0; i < n; i++){ a[pos][i] = 0; } }
Comment 3, what is d
used for - useless variable?
Comment 4, you are under the presumption that the block of memory has array subscripts [0][0]
to [4][4]
if n
had a value of 5!
Can you clarify all this?
Edit: Looking at it again... it is likely that a
got overwritten when the call to realloc
failed! I recommend this section of code to counteract this
int **tmpA; tmpA = (int**)realloc(a, n * sizeof(*a)); if (tmpA != NULL){ a = tmpA; .... a[n-1] = (int*)calloc(n, sizeof(**a)); for(i = n-1; i > pos; i--){ .... } for(i = 0; i < n; i++){ .... } }
In your function InsertZero
you have a local variable a
. This local variable is initially set with a the address of a pointer to an integer (you probably wanted the address of a pointer to an array of integers, i.e. int ***a
).
When you call realloc
you are assigning your local copy of a
a pointer to a block of memory, however once you have finished with your function it is entirely possible for your local copy of a
to be pointing somewhere different to the rest of your program. You probably wanted to say *a = (int **)realloc(a, n * sizeof(int *));
.
Dangerously, you're using n
which isn't passed to your function. It appears you've made the assumption that n
is going to be 1 bigger than the previous size of the array - otherwise your call to calloc
is superfluous and you are just rotating the array letting the last element drop off as a memory leak.
Let's use a simpler example with no arrays, no dimensions. Let's say you wanted to create a function:
void make_me_a_pointer( int **mynumber ) {
*mynumber = (int *)malloc( sizeof(int) );
**mynumber = 7; /* assign the value 7 to my allocated memory */
}
int main( void ) {
int *demoint;
make_me_a_pointer( &demoint );
printf( "Magic num is %d\n", *demoint );
}
However in your case you merely assigned a = realloc
... and thus never communicated the new address of a
outside your function.
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