Allocate struct from function in C - c

Question

I'm having issues writing a function that allocates a struct in C. Ideally, I want to have the function fill the fields of the struct with parameters passed into it.
I have defined the struct in my header file like so:
typedef struct {
char name[NAME_SIZE]; //Employee name
int birthyear; //Employee birthyear
int startyear; //Employee start year
} Employee;
And this is what I have for my function currently:
void make_employee(char _name, int birth_year, int start_year) {
Employee _name = {_name,birth_year,start_year}; //allocates struct with name
} /* end make_employee function */
Any advice on how to accomplish this?

Answer 1

The problem with your current code is that the struct your creating is created on the stack and will be cleaned up as soon as the function returns.
struct foo
{
int a;
int b;
};
struct foo* create_foo( int a, int b )
{
struct foo* newFoo = (struct foo*)malloc( sizeof( struct foo ) );
if( newFoo )
{
newFoo->a = a;
newFoo->b = b;
}
return newFoo;
}
This will get you a heap allocated object. Of course, you'll need a function to free that memory or this is a memory leak.
void destroy_foo( struct foo* obj )
{
if( obj )
free( obj );
}
void print_foo( struct foo* obj )
{
if( obj )
{
printf("foo->a = %d\n",obj->a);
printf("foo->b = %d\n",obj->b);
}
}
(btw, this style gets you part of the way toward an "object oriented" C. Add some function pointers to the struct (to get polymorphic behavior) and you have something interesting; though I'd argue for C++ at that point.)

Answer 2

You have to return a pointer allocated via malloc:
Employee* new_employee(char *_name, int birth_year, int start_year) {
struct Employee* ret = (struct Employee*)malloc(sizeof(struct Employee));
ret->name = _name;
ret->birth_year = birth_year;
ret->start_year = start_year;
return ret;
}
two more things: (1) you should make the struct definition of name a char* instead of char[NAME_SIZE]. Allocating a char array makes the struct much bigger and less flexible. All you really need is a char* anyway. And (2) change the function definition to char*.

Answer 3

Why does the make Employee return void? You need to return the Employee from the make_employee function!
Are you having trouble with the compiler complaining about the x = {a,...} syntax? Write it the long way then: Emp e; e.field1 = a; ...
Are you having weird overwriting / bogus numbers problems? If you allocate a struct in the function it will become invalid (and prone to being overwriten) as soon as the function returns! To go around this you either have to:
Return a copy of the struct (this is OK for small structs):
Employee make_emp(int a){
Emp emp; //Allocate temporary struct
emp.filed1 = a; //Initialize fields;
return emp; // Return a copy
}
Allocate the struct in the heap instead and deal with it through references (ie.: pointers) instead:
Employee* make_emp(int a){
Emp* emp = malloc(sizeof(Emp)); //Allocate the struct on the heap
//And get a reference to it
emp->filed1 = a; //Initialize it
return emp; //Return the reference
}
Don't forget to free() the Employee after you are done with it in this case!

Answer 4

Employee * make_employee(char *_name, int birth_year, int start_year)
{
Employee *employee;
if (employee = (struct Employee *)memalloc(sizeof(Employee)) == NULL)
{
return NULL;
}
else
{
strcpy(&(employee->name), _name);
employee->birthyear = birth_year;
employee->startyear = start_year;
return employee;
}
}