structs - why gets function is not working compared to scanf?

Question

gets(edu.classes[i].students[j].name);

I would like to know why the compiler skips this line after debugging and does not input into name? Is it legal to invoke gets function on that way?

Note: Once I use scanf("%s" , ... ) - it works!

scanf("%s",edu.classes[i].students[j].name);

(I know that I did not free the memory allocations and checked if the allocations were not failed - I know that it is necessary! It's only time issue) :)

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

#define SIZE 20

typedef struct
{
char name[SIZE];
int id;
}Student;

typedef struct
{
Student *students;
int num_students;
char teacher[SIZE];
}Class;

typedef struct
{
Class *classes;
int num_classes;
}Education;

int main()
{
int i, j;
Education edu;
puts("how many classes?");
scanf("%d", &(edu.num_classes));
edu.classes = (Class*)malloc((edu.num_classes) * sizeof(Class));
if (edu.classes == NULL)
{
    printf("ERROR allocation\n");
    exit(1);
}
for (i = 0; i < edu.num_classes; i++)
{
    puts("enter num of students");
    scanf("%d", &(edu.classes[i].num_students));
    edu.classes[i].students = (Student*)malloc((edu.classes[i].num_students) 
* sizeof(Student));
    for (j = 0; j < edu.classes[i].num_students; j++)
    {
        puts("enter student's name");
        gets(edu.classes[i].students[j].name); // this is the problematic line
        puts("enter id");
        scanf("%d", &(edu.classes[i].students[j].id));
    }
}
return 0;
}

I would recommend to use `fgets` instead of `gets`, its error-prone — kocica, Aug 16 '17 at 21:17
that said, I don't know why this wouldn't work. Use `scanf("%19s" ...)` to protect against buffer overflow — Jean-François Fabre, Aug 16 '17 at 21:18
The obsolete `gets` consumes *only* the newline left in the input buffer after the previous `scanf`. Don't mix your methods. — Weather Vane, Aug 16 '17 at 21:19

E. van Putten · Answer 1 · 2017-08-16T21:49:20.450

0

I think it is seeing the newline character that was produced due to hitting the enter key. gets then stops reading. Try eating those special characters before gets sees them. Or use scanf with %s which eats any leading whitespace as you have seen it works.

edited Aug 16 '17 at 21:49

answered Aug 16 '17 at 21:24

E. van Putten

615
7
17

do you mean by fflush? – Alona Rose Bellucci Aug 16 '17 at 21:32
Actually I meant reading characters from the standard input until it is no longer a special character like newline (\n). – E. van Putten Aug 16 '17 at 21:37
well... you gave me an idea :) I just write this command line twice. – Alona Rose Bellucci Aug 16 '17 at 21:42
this is my "solution" : gets(edu.classes[i].students[j].name); gets(edu.classes[i].students[j].name); – Alona Rose Bellucci Aug 16 '17 at 21:45
But scanf with %s also skips the leading whitespace. Besides `scanf` can be told not to read more than a certain amount of characters while `gets` can crash your program. – E. van Putten Aug 16 '17 at 21:45
I have already tried scanf with %s and it worked well – Alona Rose Bellucci Aug 16 '17 at 21:49

structs - why gets function is not working compared to scanf?

1 Answers1