Allocating memory for Structure Array within a Structure

Question

I have an application to work, where i need to use an array of structure type as a member of structure . I have created models with structure but as you can see I need a member as an array inside the contact structure(dynamically depending on the user's input). I am allocating the size for it in the getContacts function by passing a variable named contactPersonLength from main,but as soon as the getContacts returns to main value of contactPersonLength variable changes and garbage value gets printed in main. So i am having trouble passing the varible to print function. Please tell me where i am wrong and how to allocate size for array which is a structure member

struct address
{
    char *name,*street,*district;
    int doorNo;
};

struct contactPerson
{
    char *name,*contactNumber;
};

struct contact
{
    char *firstName,*lastName,*emailId;
    struct address billingAddress;
    struct address shippingAddress;
    struct contactPerson contactPersons[];
};

void getContacts(struct contact *contacts,int n,int contactPersonLength)
{
    int isSame,i,j;
    for(i=0;i<n;i++)
    {
        printf(".......Enter Contact Details %d  ........\n",i+1);
        contacts[i].firstName = (char *)malloc(sizeof(char));
        printf("Enter the First Name");
        scanf("%s",contacts[i].firstName);

        contacts[i].lastName = (char *)malloc(sizeof(char));
        printf("Enter the Last Name");
        scanf("%s",contacts[i].lastName);

        contacts[i].emailId = (char *)malloc(sizeof(char));
        printf("Enter the email id");
        scanf("%s",contacts[i].emailId);

        printf(".....Billing address Details.....\n");
        contacts[i].billingAddress.name = (char *)malloc(sizeof(char));
        printf("Enter the name");
        scanf("%s",contacts[i].billingAddress.name);

        printf("Enter the DoorNo");
        scanf("%d",&contacts[i].billingAddress.doorNo);

        contacts[i].billingAddress.street = (char *)malloc(sizeof(char));
        printf("Enter the Street name");
        scanf("%s",contacts[i].billingAddress.street);


        contacts[i].billingAddress.district = (char *)malloc(sizeof(char));
        printf("Enter the District");
        scanf("%s",contacts[i].billingAddress.district);

        printf(".....Shipping Address Details....\n");
        printf("Is your Shipping Address same as Billing Address Press 1 Or else Press 0");
        scanf("%d",&isSame);
        if(isSame==1)
        {
            contacts[i].shippingAddress = contacts[i].billingAddress;
        }
        else
        {
            contacts[i].shippingAddress.name = (char *)malloc(sizeof(char));
            printf("Enter the name");
            scanf("%s",contacts[i].shippingAddress.name);

            printf("Enter the DoorNo");
            scanf("%d",&contacts[i].shippingAddress.doorNo);

            contacts[i].shippingAddress.street = (char *)malloc(sizeof(char));
            printf("Enter the Street name");
            scanf("%s",contacts[i].shippingAddress.street);

            contacts[i].shippingAddress.district = (char *)malloc(sizeof(char));
            printf("Enter the District");
            scanf("%s",contacts[i].shippingAddress.district);
        }

        printf(" ContactPersonLength %d \n",contactPersonLength);
        contacts[i].contactPersons[contactPersonLength];
        for(j=0;j<contactPersonLength;j++)
        {
            printf(".....ContactPerson %d.....\n",j+1);
            contacts[i].contactPersons[j].name = (char *)malloc(sizeof(char));
            printf("Enter Contact Person Name");
            scanf("%s",contacts[i].contactPersons[j].name);

            contacts[i].contactPersons[j].contactNumber = (char *)malloc(sizeof(char));
            printf("Enter Contact Person Contact Number");
            scanf("%s",contacts[i].contactPersons[j].contactNumber);
        }
    }

