Error on scanf with structure

**George M.** · 09-20-2008

Hi there! Wright now I am on Structures
(More Info on My Learning Diary on My Blog)

Why does the application crashes at this point? I've written the scanf form exactly like it is in the example in my book

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Code:

struct user
{
       char name, surname;
       int age;
};
struct user database[10];

void read_database()
{
     int i,age;
     char name,surname;
     for (i=1; i<2; i++)
     {
         printf ("\nGive name for user %d ", i);
         scanf ("%s", database[i].name);
         
         printf ("\nGive surname for user %d ", i);
         scanf ("%c",database[i].surname);
         
         printf ("\nGive age for user %d ", i);
         scanf ("%d",database[i].age);         
     }
}

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**tabstop** · 09-20-2008

%s reads into a char[] (or a char *), but not a char.

Edit: And I know your textbook uses & in scanf's where appropriate (into "plain" char and int variables).

**George M.** · 09-20-2008

well even if i put c its just the same

**Sebastiani** · 09-20-2008

Lookup the syntax for declaring and using character strings in your book before going any further. Refer to your C API documentation for details on using scanf properly.

**George M.** · 09-20-2008

I found that i should declare the structure this way

Code:

struct user
{
       char name[10];
       char surname[10];
       int age;
};

why? Why should i declare the variables as arrays?

**laserlight** · 09-20-2008

Why should i declare the variables as arrays?

Because names consist of one or more characters. Of course, in this case you are restricting names to no more than 9 characters (the 10th is the null terminator).

**George M.** · 09-20-2008

Its true.

Initially i thought the same think but i saw that i could write more characters than 9.

However with the latest test I see that you can write more characters, but it will store only 9

Thats actually the scanf. You give order to scan the screen and store its content to that array, so it stores the most of the array's capacities.

I'm beggining to learn! (lol)

Its a miracle that it does not crash cause scanf should attempt "overstoring" the array - trying to store all elements scaned.

**laserlight** · 09-20-2008

However with the latest test I see that you can write more characters, but it will store only 9

Thats actually the scanf. You give order to scan the screen and store its content to that array, so it stores the most of the array's capacities.

Unless you provide the length, scanf() will just as much as it can into the string, possibly leading to a buffer overflow.

Its a miracle that it does not crash cause scanf should attempt "overstoring" the array - trying to store all elements scaned.

Miracles do happen, but when they don't and you assume they do, disasters happen. Consequently, use fgets() instead of scanf() for reading in a string, unless you want to specify the number of characters to be read in the format specifier for scanf().

**George M.** · 09-20-2008

So whats actually the point of structures?

They are used for creating databases?

**laserlight** · 09-20-2008

So whats actually the point of structures?

To group related objects (variables) together into a data structure (or from a C++ish point of view, to create a user defined type).

They are used for creating databases?

Yes, but that is just an example, not their primary purpose.

**George M.** · 09-20-2008

So their actually objects like the classes from C++ just more specified and restricted

**laserlight** · 09-20-2008

So their actually objects like the classes from C++ just more specified and restricted

Yes, though "restricted" in the sense that they lack native object oriented features that are present with the syntax of C++ classes and structs. However, read Stroustrup's glossary for the definitions of object.

**Sebastiani** · 09-20-2008

>> However with the latest test I see that you can write more characters, but it will store only 9

Actually, no - using scanf in that way can lead to nasty buffer overflows. Instead:

scanf("%9s", database[i].name);

[edit]
I guess I was slow to post.
[/edit]

**George M.** · 09-20-2008

This forces scanf to read only te first 9 characters?

Handy!

**laserlight** · 09-20-2008

This forces scanf to read only te first 9 characters?

That is what I mean by "specify the number of characters to be read in the format specifier for scanf()". However, fgets() is somewhat easier to use since the number is characters is just specified in an argument.

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