Write pointer to file in C

Question

I have a structure:

typedef struct student {
  char *name;
  char *surname;
  int age;
} Student;

I need to write the structure into a binary file.
Here is my attempt :

Student *s = malloc(sizeof(*s));

I fill my structure with data and then I write the struct into the file with :

fwrite(s, sizeof(*s), 1, fp);

In my file, name and surname don't exist. Instead they have their respective address of their char * pointer.
How can I write a char * into a file instead of the address of the pointer ?

Answer 1

You need to dereference your pointer and write the parts of the struct. (You should not fwrite a struct directly, but rather encode its parts and write those:

Student *s = malloc(sizeof(*s));
s->name = "Jon";
s->surname = "Skeet";
s->age = 34;

// ....

fwrite(s->name, sizeof(char), strlen(s->name) + 1, fp);
fwrite(s->surname, sizeof(char), strlen(s->surname) + 1, fp);

//This one is a bit dangerous, but you get the idea.
fwrite(&(s->age), sizeof(s->age), 1, fp); 

Answer 2

You will have to do some extra work to write out the data pointed to by the structure elements - probably just write it out element by element.

Alternatively change your structure to something like this:

typedef struct
{
    char name[MAX_NAME_LEN];
    char surname[MAX_SURNAME_LEN];
    int age;
} Student;