Taking user input with getline() and comparing to another string in a while loop

Question

I am using the code below to take in user input and then compare it with the string exit so that users can exit out of a game by typing that into the command line. However it doesn't work unless I use *buffer in the comparison with exit. It does that because it points to the first letter in exit. The problem is that means any word starting with e will cause my while loop to exit.

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

int input( char *s, int length);

int main(){

    char *buffer;
    size_t bufsize = 32;
    size_t characters;

    do{
        buffer = (char *)malloc(bufsize * sizeof(char));

        if(buffer == NULL){
            perror("Unable to allocate buffer");
            exit(1);
        }

        printf("Enter a command: ");
        characters = getline(&buffer, &bufsize, stdin);
        printf("%zu characters were read.\n", characters);
        printf("You typed: '%s' \n",buffer);

    }while(buffer != "exit");
}

buffer != "exit" -> (strcmp(buffer, "exit")). Please use a search engine to look up 1. why you should now cast the return value of malloc() 2. How to declare the main() function properly. — babon
– babon, Commented Jan 21, 2017 at 4:26
getline() stores the newline in the buffer, so you'll need to either include that in your comparison string, or chomp it off. — Dolda2000
– Dolda2000, Commented Jan 21, 2017 at 4:37
Possible duplicate of How do I properly compare strings in C? — Govind Parmar
– Govind Parmar, Commented Jan 21, 2017 at 7:16

Govind Parmar · Accepted Answer · 2017-01-21 13:11:51Z

1

buffer != "exit" -> (strcmp(buffer, "exit\n"))

edited Jan 21, 2017 at 13:11

Govind Parmar

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answered Jan 21, 2017 at 4:37

Nazzu

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Comments

Govind Parmar · Accepted Answer · 2017-01-21 07:16:18Z

0

Replace

while(buffer != "exit");

with

while(strcmpi(buffer, "exit"));

In C, you can't check logical equality of string with == or != operators. You need to use functions like 'strcmp' or strcmpi.

edited Jan 21, 2017 at 7:16

Govind Parmar

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answered Jan 21, 2017 at 4:30

VHS

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2 Comments

chux Over a year ago

Note: strcmpi() is not a standard C function.

VHS Over a year ago

@chux, yes, that's why I mentioned both strcmp and strcmpi as options. In case strcmpi is not available in OP's C library, he can use strcmp. Worth noting nevertheless.

RoadRunner · Accepted Answer · 2017-01-21 05:29:26Z

If you compare strings with ==, you are only comparing the base address of the strings, and not the actual contents of the strings. Use strcmp from <string.h> to compare strings. You while statement should be this instead:

while (strcmp(buffer, "exit"));

Note: You should also compile with -Wall -Wextra in your code, and you would have seen some warnings with using == to compare strings.

However, since getline appends a \n at the end of the buffer, similarily to fgets, you might need to remove this \n character. This would allow your code to work as intended. Otherwise, your code is actually doing this:

while (strcmp("exit\n", "exit"));

which will not allow the loop to terminate when "exit" is found, as strcmp() only returns 0 for equal strings.

You also need to check errors with getline(), as it returns -1 on failure to read a line. Their is also no need to allocate memory with malloc() in this case, as getline() stores the address of the buffer within *buffer. The function getline() basically allocates memory for you, so doing it yourself in this case would be redundant. Having said this, you do need to free()this buffer at some point. See this man page for more details.

Considering these points, you can construct your code like this:

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

int main(void ){
    char *buffer = NULL;
    size_t buffsize;
    ssize_t read = 0;
    const char *exits = "exit";

    while (1) {
        printf("Enter a command: ");

        read = getline(&buffer, &buffsize, stdin);
        if (read == -1) {
            printf("Failure found from getline()\n");
            exit(EXIT_FAILURE);
        }

        if (read > 0 && buffer[read-1] == '\n') {
            buffer[read-1] = '\0';
        }

        if (!*buffer || strcmp(buffer, exits) == 0) {
            break;
        }

        printf("%zu characters were read.\n", read);
        printf("You typed: '%s' \n", buffer);

        free(buffer);
        buffer = NULL;
    }

    return 0;
}

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