Question

So I’m trying add malloc to a phonebook application that I created, but since I’m kind of new to C I’m not sure if what I’m doing is correct. I’ve run into a small problem, but I’ve read through the beginner book that I have, and it doesn’t go though as much detail as I would like, I can’t tell by searching Google if I’m just completely wrong in how I set up the malloc or if there is something else I missed.

Basically what I’ve got are 4 arrays in my structure, First_Name, Last_name,home,cell. Each one of these have 2 functions, a function that gets the info from the user and a function that prints and adds the user info to the phonebook. What I’ve got right now is a small snipit of the original code that only adds the first name to the phonebook(so it’s not the entire code) and in each function that gets the user input, I want to add the malloc function. Right now I’ve only got the first name and the first malloc set up, but the issue I have is that when I go to check the phonebook to see if the name was entered successfully, the program quits. If I take out the malloc, it works successfully.

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

#define BUFFER 50
    //Structure for contacts
typedef struct friends_contact {

    char *First_Name;
    char *Last_Name;
    char *home;
    char *cell;
} fr;

void menu(fr * friends, int *counter, int user_entry, int i);
void setFirst(fr *, int *, int i);
char getFirst(fr *, int i);
void add_contact(fr * friends, int *counter, int i);
void print_contact(fr * friends, int *counter, int i);

int main()
{

    int user_entry = 0;
    fr *friends;
    int counter = 0;
    int i = 0;
    menu(friends, &counter, user_entry, i);
    getch();
    return 0;
}

//Menu function
void menu(fr * friends, int *counter, int user_entry, int i)
{
    do {
        int result;

        printf("\nPhone Book Application\n");
        printf
            ("1) Add friend\n2) Delete friend\n3) Show a friend\n4)Showphonebook\n5)Exit\n");
        scanf("%d", &user_entry);

        if (user_entry == 1) {
            add_contact(friends, counter, i);
        }
        if (user_entry == 2) {

        }
        if (user_entry == 3) {

        }
        if (user_entry == 4) {
            print_contact(friends, counter, i);
        }
    } while (user_entry != 5);
}

void setFirst(fr * friends, int *counter, int i)
{
    // THE MALLOC FUNCTION!
    friends = (fr *) malloc(BUFFER * sizeof(fr));
    printf("Enter a first name \n");
    scanf("%s", friends[*counter].First_Name);
    if (friends != NULL) {

        free(friends);
    }
}

char getFirst(fr * friends, int pos)
{
    printf("%s ", friends[pos].First_Name);
    return *friends[pos].First_Name;
}

void add_contact(fr * friends, int *counter, int i)
{
    setFirst(friends, counter, i);
    (*counter)++;
}

void print_contact(fr * friends, int *counter, int i)
{
    for (i = 0; i < *counter; i++)
        if (strlen(friends[i].First_Name)) {
            getFirst(friends, i);
        }
}

Looking to give a big green check mark to whoever can help me out here.

Answer 1

You need to allocate memory both for the record as a whole and separately for each field. For example:

void string_realloc_and_copy (char **dest, const char *src)
{
  size_t len = strlen (src);
  *dest = realloc (*dest, len + 1);
  memcpy (*dest, src, len + 1);
}

typedef struct
{
  char *name;
  char *title;
} record;

record * record_new ()
{
  record *r = malloc (sizeof (record));
  r->name = NULL;
  r->title = NULL;
  return r;
}

void record_free (record *r)
{
  free (r->name);
  free (r->title);
  free (r);
}

void record_set_name (record *r, const char *name)
{
  string_realloc_and_copy (&r->name, name);
}

void record_set_title (record *r, const char *title)
{
  string_realloc_and_copy (&r->title, title);
}

Now to create a record and fill it with values read from the user:

record *r;
char buffer[100 + 1];

r = record_new ();

printf("Enter a first name \n");
if (scanf ("%100s", buffer) == 1) {
  record_set_name (r, buffer);
}

...