Casting Const Void to String Errors-CodePudding

I have this line of code here where I am trying to type cast two void arguments as strings and then use the strcmp function to compare the two:

int compareFirstName(const void * p1, const void * p2)
{
  const char **str1, **str2;
  str1 = (char **) &p1;
  str2 = (char **) &p2;
  return strcmp(*str1, *str2);
}

However, I keep getting the error,

assignment from incompatible pointer type [-Werror]
   str1 = (char **) &p1;
        ^

I am pretty sure I am casting right, so I do not know what the problem seems to be. Before, I had a segmentation fault error but resolved it and am now stuck here.

EDIT: This is the struct I am using to compare the first names, IDS, and last names. Turns out I have to convert the void * into Student *, which I am a bit confused about

typedef struct
{
  int ID;
  char firstname[NAME_LENGTH] ;
  char lastname[NAME_LENGTH] ;
} Student;

CodePudding user response：

You are not trying to cast as "strings" only to pointer to pointer to char which has nothing in common with the C strings.

You also do not want to get the reference to the local parameters p1 and p2

You want:

int compareFirstName(const void * p1, const void * p2)
{
  const char *str1, *str2;
  str1 = p1;
  str2 = p2;
  return strcmp(str1, str2);
}

or just enough

int compareFirstName(const void * p1, const void * p2)
{
  return strcmp(p1, p2);
}

CodePudding user response：

Considering the question is slightly different than what it was before, I have decided to post a new answer:

int compareFirstName(const void * p1, const void * p2)
{
    const Student *s1, *s2;
    s1 = p1;
    s1 = p2;
    return strcmp(s1->firstname, s2->firstname);
}

This is the way to cast the void * arguments to the structs that you typedef'ed as Student. Then because p1 and p2 are pointers to structs, you need to use -> to access the members.