help with structs and looping in C - c

Question

beginner in C here. Is this legal here? I keep getting status access violation when i run this code:
struct point {
char *x;
char *y;
}
int main()
{
....
struct point *pt;
for (;;)
{
....
pt = process_message(message);
printf("%s",pt->x);
...
}
}
struct point* process_message(char* message)
{
struct point *pt;
pt = malloc(1*sizeof(struct point))
strncpy(pt->x, message, 4);
return pt;
}
EDIT
Hi guys i made some changes...but still getting the access violation. pls advise!

Answer 1

You need to allocate memory for struct point in process_message.
Something like this:
struct point* process_message(char* message)
{
struct point *pt;
pt = malloc(1*sizeof(struct point));
// ... do some processing ...
return pt;
}
Of course, you should free the pointer at some point afterwards.
Also you should return a struct point in process_message or check your type concordance. If you look carefully, you will see you are returning a struct price* but you are expecting (on the calling side) a struct point *.
UPDATE:
Now that question was updated, you will also need to allocate memory for char *x and char *y if you want to do this:
strncpy(pt->x, message, 4);
I would recommend to read a little bit (or a lot) about memory management and pointers in C++. Pointers are not the only way of dealing with data in C Programming Language.
(NO ERROR CHECKING DONE FOR CODE SIMPLICITY)

Answer 2

Your main function is OK. Your problem is in process_message, where you're using a pointer-to-a-struct but not allocating any memory for it, accessing it, then returning it. Declaring a pointer alone does not create the memory to hold the struct. So you should allocate (use malloc for example) the memory for the struct, so that the memory will "exist" after the end of the function. Then you can return the pointer to it as you're doing, and then you would free it in main when you were done with it.
There are possibly better ways to accomplish the goal here given the simple operation you're doing. One is to keep one "scratch" struct local in your main function, and pass its address to process message:
struct point pt;
process_message(message, &pt);
printf("%s", pt.x);
This will "Reuse" the local memory and not require the alloc/free. Notice here that you don't return anything, you just "fill in" the struct in the process function.
Finally, if you're using a modern version of C, you can actually just return the full structure from the function if you want:
struct point process_message(char* message)
{
struct point pt;
... do some processing ...
return pt;
}
And call it like this:
struct point pt = process_message(message);

Answer 3

Remember to allocate memory in the function process_message
pt = malloc (sizeof (struct point));
/* do processing */
return pt;
also after you have finished working with pt remember to free it free (pt).
EDIT
Also you need to allocate the memory blocks to store the strings to x and y in the function, after allocation of the pt. When you have done working, you need to free the strings first (memory blocks) and then the structure.

Answer 4

Whenever you create pointer, it's just a sizeof(ptr) memory , which points to some data. So you must have some memory where your data is stored.
So either allocate memory in your function ( process_message), or in calling function.
it should go like
struct point *pt = (struct point*)malloc(sizeof(struct point));

Answer 5

As written now, you're allocating the struct point, but that structure contains pointers to strings, and not storage for the strings themselves. You need to allocate space for the strings before you copy into them:
pt = malloc(1*sizeof(struct point));
pt->x = (char *)malloc( strlen(message) + 1);
Don't forget to do the same for pt->y before you use it, and don't forget to separately free the storage allocated for the strings, and then that allocated for the struct.