Calling a function with a pointer does not change value of that pointer?

Question

When I call the "InitAnimation" function, I pass the the address of my object RG. When I assign the "animationName" field of that object, I can successfully print that field. But when I return to main and call the function "ReportAnimation", I am unable to print that value and my program crashes ? How come when I assigned that objects field it is not changed globally but only in the local function ?

I have tried allocating memory for the animationName field as well but that does not work.

struct Frame {
    char* frameName;
    struct Frame* pNext;
};

typedef struct {
    char* animationName;
    struct Frame* frames;
}Animation;


int main(void) {

    char response;

    BOOL RUNNING = TRUE;

    Animation RG;

    InitAnimation(&RG);

    while (RUNNING) {
        printf("MENU\n Enter 1 to ReportAnimation\n");
        scanf("%c", &response);

        switch (response) {
        case '1':InsertFrame(&RG); 
        break;
    }
    }

    return 0;
}

void InitAnimation(Animation* pointer) {

    pointer = (Animation*)malloc(sizeof(Animation));

    char* input;
    input = (char*)malloc(sizeof(input));

    printf("Please enter the Animation name:");
    fgets(input, 32, stdin);

    //pointer->animationName = (char*)malloc(sizeof(char)*10);

    //Setting animation name
    pointer->animationName = input;


    //This print function works
    printf("\nThe name is %s", pointer->animationName);

}

void ReportAnimation(Animation* pointer) {

    //This print function does not work
    printf("Animation name is %s\n", pointer->animationName);

}

I want the initAnimation function to change the field of the Animation struct and I want the reportAnimation function to print out the field, proving its changed

["*Do I cast the result of malloc?*"](https://stackoverflow.com/questions/605845/do-i-cast-the-result-of-malloc) **No.** — Deduplicator, Sep 12 '19 at 23:50
In `InitAnimation`, `pointer` is already pointing to an `Animation` variable from the caller, so the `pointer = malloc(sizeof(Animation));` is superfluous, and wrong, and a memory leak. Also `input = malloc(sizeof(input));` will allocate a pointer sized chunk of memory (probably 4 or 8 bytes long), which is probably not the size you want since you pass the size 32 to `fgets`. — Ian Abbott, Sep 12 '19 at 23:58

ikegami · Accepted Answer · 2019-09-13T00:02:20.803

Changing the value of a function's variable (including those declared as parameters) has no effect on the caller.

void bad1(int i) {
   i = 1;  // No effect on the caller.
}

void bad2(void* p) {
   p = NULL;  // No effect on the caller.
}

If you want to change a variable in the caller, you will need to pass a pointer to it.

void good1(int* i_ptr) {
   *i_ptr = 1;
}

void good2(void** p_ptr) {
   *p_ptr = NULL;
}

So either pass a pointer to a pointer, or pass a pointer to an-already allocated structure. You could use

Animation ani;
Animation_init(&ani, name);
...
Frame* frame = Frame_new(name);
Animation_append_frame(&ani, frame);
...
Animation_destroy(&ani);

or

Animation* ani = Animation_new(name);
...
Frame* frame = Frame_new(name);
Animation_append_frame(ani, frame);
...
Animation_delete(ani);

assuming you had

typedef struct Frame {
    char* name;
    struct Frame* next;
} Frame;

typedef struct {
    char* name;
    Frame* first_frame;
    Frame* last_frame;
} Animation;

void Animation_init(Animation* self, const char* name) {
    self->name = strdup(name);
    self->first_frame = NULL;
    self->last_frame = NULL;
}

void Animation_destroy(Animation* self) {
    Frame* head = self->first_frame;
    while (head != NULL) {
       Frame* next = head->next;
       Frame_delete(head);
       head = next;
    }

    free(self->name);
}

Animation* Animation_new(const char* name) {
    Animation* self = malloc(sizeof(Animation));
    Animation_init(self, name);
    return self;
}

void Animation_delete(Animation* self) {
    Animation_destroy(self);
    free(self);
}

void Animation_append_frame(Animation* self, Frame* frame) {
   if (self->last_frame == NULL) {
      self->first_frame = frame;
   } else {
      self->last_frame->next = frame;
   }

   while (frame->next != NULL)
      frame = frame->next;

   self->last_frame = frame;
}

Calling a function with a pointer does not change value of that pointer?

1 Answers1