Question

This is my code:

char *genWord(int wordLen){
    char array[wordLen + 1];
    char *word;
    int i;

    for(i = 0; i <= wordLen; i++){
        array[i] = 'a';
    }

    array[wordLen] = '\0';

    //Test1 printf
    printf("%s \n", array);

    word = array;

    //Test 2
    printf("%s \n", word);

    return word;
}

main(){
    char *word;
    int wordLen = 10;

    word = (char *)genWord(wordLen);

    //Test 3
    printf("%s", word);       
}

And this is my output of 3 (same prog) executes in Linux gcc:

1st:

aaaaaaaaaaaaaaaaaaaaa 
aaaaaaaaaaaaaaaaaaaaa 
aaaaaaaaaaaaT��aaaaa

2nd:

aaaaaaaaaaaaaaaaaaaaa 
aaaaaaaaaaaaaaaaaaaaa 
aaaaaaaaaaaa�ƃ�aaaaa 

3rd:

aaaaaaaaaaaaaaaaaaaaa 
aaaaaaaaaaaaaaaaaaaaa 
aaaaaaaaaaaa����aaaaa   

Can’t figure out what’s wrong and even worse, can’t imagine how can i get a different output on every run? variable word can’t have garbage in a part of the string right?!

Answer 1

You are returning local address, which is not anymore guaranteed to be allocated after the return from call. Therefore it is an undefined behaviour.

When you do word = array the local address of the array is copied into word variable. After return the calling function has the address, which was the local address in the function. But after the function return the address is not anymore valid, and may be allocated for other purpose, or overwritten or anything.

Do word = strdup (array); OR

word = malloc (sizeof (char) * (strlen (array) + 1));
strcpy (word, array);
return word;