C: why doesn't fgets function work? [duplicate] - c

Question

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scanf() leaves the newline character in the buffer
(7 answers)
Closed 5 years ago.
I'm writing a program to create a linked list node. After that, i add some functions (insert,search,update,and print). The linked list contains number, name, and quantity on hand.
The main function prompts users to enter an operation code, then calls a function requested actions.
main()
{
char code;
int c;
for(;;)
{
printf("i: insert\ns: search\nu: update\np: print\n");
printf("Enter operation code: ");
scanf("%c",&code);
while((c=getchar() )!= '\n'&& c!=EOF);
switch(code)
{
case 'i': insert();
break;
case 's': search();
break;
case 'u': update();
break;
case 'p': print();
break;
case 'q':return 0;
default: printf("Illegal code\n");
break;
}
printf("\n");
}
}
All function work correctly. However, in the insert function, I use fgets statement to get the string input from the user. (NAME_LEN = 25)
void insert()
{
node *previous,*current,*new_node;
new_node = malloc(sizeof(node));
printf("Enter part number: ");
scanf("%d",&new_node->number);
for(current = inventory,previous=NULL;
current != NULL&& new_node->number > current->number;
previous = current,current = current -> next);
if(current != NULL && new_node ->number == current->number)
{
printf("Part already exists.\n");
free(new_node);
return;
}
printf("Enter name part: ");
fgets(new_node->Name,NAME_LEN+1,stdin); // i use fgets to input string name
printf("Enter quantity on hand: ");
scanf("%d",&new_node->on_hand);
new_node -> next = current;
// move to the next node
if(previous == NULL)
inventory =new_node;
else
previous->next = new_node;
}
Unfortunately, this code doesn't work. The program shows that
i: insert
s: search
u: update
p: print
Enter operation code: i
Enter part number: 2
Enter name part: Enter quantity on hand: 3
As you can see that, the name part was missed.
Moreover, after inserting a new node, the program automatically shows the default case in the switch.
i: insert
s: search
u: update
p: print
Enter operation code: i
Enter part number: 2
Enter name part: Enter quantity on hand: 3
i: insert
s: search
u: update
p: print
Enter operation code: Illegal code
Can you explain to me what happens, pls?.

Answer 1

Well the thing is fgets is consuming the \n that is left in stdin from previous input. Easy way would be to use dummy getchar() or using fgets until you get a non-whitespace input.
Do one thing, after scanf put a getchar().
scanf("%c",&code);
getchar();
^^^^
Will consume the `\n`.
To give you a more clear explanation suppose you enter iEnter
Then i is stored into the variable code. But what about the \n. fgets() when starts reading input from stdin finds that \n and it stops. By using the getchar() you have consumed the \n. Now all non-whitespace characters will be consumed by fgets until it finds a \n or EOF or buffer gets full.
The most portable and clean way to flush the stdin would be (To be correct the stdin must have atleast the \n character otherwise it will go on eating other charcaters as well (maybe your valid inputs)).
int c;
while ((c = getchar()) != '\n' && c != EOF);
This you can use whenever you encounter some garbage input or clear the stdin.
Also to really understand whether fgets function works or not you need to check what it returns. Does it return NULL? If not then check it and then try to understand. Simply because you didn't see any output when printed the buffer doesn't mean fgets failed.
Edit
The question at first posted mentioned that there is fgets right after the scanf("%c",&code). Here it is clear on the edit that scanf("%d",&new_node->number); is there before fgets but that doesn't change the scenario. Now the \n after the entered number (you input 2ENTER) is still there and fgets consumes it. So we need to place dummy getchar() over there or stdin flushing technique shown above.
scanf("%d",&new_node->number);
getchar();
^^^^
This consumes the stray `\n`