unexpected behavior of scanf() and gets()

Question

I want to write a line ( including spaces ) to the file in one go with following code:-

//in main
char ch[40];
FILE *p;
char choice;
p=fopen("textfile.txt","w");
printf("%s\n","you are going to write in the first file" );
while (1)
{
    gets(ch);// if i use scanf() here the result is same,i.e,frustrating
    fputs(ch,p);
    printf("%s\n","do you want to write more" );
    choice=getche();
    if (choice=='n'|| choice=='N')
    {
        break;
    }
}

Result of above code is frustrating to me and is hard to explain.but still i will try. If i enter,say,

"my name is bayant."

and press enter the statment comes to screen is

"do you want to write more"

it is good till now,but when i prees a key other than 'n' or 'N' (as required by the logic of program to write more line) then the message

"do you want to write more"

prints again.Now if i press other key than 'n' or 'N' the same line prints on screen.this procedure is followed and prints the statement

"do you want to write more"

4 times,which is the number of words,i.e, 4 in this case.By following this inflexible procedure i get the desired line on my file but if in response to first time printing of statement

"do you want to write more"

i press 'n' or 'N' then only first word ,i.e, "my" in this case prints on file. so what is the solution to write a complete line on the file in one go?and why gets() and fputs() seems ineffective in this case? thanxxx in advance.

gets() buffer overflow heaven!!, please don't use gets, use fgets instead. on windows use gets_s() — tesseract, Mar 07 '14 at 12:00
@tesseract i know it but even i use scanf() the problem persists. — OldSchool, Mar 07 '14 at 12:01
you haven't allocated space for the string. you haven't made the pointer point to valid memory — tesseract, Mar 07 '14 at 12:02
@tesseract what is the solution to write a complete line ( including spaces ) to the file? — OldSchool, Mar 07 '14 at 12:02
what if you want to write 'n' to the file? may better check for empty line as NO; also you should ask first and then print, otherwise you might end up consuming the first character of the input line — jev, Mar 07 '14 at 12:25
@Manu343726 `gets()` wasn't deprecated in C11, it was outright *removed*. It doesn't appear in the standard *at all* except foot notes and the official note it was removed from the `stdio.h` header file. Its gone (*finally*). — WhozCraig, Mar 07 '14 at 12:50

score 2 · Answer 1 · edited May 23 '17 at 12:03

Do something like this, its a very crude program but should give you an idea

your mistakes, you had only made a pointer to a char in your program, you need to allocate memory for the pointer using malloc, or the other option is just to make an array of chars. which I have done.

#include <stdio.h>
#include <stdlib.h>
int main(void){

char ch[100];
FILE *p;
char choice;
p=fopen("textfile.txt","w");
printf("%s\n","you are going to write in the first file" );
while (1)
{
// gets(ch);// if i use scanf() here the result is same,i.e,frustrating
int c =0;

fgets(ch,100,stdin);
fputs(ch,p);
printf("%s\n","do you want to write more" );
choice=getchar();
if (choice=='n'|| choice=='N')
    {
    break;
    }
while ((c = getchar()) != '\n' && c != EOF);
}
return 0;
}

you program was repeating the printf("%s\n","do you want to write more" ); because the input buffer had a \n written to it, you need to clear the buffer before reading. this line removes the newline from buffer while ((c = getchar()) != '\n' && c != EOF);

check this scanf() leaves the new line char in buffer?

Any particular reason for using `unsigned int c =0;` instead of `int c = 0;`? — chux - Reinstate Monica, Mar 07 '14 at 18:39
I suspect `unsigned int c` will work too. The `c != EOF` will cast the EOF to `unsigned` and will perform as expected. Not that I recommend `unsigned int c`, just thought it was novel. — chux - Reinstate Monica, Mar 07 '14 at 19:04

davir · Answer 2 · 2014-03-07T17:50:56.900

If you use

scanf("%s",ch);

(Which I assume is what you mean by "scanf"), this will read one string. If you type

"my name is bayant."

This will result in 4 strings: "my", "name", "is" and "bayant.".

Notice that, from your description, you don't want to read strings, you want to read lines. To read a whole line of text with scanf, you could use:

scanf("%[^\n]", ch);
scanf("%*c");

Which means: Line 1: "read everything until you find a \n character".

Line 2: "read and ignore that '\n' character (which was left in the buffer)".

I should say that this is not a secure solution, as the user can easily overflow the "ch" buffer, but I'm sure you can find better ways to do that if this is a real concern in your particular case.

If one uses `scanf()`, suggest `if (1 == scanf("%39[^\n]%*c", ch)) Success();` — chux - Reinstate Monica, Mar 07 '14 at 18:34

unexpected behavior of scanf() and gets()

2 Answers2