CODEWITHSUNDEEP
c
charliekelly
charliekelly

Reputation: 464

C program: need help using pointers

The purpose of this program is to assign the names to their corresponding ages using pointers.

#include <stdio.h>
#include <stdlib.h>

/* these arrays are just used to give the parameters to 'insert',
   to create the 'people' array 
*/

#define HOW_MANY 7
char *names[HOW_MANY]= {"Simon", "Suzie", "Alfred", "Chip", "John", "Tim",
              "Harriet"};
int ages[HOW_MANY]= {22, 24, 106, 6, 18, 32, 24};

/* declare your struct for a person here */
typedef struct { 
  char *name;
  int age;
} person;

static void insert(person *people[], char *name, int age, int *nextfreeplace) 
{
  /* creates memory for struct and points the array element to it. */
  people[*nextfreeplace] = malloc(sizeof(person));

  /* put name and age into the next free place in the array parameter here */
  (*people[*nextfreeplace]).name = name;
  (*people[*nextfreeplace]).age = age;

  /* modify nextfreeplace here */
  (*nextfreeplace)++;
}

int main(int argc, char **argv) 
{

  /* declare the people array here */
  person *people[HOW_MANY];
  int nextfreeplace = 0;

  for (int i = 0; i < HOW_MANY; i++) 
  {
    insert (&people, names[i], ages[i], &nextfreeplace);
  }

  /* print the people array here*/
  for (int i =  0; i < HOW_MANY; i++) {
    printf("Name: %s. Age: %d\n", (*people[i]).name, (*people[i]).age);
  }

  /* Releases the memory allocated by malloc */
  for (int i = 0; i < HOW_MANY; i++) {
    free(people[i]);
  }

  return 0;
}

It works perfectly, but when I compile it I get two warnings.

arrays.c: In function ‘main’:
arrays.c:41:13: warning: passing argument 1 of ‘insert’ from incompatible pointer type [-Wincompatible-pointer-types]
     insert (&people, names[i], ages[i], &nextfreeplace);
             ^
arrays.c:19:13: note: expected ‘person ** {aka struct <anonymous> **}’ but argument is of type ‘person * (*)[7] {aka struct <anonymous> * (*)[7]}’
 static void insert(person *people[], char *name, int age, int *nextfreeplace)

I'm new to pointers and C in general and would like some help explaining why I get these warnings and how to get rid of them. Thanks!

Upvotes: 1

Views: 174

Answers (4)

anatolyg
anatolyg

Reputation: 28241

TL; DR

Use people instead of &people.

Long explanation

Here is what the warning messages say:

Your function insert expects a parameter of type person ** (a pointer to pointer to person). Your code sends it a parameter of different type: person * (*)[7], which is a C way for "a pointer to an array of 7 pointers to person".

(you can use the site http://cdecl.org to discover that: enter struct person * (*people)[7] in its field, and it will translate it to English)

If you send your array, and not a pointer to it, to your insert function, the compiler will regard the name people as a "pointer to pointer to person", which in this context is a special case of "array of pointers to person". This process is called "decay" of array to a pointer, and is explained here.

Upvotes: 2

kdopen
kdopen

Reputation: 8215

This is actually a single warning, shown over two lines (the source of the warning at line 41, and the declaration causing the problem at line 19).

You can clear the warning by removing the ampersand from the call to insert, thus

insert(people, names[i], ages[i], &nextfreeplace);

In C, the name of an array is synonymous with its address.

Also, to clear up the <anonymous> tag in the warnings, a common idiom when typedefing structures is the following:

typedef struct <name>
{
    ...
} <name>;

which in your case would be:

typedef struct person { 
  char *name;
  int age;
} person;

Upvotes: 1

TheRealChx101
TheRealChx101

Reputation: 1544

You can also do this:

/* declare your struct for a person here */
typedef struct { 
  char *name;
  int age;
} person, *people_t;

And then

static void insert(people_t people[], char *name, int age, int *nextfreeplace){...}

Then 

  people_t people[HOW_MANY];

  int nextfreeplace = 0;

  for (i = 0; i < HOW_MANY; i++){
    insert (people, names[i], ages[i], &nextfreeplace);
  }

Compiler: Visual Studio 2010

Upvotes: 1

Aleksi Romanov
Aleksi Romanov

Reputation: 1

You get these warning because the "insert" expects pointer-to-pointer-to-person, but gets just a pointer-to-pointer-to-pointer-to-person.

In line 41, get rid of "&" from "&people", like in:

insert (people, names[i], ages[i], &nextfreeplace);

Upvotes: -1

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