C++ Random String return unexpected results

Question

I am trying to use a function to generate a char[] :

char* randString(){
    const int len = 5;
    char s[len] = {0};

    static const char alphanum[] =
        "0123456789"
        "!@#$%^&*"
        "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
        "abcdefghijklmnopqrstuvwxyz";

    int stringLength = sizeof(alphanum) - 1;
    for (int i = 0; i < len; ++i) {
        s[i] = alphanum[rand() % stringLength];
    }

    return s;
}

the result is random and expected at return s

+       s   0x002df990 "XnQWp...    char[5]

however, when i look at the value here:

char* rand = randString();

it contains strange values:

+       rand    0x002df990 "XnQWpÌÌÌÌÌÌÌÌÌÌÌ\x5"    char *

did I do something wrong in the return?

@ApproachingDarknessFish what do you mean? @Mat how is char s[len] = {0} out of bounds? I thought that was how you are supposed to init things — Epicblood
– Epicblood, Commented Feb 8, 2015 at 7:39
possible duplicate of Can a local variable's memory be accessed outside its scope? — ApproachingDarknessFish
– ApproachingDarknessFish, Commented Feb 8, 2015 at 7:39
As a side note, this would be more accurately tagged C than C++. — flogram_dev
– flogram_dev, Commented Feb 8, 2015 at 8:11
@Epicblood yes, pass a buffer and its length (a size_t ideally) as input params and let the function sort it out, including setting a terminating null (which you didn't forget, right ?). — WhozCraig
– WhozCraig, Commented Feb 8, 2015 at 8:18

flogram_dev · Accepted Answer · 2015-02-08 08:10:00Z

2

You're declaring char s[] as a local variable. Local variables are destroyed when the function returns so returning a pointer to that variable returns a pointer pointing to junk data.

Fix this by allocating s dynamically:

char* s = new char[len];

Or you could pass a char* as a parameter and write your character to that array.

Just remember to add the terminating null character before returning.

edited Feb 8, 2015 at 8:10

answered Feb 8, 2015 at 8:03

flogram_dev

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1 Comment

David Haim Over a year ago

and remember to delete[] it in the end, don't forget

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