C - Change variables values from a structure in a void* function - c

Question

I have an exercise to do for school in C where I can't change the content of the main function and the type and parameters of the two others functions. I have searched Google already but found no answers my problem that I did understand.
The goal is simply to display the content of the structure with printf but I have an erreur due to not knowing how to return the structure to the main function.
This is the code :
void *create_mage(t_character *perso)
{
if ((perso = malloc(sizeof(t_character))) == NULL)
exit(84);
perso->niveau = 1;
perso->pdv = 100;
perso->mana = 200;
perso->attaque = 40;
perso->defense = 3;
perso->crit_chance = 10;
perso->vitesse = 4;
return (perso);
}
void *create_warrior(t_character *perso)
{
if ((perso = malloc(sizeof(t_character))) == NULL)
exit(84);
perso->niveau = 1;
perso->pdv = 200;
perso->mana = 50;
perso->attaque = 10;
perso->defense = 8;
perso->crit_chance = 10;
perso->vitesse = 3;
return (perso);
}
int main()
{
t_character p_ava;
t_character p_thi;
p_ava = create_mage(&p_ava);
p_thi = create_warrior(&p_thi);
printf("def %d - atk %d - vit %d - crit %d\n", p_ava.defense,
p_thi.attaque, p_ava.vitesse, p_thi.crit_chance);
return (0);
}
This is the error I get when I compile my project :
gcc *.c -Wall -Wextra
create_class.c: In function ‘main’:
create_class.c:48:9: error: incompatible types when assigning to type ‘t_character {aka struct s_character}’ from type ‘void *’
p_ava = create_mage(&p_ava);
^
create_class.c:49:9: error: incompatible types when assigning to type ‘t_character {aka struct s_character}’ from type ‘void *’
p_thi = create_warrior(&p_thi);
^
I do not have a good concept of how structures work and did not found anything concerning returning structures from void* functions so if you have an answer or a link where I could find one to my problem it would be very welcomed. Thank you in advance !

Answer 1

If you cannot change main, you cannot compile your code -- period (see comment by J. Leffler, above). However, if "change" means you "cannot add to", then you are OK. Why?
All you really need to do is to declare a struct (or better simply declare a typedef of the required structure as t_character) outside of main, and then within main remove p_ava = and p_thi = and remove the calls to malloc in each of your functions and you are fine.
Why? When you declare your structures:
t_character p_ava;
t_character p_thi;
You have created storage for a type t_character for each variable. The compiler simply needs to know what that type is. So you define the t_character struct (or typdef) and your compiler will provide adequate storage for the variables, e.g.
typedef struct {
int niveau, pdv, mana, attaque, defense, crit_chance, vitesse;
} t_character;
When you pass the variables to your functions (e.g. create_mage(&p_ava);) you are passing the address of p_ava to your function. There is already adequate storage created for a t_character at that address. So all you need to do in your functions is validate that you have a valid address for your variable passed to the function, and then just fill the member values for the struct.
Putting those pieces together, you could (not 'change', but 'remove') and do the following:
#include <stdio.h>
#include <stdlib.h> /* for exit() */
typedef struct {
int niveau, pdv, mana, attaque, defense, crit_chance, vitesse;
} t_character;
void *create_mage (t_character *perso)
{
if (!perso) {
fprintf (stderr, "create_mage() error: invalid parameter.\n");
exit (84);
}
perso->niveau = 1;
perso->pdv = 100;
perso->mana = 200;
perso->attaque = 40;
perso->defense = 3;
perso->crit_chance = 10;
perso->vitesse = 4;
return (perso);
}
void *create_warrior (t_character * perso)
{
if (!perso) {
fprintf (stderr, "create_warrior() error: invalid parameter.\n");
exit (84);
}
perso->niveau = 1;
perso->pdv = 200;
perso->mana = 50;
perso->attaque = 10;
perso->defense = 8;
perso->crit_chance = 10;
perso->vitesse = 3;
return (perso);
}
int main (void)
{
t_character p_ava;
t_character p_thi;
create_mage (&p_ava);
create_warrior (&p_thi);
printf ("def %d - atk %d - vit %d - crit %d\n", p_ava.defense,
p_thi.attaque, p_ava.vitesse, p_thi.crit_chance);
return 0;
}
note: you can get rid of the fprint statements and the test for the valid pointers as that 'change' was a matter of habit to validate function parameters, but is not necessary to the successful operation of the functions.
Compile
$ gcc -Wall -Wextra -pedantic -std=gnu11 -Ofast -o bin/magewarrior magewarrior.c
Example Use/Output
$ ./bin/magewarrior
def 3 - atk 10 - vit 4 - crit 10
Look things over and let me know if you have any questions. If you cannot get away with remove, you are pretty much up a creek without a paddle...
Your alternative is to make p_ava and p_thi pointers as noted in the comments (replacing the '.' operator with the '->' arrow operator in your printf), then you can allocate with malloc within the functions, but will receive a warning for passing p_ava and p_thi uninitialized, unless the pointers are initialized NULL before calling:
p_ava = create_mage (p_ava);
This would require more change to main than the alternative, e.g.
int main (void)
{
t_character *p_ava = NULL;
t_character *p_thi = NULL;
p_ava = create_mage (p_ava);
p_thi = create_warrior (p_thi);
printf ("def %d - atk %d - vit %d - crit %d\n", p_ava->defense,
p_thi->attaque, p_ava->vitesse, p_thi->crit_chance);
return 0;
}
You will have to do one or the other.