Returning char array pointer

[BWiz]

Hi!

In an effort to expand my C++ knowledge, I'm writing a little application that generates a random string of lowercase characters, and then returns a pointer that that array. I've got a majority of the app working, it generates the character array fine, but when I try to read the character array outside of the function, I get a bunch of jargon characters.

Here is my code:

/*
	 	 * Create a random string
	 	 */
	 	char* randfname() {
		 	int len = 16;				// len of str 
		 	srand(time(0));				// to get a true random number
		 	char fname[len];			// char arr that stores file name
	 		char* ptr = fname;			// pointer to fname
		 	for(int i = 0; i < len; i++) {
			 	int ascii = (rand() % 25) + 97;		// Randomally generate ascii lower case
			 	*(ptr+i) = ascii;
//			 	fname[i] = ascii;
		 	}
		 	for(int i = 0; i < len; i++) {			// Test print char arr w/ ptr
			 	cout << *(ptr+i);					// Works here
		 	}
	 		return ptr;
	 	}

...

/*
	 	 * Test function to ensure that randfname()
	 	 * is working.
	 	 */
	 	 void testrandfname() {
		 	 char* ptr = randfname();	// call randfname
		 	 cout << endl;
		 	 for(int i = 0; i < 16; i++) {
			 	 cout << i << ": " << *(ptr+i) << endl;
		 	 }
	 	 }

When I print out the array in the randfname() function, it works fine, however, when I pass the pointer to the testrandfname() function and I attempt to print out the array, it doesn't work and I get rubbish data.

I think there is an error where I assign the char* ptr to the pointer returned from randfname() but I'm not sure.

I've tried several different things, but I always arrive at printing rubbish data. Could anybody help me properly read the character array that is returned?

Thanks in advice!
BWiz

[laserlight]

The problem is that ptr points to the first element of the local array named fname. When control returns from this function, the array is destroyed, so the caller gets a pointer to an element that no longer exists.

[BWiz]

Thanks for the response laserlight.

So if the array is destroyed, how would I save the array so that other functions could read it? And how would I point to that array?

[laserlight]

Use and return a std::vector<char> instead.

Oh, incidentally, you were using a compiler extension earlier: fname is actually a variable length array. If you really intend a fixed size array, you could consider a std::tr1::array<int, 16>, but a TR1 implementation may not be available for you to use.

[BWiz]

Actually, if I passed a pointer to the array (which is set by the calling function) to randfname, and the length of the array, the array would be preserved right? And I could use that array in the calling function right?

[laserlight]

Actually, if I passed a pointer to the array (which is set by the calling function) to randfname, and the length of the array, the array would be preserved right? And I could use that array in the calling function right?

Yes.

[BWiz]

Great! Thanks a lot laserlight!

[treuss]

For completeness: Another "solution" would be to declare fname as static, thus it would continue to exist after the function exited.

Solution is written in quotes, as this approach imposes a complete new set of problems, i.e. dependencies between multiple calls to this function and the unability to use this function in multi-threaded program. Therefore the "For completeness".