C - Strcmp() not working

Question

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

int main()
{

const int SIZE = 100;

char input[SIZE];

while(1)
{
    fgets (input, SIZE - 2, stdin);          // input
    printf("%d", strcmp(input, "exit"));    //returining 10 instead of 0

    if(strcmp(input, "exit") == 0)
    {
        printf("SHELL Terminated\n");
        exit(0);    
    }

return 0;
}

I am facing a problem. If I enter exit in input variable, the function strcmp() returns 10, but it should return 0 and exit the program as exit is equal to exit. But it does not.

I can't find the problem.

Googling your exact title gives: 'About 60,500 results', with three SO Q&A about the 'newline' issue as the first three entries. — Martin James, Mar 29 '16 at 18:08
@Muzahir Hussain: Firstly, why would you reserve *two* bytes for *one* null character? Secondly, `fgets` already knows to reserve space for null character internally. — AnT stands with Russia, Mar 29 '16 at 18:19

score 8 · Answer 1 · answered Mar 29 '16 at 17:55

8

You get 10 because there is a newline character in your input string. the 10 return value is the difference between the ascii value of that newline character and the terminating null character of the "exit" string literal you're comparing to.

answered Mar 29 '16 at 17:55

Carl Norum

219,201
40
422
469

Vlad from Moscow · Answer 2 · 2016-03-29T18:08:38.423

Function fgets also includes the new line character '\n' that corresponds for example to the pressed Enter key if there is enough space in the array.

You should remove it for example the following way

fgets( input, SIZE, stdin );
input[strcspn( input, "\n" )] = '\0';

or more safe

if ( fgets( input, SIZE, stdin ) != NULL ) input[strcspn( input, "\n" )] = '\0';

Take into account that this code

*strchr(input, '\n') = '\0';

in general is invalid because the new line character can be absent in the array and the function strchr will return NULL.

score 5 · Accepted Answer · edited May 23 '17 at 12:24

5

fgets appends a newline (\n) character to the end of the string read into the buffer.

Delete it using

char* newline = strchr(input, '\n');
if (newline)
    *newline = '\0';

As @WeatherVane mentioned, some calls of fgets might not set a newline in the buffer, so we need to check if strchr returns NULL (no newline found).

edited May 23 '17 at 12:24

Community

1
1

answered Mar 29 '16 at 17:55

cadaniluk

15,027
2
39
67

This method will break if the last line of a file does not have `newline`, or if the input buffer passed to `fgets` was shorter than the text line length. – Weather Vane Mar 29 '16 at 18:10
Yes, but, alk and vlad have a better way. – Weather Vane Mar 29 '16 at 18:16
@WeatherVane ...which I wholeheartedly agree with. :-) – cadaniluk Mar 29 '16 at 18:17
Yes, `fgets` can append a newline character. But using `strchr` to search for it is an atrocity. – AnT stands with Russia Mar 29 '16 at 18:18
@AnT How is it an "atrocity"? – cadaniluk Mar 29 '16 at 18:19
@cad: It generates more heat than light, speaking figuratively. Why are you *searching* for something, if you already know where it shall be (if it is present). – AnT stands with Russia Mar 29 '16 at 18:21
@AnT `fgets` doesn't provide information about the length of the string and the location of the newline, so why **not** search it? – cadaniluk Mar 29 '16 at 18:23
@cad: OK, makes sense, I guess. Still, `strlen` is theoretically (and often practically) a more efficient way to find the end of the string, and then check for newline manually. – AnT stands with Russia Mar 29 '16 at 18:24

score 4 · Answer 4 · answered Mar 29 '16 at 18:00

4

fgets() keeps the '\n'. You either remove it from input (see other answers) or add it to the literal

strcmp(input, "exit\n")

answered Mar 29 '16 at 18:00

pmg

106,608
13
126
198

C - Strcmp() not working

4 Answers4