(newbie question)
Look at this struct:
struct Atom_data{
char *name;
char symbol;
double weight;
};
I want to write a function that gets a struct pointer as input, in this function with the usage of scanf(), I want to give each member of struct a value, which one of them is string. So far I have written this function:
void data_entry(struct Atom_data* ptr){
printf("Please write the Atom's Name:\n");
ptr =(struct Atom_data *)malloc(sizeof *ptr);
ptr->name = malloc(20);
scanf("%s", ptr->name);
printf("Please write the Atom's symbol:\n");
scanf("%c", &ptr ->symbol);
printf("please write the Atom's weight:\n");
scanf("%lf", &ptr ->weight);
}
for symbol and weight there was no problem (when I mask the codes related to name). But for name there is a crash without Error. these codes inside the function (specially codes related to name) worked perfectly outside of the function. (I tried to test each part of them on the main function)
I learnt to write this line of code ptr =(struct Atom_data *)malloc(sizeof *ptr); from a question about passing values to pointer! but for the line after that which is ptr->name = malloc(20);, I just did it based on some ideas about creating dynamic memory for name, which worked outside of the function! (Any clarification about it would show your great favour).
void data_entry(struct Atom_data* ptr){
printf("Please write the Atom's Name:\n");
ptr =(struct Atom_data *)malloc(sizeof *ptr);
Will do nothing useful for you. You are setting ptr to a dynamically allocated piece of memory, but it won't change the pointer you pass into the function:
struct Atom_data *data = NULL;
data_entry(data);
// data will still be NULL
If the desired behaviour is in fact, to set a pointer from within the function you need to use a "pointer to a pointer" like this:
void data_entry(struct Atom_data **ptr) {
*ptr = malloc(sizeof(** ptr));
}
And then later:
struct Atom_data *data = NULL;
data_entry(&data);
// data will not be NULL (if malloc worked.)
However, this is unecessarily complicated, the common way to do it is like this:
struct Atom_data *data_entry() {
struct Atom_data *ptr = malloc(sizeof(* ptr));
// check for NULL pointer returned by malloc (out of memory)
if (!ptr) return NULL;
// initalisation
ptr->name = NULL;
ptr->symbol = 'a';
ptr->weight = 0;
return ptr;
}
And use it like this:
struct Atom_data *data = data_entry();
Of course, if you want to initialize the struct within the function, you'll need to pass those as well:
struct Atom_data *data_entry(char *name, char symbol, double weight)
It really depends on how you want to initialize the struct.
Note: memory allocated with malloc can/should be "released" (or given back to the OS) with free. Failing to do so, can lead to memory leaks.
Also, be careful with scanf:
Some other resources, that might be of interest to you:
• Difference between pointer and array
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