}



void main()
{   
    struct contact contacts[n];

    getContacts(contacts,n,contactPersonLen);
}

In C, a string is an array of characters with a NUL byte at the end. The line `contacts[i].firstName = (char *)malloc(sizeof(char));` allocates only enough room for the NUL byte and nothing else. — user3386109, Aug 05 '18 at 16:59
input is working fine only problem i have is that if i allocate the array size for contactPerson member in getContacts function and when it returns contactsPersonLength variable gets filled with garbage value — Sibi, Aug 05 '18 at 17:03
That's the thing about [undefined behavior](https://en.wikipedia.org/wiki/Undefined_behavior). Just because your code appears to work doesn't mean it's correct. — user3386109, Aug 05 '18 at 17:06
You had bad luck that `scanf` didn't segfaulted as @user3386109 mentioned. Using `scanf` without bound checks `nParsed = scanf("%SIZEs");` is potentially dangerous. Either fix it or use char Arrays instead. (Where `SIZE` is an actual number of your allocated size-1 to store the NUL) — Inrin, Aug 05 '18 at 17:07
You can read more about undefined behavior [here](https://stackoverflow.com/questions/33797630) and [here](https://stackoverflow.com/questions/2397984) — user3386109, Aug 05 '18 at 17:08
Possible duplicate of [C flexible arrays vs pointers](https://stackoverflow.com/questions/34249886/c-flexible-arrays-vs-pointers) — Inrin, Aug 05 '18 at 17:36

score 3 · Answer 1 · answered Aug 05 '18 at 17:12

I would suggest that your function should work as follows:

char buffer[256];
for(i=0;i<n;i++)
{
    printf(".......Enter Contact Details %d  ........\n",i+1);
    do {
        printf("Enter the First Name");
    } while (scanf("%255s",buffer)!=1);
    contacts[i].firstName = malloc(strlen(buffer)+1);
    strcpy(contacts[i].firstName, buffer);
    //...etc

So you use a single buffer to get the input, then allocate memory exactly the size of the input, plus a terminating null character, then copy the buffer to that memory and reuse the buffer for the next input.

Notes: the size of a char is always 1, so sizeof(char) is not needed. Also, the result of malloc should not be cast.

You ask the input in a do...while loop and check that scanf was able to read input. In scanf, the format specifier limits the input to 255 characters so the buffer will not overflow.

Could you also answer the actual main question, how to allocate space for the flexible array member in the flexible struct? — Inrin, Aug 05 '18 at 17:58

Susmit Agrawal · Answer 2 · 2018-08-05T17:19:12.277

0

(char *)malloc(sizeof(char)); allocates only one byte of memory. If you want to store a string, say 20 chars long, the code is: (char *)malloc(sizeof(char) * 20 + 1); //explicitly showed +1 for clarity, based on comment.

EDIT

Generally it is a good idea to set the maximum possible size of the array in malloc. But if the size is too large, and/or you are concerned about memory, do the following:

contacts[i].firstName = (char *)malloc(sizeof(char) * MAX_POSSIBLE_SIZE);
printf("Enter the First Name");
scanf("%s",contacts[i].firstName);
char *temp = (char *)realloc(contacts[i].firstName, strlen(contacts[i].firstName) + 1);
if(temp!=NULL)  contacts[i].firstName = temp;

Do this for every string.

edited Aug 05 '18 at 17:19

answered Aug 05 '18 at 17:03

Susmit Agrawal

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..plus 1 for the terminating null character! – Paul Ogilvie Aug 05 '18 at 17:06
Right, my mistake. – Susmit Agrawal Aug 05 '18 at 17:07
I don't know the exact size of the input from user ,so how can I handle it – Sibi Aug 05 '18 at 17:11
Generally this wastes a lot of memory and should be discouraged. Now it is just fixed strings that could have defined in the typedef. – Paul Ogilvie Aug 05 '18 at 17:25
The longest recorded Name in history was 746 letters long. (All names + Surname). [source](https://en.wikipedia.org/wiki/Hubert_Blaine_Wolfeschlegelsteinhausenbergerdorff,_Sr.) – Inrin Aug 05 '18 at 17:27
Okay thanks for the optimization.Kindly instruct me how to allocate memory for contactPerson which is an array inside the contact Structure – Sibi Aug 05 '18 at 17:27
`contacts[i].contactPersons = malloc(sizeof(struct contactPerson) * REQUIRED_NUMBER);` You should also change `contactPersons[]` to `*contactPersons` in the definition. – Susmit Agrawal Aug 05 '18 at 17:30

Allocating memory for Structure Array within a Structure

2 Answers2