how do I input values into a structure using pointers?

Question

I have a small problem with this program I'm writing, I'm trying to input data into a structure using a pointer but the compiler just gives me an error stating: "dereferencing pointer to incomplete type"

the function of the program is simple: a program that uses functions to input data into a structure using pointers

heres the code: the main function simply calls the input function and passes the structure pointer as an argument

void input(struct test *ptr)
{
printf("Enter: \n");
fflush(stdin);
scanf("%s",&ptr->entry);

}

void print(struct test *ptr)
{

}

int main()
{
    int counter;
    struct test
    {
        char entry[20];

    }p[4];
    struct test *ptr=p;
    ptr=&p;

    for(counter=0;counter<=4;counter++)
        {
            input(ptr);
            ptr++;
        }
    return 1;
}

The print function is still empty.

"The print function is still empty." Yes, it is. We're not going to write your code for you. What is your specific question? — The Archetypal Paul, Nov 16 '14 at 09:11
And you don't want <=4, as you'll access one off the end of your `p` array. — The Archetypal Paul, Nov 16 '14 at 09:12
"the compiler just gives me an error stating: "dereferencing pointer to incomplete type"" On what line? Help us help you... — The Archetypal Paul, Nov 16 '14 at 09:13
I edited the original post stating my question but anyways my question is how do I enter values into my structure using pointers? "scanf("%c",&ptr->entry);" seems to cause an error. — Slaine, Nov 16 '14 at 09:15
@Gopi I edited that line to that I still get the same error. — Slaine, Nov 16 '14 at 09:18
[`fflush(stdin)` is Undefined Behaviour](http://stackoverflow.com/questions/18170410/what-is-the-use-of-fflushstdin-in-c-programming) — Spikatrix, Nov 16 '14 at 09:35

The Archetypal Paul · Answer 1 · 2014-11-16T09:31:02.773

First problem is that struct test is defined only inside your "main" function and is not accessible to input or print. Define it outside all the functions.

Then you want to scan a string, not a character, so you need "%s". Actually "%19s" so if the input is too long you don't walk off the end of the entry array (Hat tip to @BLUEPIXY)

And ptr->entry is the address of where you want to put the string so you don't need &ptr->entry but ptr->entry

You assign to ptr twice in main. The second is wrong (again, p is the address of the array of structs, so you have it right in the initialisation. The assignment of &p is wrong.

Finally, your for-loop executes 5 times (0,1,2,3,4) and the last iteration accesses one off the end of your 4-element p array

Putting that altogether:

#include <stdio.h>

struct test
{
    char entry[20];
};

void input(struct test *ptr)
{
    printf("Enter: \n");
    fflush(stdin);
    scanf("%19s", ptr->entry);
}

void print(struct test *ptr)
{

}

int main()
{
    int counter;
    struct test p[4];
    struct test *ptr=p;

    for(counter=0;counter<4;counter++)
    {
        input(ptr);
        ptr++;
    }
    return 1;
}

Oops, missed actually changing the `%c`. Thanks – The Archetypal Paul Nov 16 '14 at 09:29 — The Archetypal Paul, Nov 16 '14 at 09:29

score 0 · Accepted Answer · answered Nov 16 '14 at 09:30

Scanning a string using & is not required. A string should be scanned using %s not %c. Filling your empty print. Preferably change your function name print to something else.

Check the below code:

#include <stdio.h>
#include <string.h>
    struct test
    {
        char entry[20];

    }p[4];

void input(struct test *ptr)
{
printf("Enter: \n");
scanf("%s",ptr->entry);
return;
}

void print(struct test *ptr)
{
int i;
for(i=0;i<4;i++)
{
    printf("%s\n",ptr[i].entry);
}
return;
}
int main()
{
    int counter;

    struct test *ptr=p;

    for(counter=0;counter<4;counter++)
        {
            input(ptr);
            ptr++;
        }
        print(p);
    return 0;
}

how do I input values into a structure using pointers?

2 Answers2