Dynamic array in C cause segmentation fault

Question

I have defined a dynamic array this way:

double *n_data ;
int n_data_c = 0, n_cnt = 0;
n_data_c = count_lines_of_file("abnorm");
n_data = (double *)malloc(n_data_c * sizeof(double));

in a loop I calculate distance and do so:

n_cnt++;
n_data[n_cnt] = distance;

but it returns segmentation fault here : n_data[n_cnt] = distance;

I want to know if I'm doing something wrong.

• Some programmer dude: In C you should not cast the result of malloc.
• BLUEPIXY: your source has many errors.

Answer 1

Check what malloc returned, if it returned 0, then it failed. More likely, I think, is your n_cnt is out of bounds. If it's negative, or greater than or equal to n_data_c, then you'll get a segfault.

Answer 2

You are overrunning your array buffer..

Compare n_cnt with n_data_c and only access the array if n_cnt < n_data_c/

n_cnt++;
if (n_cnt < n_data_c)
{
n_data[n_cnt] = distance;
}

• Simol: n_data_c is 7 as my file has 7 lines and n_cnt is 1 at first iteration and then it returns segfault.

Answer 3

n_data_c = count_lines_of_file("abnorm");

this is generating the segmentation fault. Check the value of n_data_c

• Simol: @G one: I checked it, it returns 7 as my file has 7 lines.
• G one: int n_data_c = 7, n_cnt = 0; n_data = (double *)malloc(n_data_c * sizeof(double)); n_cnt++; n_data[n_cnt] = 10; I tried this and its not giving me any seg fault printf("%f", n_data[n_cnt])
• G one: Did you check passing of parameters into the calculatedistance() are they correct??

Answer 4

Just try printing the value of n_data_c before mallocing.

• Simol: I checked it, it returns 7 as my file has 7 lines.