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, Feb 08 '15 at 07:39
possible duplicate of [Can a local variable's memory be accessed outside its scope?](http://stackoverflow.com/questions/6441218/can-a-local-variables-memory-be-accessed-outside-its-scope) — ApproachingDarknessFish, Feb 08 '15 at 07:39
@Mat, the second line is a variable definition, not an assignment — Andras, Feb 08 '15 at 07:51
@Epicblood, return strdup(s) instead (and remember to free(s) in the caller when no longer needed) — Andras, Feb 08 '15 at 07:52
@ApproachingDarknessFish so you mean i should pass in a `char*` and modify that? — Epicblood, Feb 08 '15 at 08:03
As a side note, this would be more accurately tagged C than C++. — Emil Laine, Feb 08 '15 at 08:11
@ApproachingDarknessFish now the last val in the `char[]` is always `-52 'Ì'` — Epicblood, Feb 08 '15 at 08:13
@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, Feb 08 '15 at 08:18
it's c++, take a look at the alternative and safer ways: [rand() considered harmful](http://channel9.msdn.com/Events/GoingNative/2013/rand-Considered-Harmful) — Dmitry Ledentsov, Feb 08 '15 at 08:26

Emil Laine · Accepted Answer · 2015-02-08T08:10:00.720

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 08 '15 at 08:10

answered Feb 08 '15 at 08:03

Emil Laine

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and remember to delete[] it in the end, don't forget – David Haim Feb 08 '15 at 14:06

C++ Random String return unexpected results

1 Answers1