General question regarding allocating memory for an integer in C

Question

I am currently learning about dynamic memory allocation in C and am having difficulty with a particular concept. For using malloc for a particular integer, I need to put a previously declared integer within that space.

The following below is what I am putting in as my code.

void example(int ab)
{
int* intPtr=(int*)malloc(sizeof(int));
*intPtr = &ab;
}

I am not trying to run a program or anything. I just want to see if I have the right idea about some basic memory allocation.

Answer 1

There are few issues, firstly its not necessary to typecast the result of malloc() as it's done implicitly by compiler. Read Do I cast the result of malloc? . This

int* intPtr = malloc(sizeof(int)); /* or even malloc(sizeof(*intPtr)); is better */

is fine.

Secondly, this

*intPtr = &ab; /* *intPtr means value at allocated dynamic address, you should assign the value not address of ab*/

is wrong as *intPtr is the value at dynamic address, compiler should have warned you like

main.c:7:9: warning: incompatible pointer to integer conversion assigning to ' int ' from ' int * '; remove & [-Wint-conversion]

If you had compiled with below flags like

gcc -Wall -Wextra -Werror -Wpedantic test.c

It should be

*intPtr = ab;

Also you are not freeing the dynamically allocated memory, its leads to memory leakage. Free the dynamically allocated memory by calling free() once usage is done.

Side note , if your intention is to do like this

intPtr = &ab; /* Before this statement intPtr was pointing dynamic memory(heap), but now that gets overwritten with stack allocated space */

then you are overwriting dynamic memory with stack created memory ie address of local variable , in that case you are loosing dynamic memory as nothing is pointing to previously allocated dynamic address.