是否有任何 function 将字符串传递给 C 中的结构？

Question

(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).

Answer 1

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 :

• Disadvantages of scanf

• Reading a string with scanf

Some other resources, that might be of interest to you:

• Difference between pointer and array

• Difference between char * and char []

• Difference between "." and "->" .