dynamically allocate memory for pointer to array of structure - c

Question

hi i have this structure
typedef struct STUDENT
{
char studName[20];
int timeEnter;
int timeUpdate;
}STUDENT;
and the local pointer to array of structure
STUDENT *studArr[100];
I'm trying to allocate memory for the structure by doing reading in the first line of the file then use it to allocate memory for the structure.
fscanf(fp, "%s", &first);
**studArr = (STUDENT**) malloc(sizeof(STUDENT*)*first);
I got an error saying that no operator "=" matches these operands on the allocation line
why am I gettting the error, what did i do wrong here?
thank in advance

Answer 1

I think you're confusing things, it looks like you're declaring an array of pointers, when all you need is a single pointer. Note that as long as you're indexing properly, a pointer to "one" struct is the same as a pointer to a hundred.
You should probably have:
STUDENT *studArr;
then, once you know how many you need (I'm assuming first is the number of students to allocate room for):
studArr = malloc(first * sizeof *studArr);
Also note that no casting is needed.

Answer 2

If you want to allocate an array of 100 students, you have two choices:
struct STUDENT
{
char studName[20];
int timeEnter;
int timeUpdate;
};
struct STUDENT studArr1[100];
... OR ...
struct STUDENT *studArr2 = (struct STUDENT *)malloc (sizeof (struct STUDENT) * 100);
If you just want a pointer, you can say:
struct STUDENT *p = studArr1;
PS:
I deliberately left the "typedef" stuff out so as not to confuse the basic issue (struct vs. array of struct vs pointer to array of struct).

Answer 3

You have defined an array of pointers to struct, not a pointer to an array of struct. The memory for the array is already allocated, as a global array.
The type of **studArr is STUDENT, and hence is not compatible with the expression to the right of the assignment operator, which is casted to (STUDENT**).
You probably meant:
STUDENT **studArr;
[...]
fscanf(fp, "%s", &first);
studArr = (STUDENT**) malloc(sizeof(STUDENT*)*first);

Answer 4

As you are using STUDENT *studArr[100]; then it means you have allocated the memory for 100 pointers for STUDENT structure type.
so for allocating the memory you should try.
studArr =malloc(sizeof(struct STUDENT)*first);
then you can access all the allocated members as studArr[i] ....so on.

Answer 5

You're not exactly clear on what your overall goal is.
First of all, you want to use %d if you want to read in an integer:
int count;
if (fscanf(fp, "%d", &count) != 1) { // error... }
Next, don't use STUDENT as both the struct name and the typdef. Just leave the struct name out:
typedef struct
{
char studName[20];
int timeEnter;
int timeUpdate;
} student_t;
Now, you're going to want just a pointer to an array of student_ts:
student_t* student_array = NULL;
Finally, allocate that array:
student_array = malloc(count * sizeof(student_t));
Now you can use it, like:
strncpy(student_array[0].studName, "Bobby", 20);
Or get a pointer to an individual student:
student_t* bob = &student_array[1];
bob->timeEnter = 42;