C - Segmentation fault(core dumped)

Question

I am making a Rubik's Cube that can rotate it's rows and columns. My struct is this:

typedef struct
{
int **cell;
}face;

In this part I am allocating memory for cells. (X is user input)

face faceOne; //There are six of this faces
faceOne.cell = (int **)malloc(x * sizeof(int));
for(int i = 0; i < x; i++)
     faceOne.cell[i] = (int *)malloc(x * sizeof(int));

Then I am filling these cells:

for (int i = 0; i <  x; i++)
  for (int j = 0; j < x; j++)
     {
     faceOne.cell[i][j] = 0;
     printf("%d\n", i);
     }

If x is bigger than 3, program crashes with Segmentation fault. How can i solve this?

`faceOne.cell = malloc(x * sizeof(int));` What if `sizeof(int) != sizeof(int *)`? — Some programmer dude, Dec 10 '17 at 11:16

user2736738 · Accepted Answer · 2017-12-10T11:21:45.177

2

faceOne.cell = (int **)malloc(x * sizeof(int));

would be

faceOne.cell = malloc(x * sizeof(int*));

Otherwise you are allocating memory for x int variables and then storing int* variable on it. In case sizeof(int) = sizeof(int*) then it wont be any problem but on the system where this is not the case you have earned yoruself a undefined behavior.

To get rid of all this ** while using malloc the best would be to do this

faceOne.cell = malloc(x * sizeof *faceOne.cell);

Similarly,

faceOne.cell[i] = malloc(x * sizeof *faceOne.cell[i] );

And also don't cast the return value of malloc. This suppresses many warning generated by compiler and is unnecessary.

Apart from all this, have you ever wondered what would happen if malloc did fail and returned NULL? Then you are working on a NULL and dereferencing it(Undefined behavior). Check the return value of malloc, in case it returns NULL handle the error and move on.

edited Dec 10 '17 at 11:21

answered Dec 10 '17 at 11:16

user2736738

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Thanks for explenation, i am still bad at handling memory with pointers and trying to improve this flaw. Can you suggest a source for learning more or practice this subject? – Ayro Dec 10 '17 at 11:27
@MertcanElçi.: Well I can show you a list of good books. https://stackoverflow.com/questions/562303/the-definitive-c-book-guide-and-list Check this. – user2736738 Dec 10 '17 at 11:28
https://stackoverflow.com/questions/15222774/mallocsizeofint-vs-mallocsizeofint-vs-int-mallocsizeofint – EsmaeelE Dec 10 '17 at 12:43
@EsmaeelE.: What you want to say? Casting? Then you can check this https://stackoverflow.com/questions/605845/do-i-cast-the-result-of-malloc – user2736738 Dec 10 '17 at 12:45

score 0 · Answer 2 · answered Dec 10 '17 at 11:18

You want here

faceOne.cell = (int **)malloc(x * sizeof(int));

to allocate x pointers to int, so this should be

faceOne.cell = malloc(x * sizeof(int *));

It seems on your system the size of a pointer (to int) is bigger than the size of an int ...

Also in C don't cast malloc() (see this)

C - Segmentation fault(core dumped)

2 Answers2