在结构中的Sturts数组中初始化字符串

Question

I have a struct gradebook with(among other things) an array of student structs that has two string fields

#define MAX_NAME_LEN 50
#define MAX_EMAIL_LEN 80
#define MAX_NUMBER_OF_STUDENTS 200
#define MAX_NUMBER_OF_ASSIGNMENTS 100

typedef struct students {
   char *name;
   char *email;
} Students;

typedef struct gradebook {
   int number_of_students;
   Students students[MAX_NUMBER_OF_STUDENTS];
   int number_of_assignments;
   char assignments[MAX_NUMBER_OF_ASSIGNMENTS][(MAX_NAME_LEN + 1)];
   int scores[MAX_NUMBER_OF_STUDENTS][MAX_NUMBER_OF_ASSIGNMENTS];
} Gradebook;

I have an initialization function

 int init_gradebook(Gradebook *book) {
   int row, col, ndx, count;

   book->number_of_students = 0;
   count += book->number_of_students;

   for(ndx = 0; ndx < MAX_NUMBER_OF_STUDENTS; ndx++) {
      book->students[ndx].name = 0;
      book->students[ndx].email = 0;
   }

   book->number_of_assignments = 0;
   count += book->number_of_assignments;

   for(row = 0; row < MAX_NUMBER_OF_ASSIGNMENTS; row++) {
      for(col = 0; col < (MAX_NAME_LEN + 1); col++) {
         book->assignments[row][col] = 0;
         count += book->assignments[row][col];
      }
   }

   for(row = 0; row < MAX_NUMBER_OF_STUDENTS; row++) {
      for(col = 0; col < MAX_NUMBER_OF_ASSIGNMENTS; col++) {
         book->scores[row][col] = 0;
         count += book->scores[row][col];
      }
   }

   if (count == 0) {
      return 1;
   } else {
      return 0;
   }

}

and I need to then insert, into those two string fields, the passed in strings, with my add_student function.

int add_student(Gradebook *book, char *nom, char *mail) {
   int ndx, count;

   if (book->number_of_students == 0) {
      book->students[(book->number_of_students)].name = malloc(sizeof(51));
      book->students[(book->number_of_students)].email = malloc(sizeof(81));
      strcpy(book->students[(book->number_of_students)].name, nom);
      strcpy(book->students[(book->number_of_students)].email, mail);
      book->number_of_students++;
   } else {
   for (ndx = 0; ndx < book->number_of_students; ndx++) {
      book->students[(book->number_of_students)].name = malloc(sizeof(51));
      book->students[(book->number_of_students)].email = malloc(sizeof(81));
      strcpy(book->students[(book->number_of_students)].name, nom);
      strcpy(book->students[(book->number_of_students)].email, mail);
      book->number_of_students++;
      }
   }

   return 1;
}

My code compiles, but when I run it with the main function, I get a seg fault. The add_student function is what I am ultimately trying to do (copy the given string into book->student[ndx].name) If you need to see the main file or the gradebook.h file, let me know.

Edit: Thanks to all of you, this issue has been solved. The main problem, as abginfo pointed out, was my If Else + the For loop inside of it. But now I have other problems further along in my program. Haha, Thank You.

Answer 1

From what portion of your code I can see, I'm going to make the assumption that the init_gradebook function takes a non allocated reference to gradebook and attempts to initialize it. In this case the gradebook reference you have has no memory allocated to it just yet. Try using the malloc() function to assign the required memory to your gradebook reference before attempting to initialize the rest of its variables.

 gb = (Gradebook*)malloc(sizeof(*Gradebook));

I've changed the variable name to avoid any confusion.

Answer 2

To supplement varevarao's answer, you should allocate everything explicitly as a matter of habit instead of relying on segfaults to tell you something's not allocated. (Not that you necessarily do!) Messing with unallocated memory is undefined behavior, so in some cases this code does not trigger an error -

int main (void) {
  Gradebook mybook;
  init_gradebook(&mybook);
  printf("there are %i students\n", mybook.number_of_students);
  add_student(&mybook, "blerf", "blerf@gmail.com");

  printf("now there are %i students\n", mybook.number_of_students);
  printf("%s has an email address of %s\n", mybook.students[0].name, mybook.students[0].email);
  return 0;
}

returned (on my machine)

there are 0 students
now there are 1 students
blerf has an email address of blerf@gmail.com