Difference between dangling pointer and memory leak

Question

I don't understand the difference between a dangling pointer and a memory leak. How are these two terms related?

Answer 1

A dangling pointer points to memory that has already been freed. The storage is no longer allocated. Trying to access it might cause a Segmentation fault.

Common way to end up with a dangling pointer:

char *func() {    char str[10];    strcpy(str, "Hello!");    return str;  } //returned pointer points to str which has gone out of scope.  

You are returning an address which was a local variable, which would have gone out of scope by the time control was returned to the calling function. (Undefined behaviour)

Another common dangling pointer example is an access of a memory location via pointer, after free has been explicitly called on that memory.

int *c = malloc(sizeof(int)); free(c); *c = 3; //writing to freed location! 

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A memory leak is memory which hasn't been freed, there is no way to access (or free it) now, as there are no ways to get to it anymore. (E.g. a pointer which was the only reference to a memory location dynamically allocated (and not freed) which points somewhere else now.)

void func(){     char *ch = malloc(10); } //ch not valid outside, no way to access malloc-ed memory 

Char-ptr ch is a local variable that goes out of scope at the end of the function, leaking the dynamically allocated 10 bytes.

Answer 2

You can think of these as the opposites of one another.

When you free an area of memory, but still keep a pointer to it, that pointer is dangling:

char *c = malloc(16); free(c); c[1] = 'a'; //invalid access through dangling pointer! 

When you lose the pointer, but keep the memory allocated, you have a memory leak:

void myfunc() {     char *c = malloc(16); } //after myfunc returns, the the memory pointed to by c is not freed: leak!