double linked list in c send data from function

Question

This is my code:-

typedef struct Frame
{
    char* name;
    unsigned int duration;
    char* path;  // may need to scan (with fgets)
}frame_t;

typedef struct Link
{
    frame_t* frame;
    struct Link* next;
}link_t;

void addNewFrame(void)
{
  link_t* newLink = (link_t**)malloc(sizeof(link_t*));
  printf("                *** Creating new frame ***\n\n");
  printf("Please insert frame path:\n");

  //  newLink->frame->name = (char*)malloc(sizeof(char) * MAX_LEN);

  fgets(newLink->frame->name, MAX_LEN,stdin);
  printf("%s", newLink->frame->name);
}

I just need to add a data to name variable in the "Frame" link list, please help me by reviewing this code.

Not a clue what you are asking here. What does "without succeed" mean? Is there a error? — OldProgrammer, Jun 14 '17 at 23:38

Zakir · Accepted Answer · 2017-06-14T23:48:58.447

1

You want to allocate the right types here:-

link_t* newLink = malloc(sizeof(link_t));  //Pointer to Link st
if(newLink){
    newLink->frame = malloc(sizeof(frame_t)); //Pointer to frame member
    if(newLink->frame){
        newLink->frame->name = malloc(sizeof(char) * MAX_LEN); //Pointer to name member
        if(newLink->frame->name){
            //Rest of your code
        }
    }
}

EDIT:-
1. As pointed out in comments there's no need to cast the pointer returned by malloc()
2. Another very imp point you may want to check validity of the pointers before de-referencing them

edited Jun 14 '17 at 23:48

answered Jun 14 '17 at 23:43

Zakir

2,222
21
31

1

[Do I cast the result of malloc?](https://stackoverflow.com/questions/605845/do-i-cast-the-result-of-malloc) – Stargateur Jun 14 '17 at 23:44
I guess not needed - but did not want to differ too much from OP's original code – Zakir Jun 14 '17 at 23:45

geckos · Answer 2 · 2017-06-15T00:06:21.640

First. You don't need to cast void * so (link_t **)malloc(... can be only malloc(....

Second. You allocated enough memory for a pointer not for a struct. I think you mean malloc(sizeof(link_t)) or even better malloc(sizeof(*newLink))

Third newLink->frame is a pointer so you need to allocate data for it too, newLink->frame = malloc(sizeof(frame_t))

Fourth newLink->frame->name is still a pointer so you need to allocate data for it too. newLink->frame->name = malloc(MAX_LEN)

The confusion that you are doing is pretty common. When you say type *something you are allocating a pointer to type in the stack. A pointer needs to point to somewhere else or NULL, or bad things happen. This applies to structures too. If your structure has a pointer member you need to point it to somewhere else. The somewhere else is where the real "type" object resides.

This also applies to arrays. If you say 'int foo[10]' you are allocating ten integers in the stack. If you say int *foo[10] you are allocating ten pointers in the stack. If you say int **foo you are allocating one pointer in the stack. Again all pointers need to be initialized, I mean, they need to point to some valid object, allocated somewhere else in the memory.

I hope this helps.

Some other points.

Always check pointer coming from malloc, if allocation failed you'll receive NULL. Dereferencing NULL will break your program.
Always initialize memory coming from malloc, fill it with zeros or something.
Don't use _t suffix, is POSIX reserved.
I didn't test any of this.

double linked list in c send data from function

2 Answers2