C - Malloc or calloc…and how?

Question

i have a text file where the first number defines the size of the arrays. I know that calloc or malloc can reserve memory, but how?

this code:

typedef struct alpha {
    int* size;
    char name;
    int  tot;
    char line[60];
} ALPHA;

fgets(line, 60, fp);
tot = atoi(line);
size = (int*)calloc(name, sizeof(int);

Imagine that in the first line of text is the number 10, with this code the size of name will be 10? like name[10]???

Answer 1

Use one of the following:

ALPHA* alphas1 = calloc(tot, sizeof(ALPHA));
// or
ALPHA* alphas2 = malloc(tot * sizeof(ALPHA));

Those allocate memory for tot of your ALPHA structures.

Answer 2

There's several problems with your code.

First, you seem to be confusing a declaration of what a data type is with actually having a variable. You declare a struct , and from then on it's a data type, much like int or double . Before you assign anything to one, you need to have one. You can get one either by defining one in the function ( ALPHA a; ) or by allocating memory for one with malloc() or calloc() .

To use calloc() , you have two arguments, one being how many whatevers you want, and one being the size of a whatever. For malloc() , you multiply the two. The other difference is that malloc() returns memory with whatever used to be in it, while calloc() initializes everything to zero. (That's zero-length string or integral zeros according to the Standard. Other values are not guaranteed, but with most modern systems you'll get the equivalent of a zero.) These functions return a pointer to the memory allocated.

You seem to want tot int s, so (using calloc() ), the correct statement is something like int * a = calloc(tot, sizeof(int)); , or int * a = calloc(tot, sizeof(*a)); . No cast is required in C (it is required in C++, but you usually wouldn't use malloc() or calloc() in C++), and the only thing it can do is cover up a possible mistake (leaving out #include <stdlib.h> to be specific).

Once you have that, you can refer to the int s as something like a[3] .

Putting the result in a field of an ALPHA is doable, but you really do need an ALPHA , so something like

ALPHA a;
a.size = calloc(tot, sizeof(*a));

will work. You would therefore refer to it as a.size[3] , for example.

Also, I don't see what name is doing. It's one character, which is not enough for any non-empty string, and I don't know why you've got it in the calloc() call. You might want name to be a dynamically allocated string, with size being its size. In that case, you'd need to change the lines in the declaration of ALPHA to be

int size;
char * name;

and the code might be be

ALPHA a;
fgets(line, 60, fp);
a.size = atoi(line);
a.name = calloc(a.size, sizeof(*a.name));

Once you've done that, after entering 10, you can refer to a.name[0] through a.name[9] , and that's your ten characters. a.name[10] would be one past the end. Note that you can only put a nine-character string in a ten-character array, since you need to have room for the null terminator (the '\\0' which is the last character of any C-style string). If you want to be able to enter the specified number of characters, you'd want to add 1 to a.size after putting the user-entered number in.

Answer 3

You might just want to

ALPHA alphas[tot];

Answer 4

ALPHA *alphas1 = calloc(tot, sizeof(*alphas1));
ALPHA *alphas2 = malloc(tot * sizeof(*alphas2));

is a slightly better version of:

ALPHA* alphas1 = calloc(tot, sizeof(ALPHA));
ALPHA* alphas2 = malloc(tot * sizeof(ALPHA));

Now you can access alphas1 and so on, "array-like":

alphas1[0], alphas1[1], alphas1[2], ..., alphas1[tot] 
alphas2[0], alphas2[1], alphas2[2], ..., alphas2[tot] 

alphas1[...] is of type ALPHA
alphas2[...] is of type ALPHA .

PS: To make your code more reliable, don't forget to check if a malloc / calloc failed. To this verify that (in your case) check if alphas1 or alphas2 are different from NULL . In case they are NULL , you won't be able to access them as presented, and you should design a mechanism to recover from this, or simply exit the program (+error message).