-2

Got a small problem with C. Restricting myself to simple C (i.e. OS instructions), and two strings seem to not be the same. Here is my code:

    char inputData[256];
    int rid;
    rid = read(0,inputData,256);
    // Strip input
    char command[rid];
    int i;
    for (i = 0; i<=rid-2; i++) {
        command[i] = inputData[i];
    }
    command[rid-1] = '\0';
    if (command == "exit") {
        write(1,"exit",sizeof("exit"));
    }

Now, if a user enters "exit" into the terminal when queried and hits enter, the if for detecting "exit" never gets run. Any ideas?

Thanks,

EDIT: I am commiting to git as I go, so the current version can be found at github.com/samheather/octo-os. It's very obviously not complete code, but it demonstrates the problem.

Improve this question
2
  • 1
    Why's everyone downvoting my question? It may be obvious to you guys, but this stuff is a bit new to me at this low level and the question is not worded badly, nor is it easy to find the answer online about pointers to char arrays (believe me, I tried - google throws up all kinds of other stuff with the same keywords). Commented Oct 16, 2013 at 16:02
  • @TimCooper, possibly, I wasn't even thinking of this bug, but not completely as there's the pointer issue in this as well. Commented Oct 16, 2013 at 16:03

5 Answers 5

Reset to default
6

You can't compare strings with ==. You need to use strcmp.

if (strcmp(command, "exit") == 0) {

C strings are actually character arrays. You can think of "command" as a pointer to the first character. You want to compare every character in the string, not just the location of the first characters.

Improve this answer
Sign up to request clarification or add additional context in comments.

Comments

2

You should use strcmp to compare strings in C.

if(strcmp(command, "exit") == 0) //strcmp returns 0 if strings are equal

To quote:

A zero value indicates that both strings are equal. A value greater than zero indicates
that the first character that does not match has a greater value in str1 than in str2.
a value less than zero indicates the opposite.
Improve this answer

Comments

2

As it stands right now, you're comparing the address of command with the address of the string literal "exit", which pretty much can't be the same.

You want to compare the contents, with either strcmp, or (if "only OS instructions" means no standard library functions) an equivalent you write yourself that walks through the strings and compares characters they contain.

Improve this answer

7 Comments

This is something I wasn't certain of from reading the passage in my book - when I create an array of chars (say, called 'myArray'), myArray is just a pointer to the first char? Or have I got that mixed up? Which part is the pointer?
@SamHeather: Yes and no. myArray is an array, but in most situations, the name of an array will "decay" to the address of the first element of the array. The exceptions are when you use sizeof(array) or &array.
So, when it decays to the first item, and I do myArray[2], it knows to shift 2*sizeof(myArray[0]) further along in memory? Doesn't that mean the length of the string is not stored, it's just however far you go until you reach the \0 ?
@SamHeather: Yes -- correct on both counts.
@SamHeather The length on the string is available - but only at the level where you have access to myArray: there you can use sizeof myArray. If you subtract 1 (the space used by the NUL terminator), you have the length. And if you have an array containing arbitrary data, you'll have to pass the length along with the data to called functions, because the caller needs to know the length. If you have a string, which has no NUL byte in the middle, this length information can be omitted, because the callee can obtain it himself via strlen().
|
1

As others said, == doesn't work with strings. The reason is that it would compare the pointers given.

In the expression

command == "exit"

command is a pointer to your array variable, while "exit" is a pointer to that string which resides in read-only data space. They can never be identical, so the comparison always is false.

That's why strcmp() is the way to go.

Improve this answer

Comments

1

Use strcmp from the standard library.

Improve this answer

Comments

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.