how to get program to terminate when user types in "exit" in C

Question

When I type in "exit" the program doesn't seem to terminate from my code below; is there something I am missing or is there better way to terminate the program when the user types "exit"?

In my code you can also see it terminates when an EOF occurs this works properly, it just does not terminate when I type in "exit".

MY CODE:

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

int main () {
   char input[512];
   char * charp;
   int len = 0;

   printf("> ");
   if(fgets(input, 512, stdin) == 0 || input == "exit") {    /* reads in input from user no more than 512 chars including null terminator*/
     printf("exitting program");
       exit(0);
   }

   printf("You entered: %s", input);
   len = strlen(input);

    charp = strtok(input, "  ' '\t"); /* splits the string on delimiters space '' and tab*/
    while (charp != NULL) {
        printf("%s\n", charp);
        charp = strtok(NULL, "  ' '\t"); 
    }

   printf("The len of input = %d ", len);

   return(0);
}

`input == "exit"` does not do a string comparison but a comparison of the pointers to the first element. `"exit"` is a string literal and has a completely different address from the stack variable `input`. — flowit, Jan 23 '21 at 18:33

score 4 · Accepted Answer · edited Jan 23 '21 at 18:39

4

input == "exit"

Is wrong. It should be strcmp(input,"exit")==0, otherwise you compare the address of the inputted string to the address of the string "exit" in memory, not the contents.

Make sure you remove the newline from your fgets call first, or use strncmp(input, "exit", 4)

edited Jan 23 '21 at 18:39

Robert Harvey

178,213
47
333
501

answered Jan 23 '21 at 18:30

JCWasmx86

3,473
2
11
29

1

i just tried that with this line -> if(fgets(input, 512, stdin) == 0 || strcmp(input, "exit") == 0) { that still didn't seem to work any ideas why?? @JCWasmx86 – program.exe Jan 23 '21 at 18:33
3

Don't forget [Removing trailing newline character from fgets() input](https://stackoverflow.com/questions/2693776/removing-trailing-newline-character-from-fgets-input/28462221#28462221) – Weather Vane Jan 23 '21 at 18:33
2

@JamesTierney - use `strncmp(input, "exit", 4)` instead. – Bob Jarvis - Слава Україні Jan 23 '21 at 18:35
5

Our job here isn't to completely fix people's broken programs. – Robert Harvey Jan 23 '21 at 18:36
1

@BobJarvis-ReinstateMonica that worked thanks can you explain what this does im a bit of a beginner when it comes to C – program.exe Jan 23 '21 at 18:38
1

can anyone explain to me how the line below works: strncmp(input, "exit", 4) == 0 – program.exe Jan 23 '21 at 18:40
1

@JamesTierney It means, it compares the first `4` chars of the input with the second argument: `"exit"` – JCWasmx86 Jan 23 '21 at 18:41
1

The size `4` tells the function only to check 4 charcters, not the newline that follows. – Weather Vane Jan 23 '21 at 18:41
2

https://www.tutorialspoint.com/c_standard_library/c_function_strncmp.htm – Robert Harvey Jan 23 '21 at 18:43
@RobertHarvey your fix won't work if "exitt" is entered. – Weather Vane Jan 23 '21 at 18:48
1

Then just add `(strlen(input)==4&&(strncmp(input,"exit",4)==0))` @WeatherVane – JCWasmx86 Jan 23 '21 at 18:50
@JCWasmx86 there is a newline: the length is not 4. – Weather Vane Jan 24 '21 at 13:13

how to get program to terminate when user types in "exit" in C

1 Answers1