How to get rid of these fflush(stdin);

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Question:
Write a program to take data of student as input by the help of
structure student variable.
Do some changes with it.(by using pointers)
And print it.
Problem 1;
I was practicing the structures in C and encounter the problem of (Enter taken by input functions as an input).
after debugging i found that roll and gender is taking enter as an input
So I place a fflush(stdin) between

fgets(std->name, 20, stdin);

and

scanf("%d", &(std->roll));

Later I found that gender is still taking Enter as an input
So I place fflush(stdin) between

scanf("%d", &(std->roll));

and

scanf("%c", &(std->gender));

While taking the input of Second student i found that now name of the second student is taking enter and roll and gender taking garbage vlaue.
So, finally placed fflush(stdin) above
this

fgets(std->name, 20, stdin);

Isn't there any way to resolve this enter issue so that we don't have to think where to place fflush(stdin) by debugging again and again.
Problem 2;
How this buffer input used by the input functions and where actually is this buffer input located (is it in the stack or heap) or in both.

What I have tried:

Header and Global Variables

#include <stdio.h>
struct student
{
    char name[20];
    int roll;
    char gender;
};
void input(struct student *std);
void change(struct student *std);
void display(struct student std);

Main function

int main()
{
    struct student Stu1, Stu2;
    
    input(&Stu1);
    change(&Stu1);
    display(Stu1);

    input(&Stu2);
    change(&Stu2);
    display(Stu2);

    return 0;
}

Input Function : IN THIS FUNCTION I HAVE USED THE FFLUSH(STDIN);

void input(struct student *std)
{
    fflush(stdin);
    fgets(std->name, 20, stdin);
    fflush(stdin);
    scanf("%d", &(std->roll));
    fflush(stdin);
    scanf("%c", &(std->gender));
}

Change function

void change(struct student *std)
{
    std->roll += 5;
    printf("\n");
}

Display function

void display(struct student std)
{
    printf("%s", std.name);
    printf("%d\n", std.roll);
    printf("%c\n", std.gender);
}

Posted 6-Oct-22 22:32pm

Parth Vijay

Updated 7-Oct-22 6:37am

v4

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4 solutions

Solution 1

Unfortunately this is a feature of the scanf function. The function scans the input for information separated by whitespace, which it uses to delimit fields. If a newline character is left in the input stream, then the next scanf request will read that and assume that there is a null field entered. Flushing the stream, or using gets to read the newline, are the simplest ways round the problem.

Posted 6-Oct-22 23:21pm

Richard MacCutchan

Solution 2

You should be aware that fflush(stdin) as per the C standard is undefined behavior, and should probably be avoided.

If you are using Linux, take a look at getline(3) - Linux manual page[^] which will read an entire line of input at once, including the terminating new-line. You will then have to parse the input string for yourself, but that also means you have the opportunity to handle unexpected input gracefully. Don't forget to free() the buffer when you're done.

If you are using Windows, then AFAIK you are limited to fgets() or scanf(), or you could google for something like "windows C getline alternative" and use one of the solutions provided there.

Posted 7-Oct-22 4:08am

k5054

Solution 4

Since C++ was also asked for as a solution, you could also solve it this way.

C++

#define STRLEN 20

typedef struct {
	char name[STRLEN];
	int roll;
	char gender;
} student;

void change( student &std);

void display( student &std)
{
   std::cout << std.name << ", " << std.roll << ", " << std.gender << "\n";
}

void clrinput()
{
   if (std::cin.fail()) {
      std::cin.clear();
      std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
   }
}

void input( student &std)
{
	clrinput();
	std::cin.getline(std.name, STRLEN);
	clrinput();

	std::cin >> std.roll;
	clrinput();

	std::cin >> std.gender;
	clrinput();
}

C++

int main()
{
	student Stu1, Stu2;

	input(Stu1);
	change(Stu1);
	display(Stu1);

    // ... 
}

Actually, all of this belongs in one class, but it works just as well.

Posted 7-Oct-22 6:37am

merano99

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Rick York · Accepted Answer · 2022-10-07T05:28:00

The previous answers give the necessary information. I would summarize by stating you should pick one way to obtain your input and then stick with it. In my opinion, fgets is the best function to use if you accept your input one line at a time because it will accept the newline character(s). Here is its documentation : cplusplus.com - fgets[^]. Then you can use cplusplus.com - atoi[^] to convert the string to an integer value. Be aware that the newline will be contained in the returned string. That means when you get the string as an input you will need to remove it from the buffer. I usually use strchr to do this : cplusplus.com - strchr[^]. In fact, I do this so often that many years ago I made a little function to locate and remove a character. Here it is :

C++

void RemoveChar( char * buffer, int rmvchr )
{
    char * p = strchr( buffer, rmvchr )
    if( p )
       * p = 0;   // character was located - null it
}

You can call this to remove the newline character from your input string and you will be left with what you are after. You don't need to do that when accepting an integer because atoi will stop conversion when it is found and when you accept a single character you can ignore it yourself because your code will probably look something like this :

C++

std.gender = buffer[ 0 ];