How to solve "control may reach end of non-void function" in C?

Question

I am a beginner of C, so I might not understand all the terms. The is a program meant to convert some plain text into ciphered text through vigenere. It's the thrid problem here :http://cdn.cs50.net/2016/x/psets/2/pset2/pset2.html

However, I keep having the warning "control may reach end of non-void function" from the CS50 IDE. Thanks for helping out!

#include <stdio.h>
#include <stdlib.h>
#include <cs50.h> 
#include <math.h>
#include <string.h>
#include <ctype.h>

string k = "";
int testalpha ();
int main(int argc, string argv[])
{
    k = argv[1];
    testalpha ();
   if (argc !=2 || testalpha(1))
    {
        printf("Please retry.");
        return 1;
    }
    string p= GetString();
    for (int i=0;i<strlen(p); i++)
            {   int c [50];
                int a=0;
                if (isupper(p[i]))
                {   
                    c [i] =  (((int)p[i] + k[a] -65) % 26)+65;
                    printf ("%c", c[i]);
                    a+=1;
                    if (a==strlen(k))
                    {a=0;}
                    continue;
                }
                if (islower(p[i]))
                {
                 c [i] =  (((int)p[i] + k[a]-97) % 26)+97;
                    printf ("%c", c[i]);
                    if (a==strlen(k))
                    {a=0;}
                    continue;
                }
                if (isalpha(p[i])==0)
                {
                    c [i] = p[i];
                    printf ("%c", c[i]);
                }


            }
            printf("\n");
}

int testalpha (void)
{
    for (int x=0; x<strlen(k); x++)
        {
            if(isalpha(k[x])==0)
            {   
            return 1;
            }   
            else 
            {
            return 0;
            }
        }
}

Maybe you should return something from your `main` function? — Some programmer dude, Oct 21 '16 at 14:07
I think your `main()` function can end without returning an `int`. — Tim Biegeleisen, Oct 21 '16 at 14:07
By the way, your logic in the `testalpha` function is flawed. — Some programmer dude, Oct 21 '16 at 14:08
_Aside:_ Please choose a formatting style and stick with it throughout your code, as it helps readers to review and spot errors easier. This code is mind numbing to read, with all of its spacing inconsistencies. — Oka, Oct 21 '16 at 14:13

score 2 · Answer 1 · answered Oct 21 '16 at 14:07

Look carefully at the function int testalpha (void).

It doesn't return a value on all control paths: for example if strlen(k) is zero.

That's undefined behaviour in C: aside from main, if a function has a return type other than void, then you must return a value.

How to solve "control may reach end of non-void function" in C?

1 Answers1