There is a problem with the gets function in c programming language

Question

I'm creating a simple program using the C programming language. Unfortunately, I'm having a problem with a piece of source code right now. I am not able to store data in the students structure after running the program.

I think the problem is the gets() function in the ADD_Students() function. (Line 60)

Here is my source code: (Used code blocks with gnu gcc compiler in windows 10)

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

#define TRUE 1
#define MAX 3

struct Students
{
    char * Name;
    char * Field;
    char * City;

    unsigned int Student_ID;
    float Mark;
}Addr_INFO[MAX];


char Menu();
void ADD_Students();
void DEL_Students();
void EDT_Students();

int main()
{
    char Selected_ITEM;

    while (TRUE)
    {
        Selected_ITEM = Menu();

        switch (Selected_ITEM)
        {
            case '1' : ADD_Students(); break;
            case '2' : DEL_Students(); break;
            case '3' : EDT_Students(); break;
            default : printf("Wrong number!\n"); exit(0);
        }
    }
}


char Menu()
{
    char Chose;

    printf(".----------+ Students information management +----------.\n\n");
    printf("[1]. ADD student\n");
    printf("[2]. DEL student\n");
    printf("[3]. EDT student\n\n");

    printf("Please enter your choice: ");
    scanf("%c", &Chose);

    return Chose;
}


void ADD_Students()
{
    register int i;

    for (i = 0; i < MAX; i++)
    {
        printf("[S-%d]. Name: ", i);
        gets(Addr_INFO[i].Name);
        printf("[S-%d]. Field: ", i);
        gets(Addr_INFO[i].Field);
        printf("[S-%d]. City: ", i);
        gets(Addr_INFO[i].City);
        printf("[S-%d]. ID: ", i);
        scanf("%d", &Addr_INFO[i].Student_ID);
        printf("[S-%d]. Mark: ", i);
        scanf("%.2f", &Addr_INFO[i].Mark);
        printf("\n");
    }
}


void DEL_Students()
{
    printf("Test2");
}


void EDT_Students()
{
    printf("Test3");
}

what's going on?

Why this is happened?

Ouput

.----------+ Students information management +----------.

[1]. ADD student
[2]. DEL student
[3]. EDT student

Please enter your choice: 1
[S-0]. Name: [S-0]. Field: [S-0]. City: [S-0]. ID:

Please explain the nature of the problem; if you get an error message, what does it say? If it produces the wrong result, what is it an what should it be? When you run it with a debugger, what is the *first* thing the program does incorrectly? — Scott Hunter, Feb 11 '20 at 15:32
The `scanf()` call leaves the newline in the input buffer. Therefore, `gets()` reads up to the first newline. And you should NOT be using `gets()` ever! Never, never, never, *never*, **never**, ***NEVER*** use `gets()`! — Jonathan Leffler, Feb 11 '20 at 15:36
@JonathanLeffler -- Is there [a problem with the `gets()` function](https://stackoverflow.com/questions/1694036/why-is-the-gets-function-so-dangerous-that-it-should-not-be-used)? ;) — ad absurdum, Feb 11 '20 at 15:42
Jonathan Leffler , So what function should I use to get the string? — D.H, Feb 11 '20 at 15:45
Use `fgets()`, but you’ll still need to deal with the newline left over from `scanf()` and `%c`. And the newline included in the input by `fgets()`. The Q&A linked to by ex nihilo explains the problemcc GCC and the alternatives. — Jonathan Leffler, Feb 11 '20 at 15:47
You should be able to reduce this to a [mcve] of your problem, without all the extraneous parts (`struct Students` etc.). — Toby Speight, Feb 11 '20 at 20:38
Not the cause of your problem, but it's a good idea to make all your function declarations *prototypes* - i.e. specify what arguments they take; `(void)` if no arguments. — Toby Speight, Feb 11 '20 at 20:39

vlad_tepesch · Answer 1 · 2020-02-11T15:39:19.773

1

There is a problem in your code. You are passing an uninitialized pointer to gets.

You never add an actual bit of memory to your Addr_INFO entries. You should either declare the members as an array of some appropriate size or gets the data into a local buffer and then allocate memory (malloc) of sufficient size and copy the local data to that buffer and assign its pointer you to the member.

edited Feb 11 '20 at 15:39

answered Feb 11 '20 at 15:35

vlad_tepesch

6,681
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3

That’s one of the problems. There are at least two others, one being the use of `gets()` at all. – Jonathan Leffler Feb 11 '20 at 15:37
2

`gets` should be `fgets` at least – Ed Heal Feb 11 '20 at 15:42

score 1 · Answer 2 · answered Feb 11 '20 at 17:12

First problem - NEVER NEVER NEVER NEVER NEVER NEVER use gets. It has been removed from the standard library as of the 2011 standard, and it will (not might, will) introduce a point of failure in your program. It is the programming equivalent of plutonium. Use fgets instead:

if ( fgets(Addr_INFO[i].Name, MAX_NAME_LEN, stdin) == NULL )
  // error on input
else
  // process Addr_INFO[i].Name

Second problem - you never set aside any memory to actually store a name, field, or city. All you've done is declare the pointers, but you never set them to point anywhere meaningful. They initially contain some random bit pattern that very likely doesn't correspond to a valid address.

For a simple program like this, it's probably better to declare those fields as arrays of char, rather than just pointers:

#define MAX_NAME_LEN  40 // or whatever you expect the longest name to be
#define MAX_FIELD_LEN 40 // same as above
#define MAX_CITY_LEN  40 // same as above 

struct Students
{
    char Name[MAX_NAME_LEN + 1];  // +1 for string terminator
    char Field[MAX_FIELD_LEN + 1];
    char City[MAX_CITY_LEN + 1];

    unsigned int Student_ID;
    float Mark;
}Addr_INFO[MAX];

And then your fgets call changes ever-so-slightly to

if ( fgets( AddrInfo[i].Name, sizeof AddrInfo[i].Name, stdin ) == NULL )
  // error on input
else
  // process AddrInfo[i].Name

There is a problem with the gets function in c programming language

2 Answers2