Array of a struct with strings

Question

I have defined a structure

struct subject
{
    char name[100];
    int year;
};

and since I need n of these and I have to use malloc I did the following in my main function:

int n, i;
scanf("%d", &n);
struct subject *ptr = malloc(n*sizeof(struct subject));

Unfortunately when I try to input something with this code:

for(i = 0; i < n; i++)
{
   gets((ptr + i)->name);
   scanf("%d", (ptr + i)->year);
}

It crashes after I type the first name. The task requires the use of malloc.

Here's the whole code (unfortunately it's in my native language so it's a little bit different)

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

#ifndef DEBUG
#define DEBUG(...)printf(_VA_ARGS_)
#endif

struct kolegij
{
    char naziv[100];
    int semestar;
};

int main(){
    int brPredmeta, i;

    scanf("%d", &brPredmeta);

    struct kolegij *ptr = malloc(brPredmeta*sizeof(struct kolegij));

    if(ptr == NULL)
    {
       printf("error\n");
       return 0;
    }

    for(i = 0; i < brPredmeta; i++)
    {
       //gets(ptr->naziv);
       gets((ptr + i)->naziv);
       scanf("%d", &(ptr + i)->semestar);
       getchar();
    }

    for(i = 0; i < brPredmeta; i++)
    {
       printf("%s\n", ptr[i].naziv);
       printf("%d\n", ptr[i].semestar);
    }

    return 0;
}

With regards to the duplicate issue. I believe this shouldn't be a duplicate since it's related to structs and pointers. I had issues with scanfs before and I haven't considered this as a solution so I think it shouldn't be flagged as a duplicate.

Answer 1

I believe the statement scanf("%d", (ptr + i)->year); to be responsible for your unstable behaviour, as you're passing an int where scanf expects an int *. Perhaps you meant something like scanf("%d", &(ptr + i)->year) (note the additional ampersand), though this also has perils associated. Namely, you're discarding the return value, so you can never be sure that the value stored will be sane. If the user enters invalid input, it might be best to re-prompt or exit:

int n;
do {
    puts("Enter year: ");
    n = scanf("%d", &(ptr + i)->year));
    scanf("%*[^\n]"); // more on this later
    getchar();
} while (n == 0);
if (n < 0) exit(EXIT_FAILURE);

As part of handling input is expecting the enter keystroke, we need to clear that before the next gets call, and I use scanf("%*[^\n]") followed by getchar() for that.

On the topic of the code which you presented near the end, that does indeed look very different. I notice you fixed the scanf type error (the additional ampersand), and you have getchar() after the scanf("%d", ...) so you won't usually get the trailing newline hanging around. I see no reason to be suspect of that (code at the end of your post); if you're concerned about a reason for closure, it appears that you don't have a problem anymore (we can't reproduce your crash)... I mean, aside from using gets; that's a minor problem, I guess (gets is deprecated; use fgets instead, trim the trailing '\n' and if there is none use scanf("%*[^\n]") followed by getchar() again to effectively truncate the input).

Answer 2

Malloc returns a void pointer type. In order to use it as a pointer to your structure you have to cast it first. Try changing the line with malloc into:

int n, i;
scanf("%d", &n);
struct subject *ptr = (struct subject*) malloc(n*sizeof(struct subject));

You also have to change your input into:

for(i = 0; i < n; i++)
{
   gets((ptr + i)->name);
   scanf("%d", &((ptr + i)->year));
}