Function would not return 0, cant find error in logic

Question

I am not sure why the following code isnt working.

I try to use gcc to compile it gcc -Wall -o test test.c, it gives me warning message: control reaches end of non void function.

But I couldn't find the error so I tried to compile it ignoring the warning messages, and tested the program out using ./test 1234321 after compiling it using gcc -o test ispalidrome.c

But instead of returning 0 (that it should to my understanding because it didn't trigger any of the if statements to return 1), it returned a random value, 46, 52, 84 etc.

The main logic behind the program is exactly as required, I just can't tell why it isn't exiting properly.

#include <stdio.h>
int main(int argc, char **argv){
    if (argc != 2){
        fprintf(stderr,  "Usage: %s \"string\"", argv[0]);
        return 1;
    }
    else{
        int i = 0;
        while( '\0' != argv[1][i] ) i++;
        char *start=argv[1];
        char *end=&start[i-1];

        while (start<end){
            if (*start != *end){
                break;
                return 1;
            }
            printf("%c is the same as %c\n",*start, *end);
            start =start+1;
            end=end-1; 
        }
    }
}

Note that your code will never return 1, because the line before `return 1;` is `break;` which jumps to just after the loop. — user253751, Feb 25 '14 at 06:20
@immibis Well it *will* `return 1` if `argc != 2`. But yes, that spot is most likely a mistake. — Jonathon Reinhart, Feb 25 '14 at 06:20
specify `std=c99` then you don't need a `return 0` at the end — phuclv, Feb 25 '14 at 06:31

score 7 · Accepted Answer · edited May 23 '17 at 12:20

7

Your main function is very obviously missing a return 0 at the end.

The random value it is returning is whatever happens to be in the return value register (eax on x86) when main returned.

Never turn off -Wall, and never ignore the warnings. They are your friend.

As Jim Balter and Lưu Vĩnh Phúc mentioned, the return 0 is optional in C99.

edited May 23 '17 at 12:20

Community

1
1

answered Feb 25 '14 at 06:15

Jonathon Reinhart

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Many warnings are unnecessary. Always using `-Wall` might be a good idea for finding issues in production code, but for education it is likely to just add confusion. – user253751 Feb 25 '14 at 06:18
1

@immibis I disagree. `-Wall -Werror` sets good habits for beginners. I wish `-Wall` were the default. – Jonathon Reinhart Feb 25 '14 at 06:19
I also disagree. The warnings may not always be important to beginners, but they are often the cause of subtle programming flaws that will cripple complex programs. – dckuehn Feb 25 '14 at 06:19
I did say they should be turned on when making complex programs (which is what I meant by production code). But it is usually better to learn from things not working than to learn from compiler warnings. – user253751 Feb 25 '14 at 06:23
@immibis Still disagree, but to each his own. It doesn't really matter because the OP had a clear error message and it still didn't help. – Jonathon Reinhart Feb 25 '14 at 06:24
1

`return 0` isn't required in C99 or later. – Jim Balter Feb 25 '14 at 06:31
@JimBalter Woah, what? Citation, please. – Jonathon Reinhart Feb 25 '14 at 06:32
No, you take the 10 seconds to google it if your knowledge of C is that poor. – Jim Balter Feb 25 '14 at 06:32
@JimBalter I was already googling it. I can honestly say I feel foolish for not knowing that (I work strictly in a C89 environment), but I'm mostly surprised they introduced such a feature. – Jonathon Reinhart Feb 25 '14 at 06:34
"They" adopted it from the 1998 C++ standard, so this probably originates from Stroustrup. – Jim Balter Feb 25 '14 at 06:38
Such an obvious problem! I cant believe I looked at this thing for an hour and wasnt able to find it! thank you guys~ – Nick Feb 25 '14 at 06:47
@NickGong Not a problem. And some of us learned something from it. – Jonathon Reinhart Feb 25 '14 at 06:49

score 1 · Answer 2 · answered Feb 25 '14 at 06:18

You're method declaration specifies that it returns an int

int main(int argc, char **argv){

You do correctly return an int at the end of your if block when you do

return 1;

but at the end of your else block there is no int returned. You can return any integer, but 0 is common for programs that naturally terminate, or you can specify a different integers to specify different exit situations.

score 1 · Answer 3 · answered Feb 25 '14 at 06:48

1

Add line return 0 at the end of code. It will return proper value.

answered Feb 25 '14 at 06:48

Amit

81
5

Function would not return 0, cant find error in logic

3 Answers3