Using strncpy on struct - c

Question

Let's say I have this student struct defined:
struct student {
char *name;
};
typedef struct student Student
Now I have the following function:
void add_student(const char *student_name) {
// create new student
Student *new_s;
new_s = malloc(sizeof(Student));
strncpy(new_s->name, student_name, sizeof(new_s->name) - 1)
}
I want to add the student_name to the name of the new student struct. However because const char and char are different I have to use strncpy.
I tried it this way, however I get a segmentation fault, what's wrong?

Answer 1

You are only allocating memory for the structure new_s in this line
new_s = malloc(sizeof(Student));
This includes the variable char* name, which is a pointer to a char. Although, you also need memory to which this pointer will point at.
So, you need to allocate memory for the character pointer name inside the structure.
// create new student
Student *new_s;
new_s = malloc(sizeof(Student));
new_s->name = malloc(100); //assuming you need to store a string of len 100

Answer 2

As Johan Wentholt correctly outlined in his answer, you must allocate memory for both the Student structure and the string its member name points to, but you must return the new structure so the caller can do something with it:
Student *add_student(const char *student_name) {
Student *new_s = malloc(sizeof(Student));
if (new_s) {
new_s->name = strdup(student_name);
}
return new_s;
}
You code invokes undefined behavior because you did not allocate memory for the string, worse even, you left the name member uninitialized (malloc does not initialize the memory it returns).
Furthermore, you should not use strncpy. It is not some safe version of strcpy, it is a very error prone function, whose semantics are poorly understood by most programmers. NEVER use this function. If you see it used, there are chances you are either in front of a bug or there is a better method to replace it.
For completeness, your code:
strncpy(new_s->name, student_name, sizeof(new_s->name) - 1);
Would attempt to copy at most sizeof(char*)-1 characters from student_name into the array pointer to by new_s->name.
If the student_name is longer, the destination will not be null terminated,
If it is shorter, the destination will be padded with null bytes upto the given size.
Here the destination pointer is uninitialized and the size information is bogus anyway: you really want to copy all characters in the string plus the null terminator, which is exactly what strcpy does. But you need to allocate enough memory for that. You could use:
new_s->data = malloc(strlen(student_name) + 1);
strcpy(new_s->data, student_name);
The Posix function strdup() does both operations in one call:
new_s->data = strdup(student_name);

Answer 3

Haris gave a good solution. But like Florian Zwoch said in the comments you can also use strdup like so:
void add_student(const char *student_name)
{
Student *new_s = malloc(sizeof(Student));
new_s->name = strdup(student_name);
}
Keep in mind that you have to free new_s->name and than free new_s.
You should also check the return value of malloc and strdup for NULL value. Since it returns NULL if insufficient memory is available.
As side note, you can shorten up the struct and typedef to one statement like so:
typedef struct student {
char *name;
} Student;