Segmentation fault. gdb does not give me a clue

[dbachito]

Hi there, I have a segmentation fault caused by a function. I display the code below. Any suggestions as to why this might be happening? Thanks a million!

#include <iostream>
#include <ctime>
#include <cstdlib>


int * aRand(const int &nL_bound, const int &nU_bound, const int &nSize, const unsigned int &nSeed = time(NULL)) {
//return a pointer to an array of size N of p-random numbers.
//parameters: boundaries, size and seed (by default set to time(NULL).


//initialise seed
	srand(nSeed);

//declare range and array to store p-random numbers.
	int nRange = nU_bound - nL_bound + 1;

	int * pnRandom = new int[nSize];

//loop over size to assign random number to each element of array.
	for (int i = 0; i < nSize; i++) pnRandom[i] = nL_bound + (rand()/100%nRange);

	delete[] pnRandom;

//line 24  pnRandom = 0 //???
	return pnRandom;
}

//*******test driver

int main () {

	int * p;

	p = aRand(1,100,10);

	for ( int i = 0; i < 10; i++ ) {
		std::cout << "*(p + " << i << ") : ";
		std::cout << *(p + i) << std::endl;
	}

	return 0;
}

The code as it is will produce always a zero in the first printed element. If I leave uncommented pnRandom = 0; on line 24 then it will return a segmentation fault (core dumped).
Both versions compile without any warning message.

When I use gdb to debug, it does not tell me useful info as to where the problem is.

Any hints?

Many thanks!

[Peter87]

valgrind gives a somewhat more helpful error message.

valgrind wrote:

Invalid read of size 4
at 0x400F1E: main (b.cpp:38)
Address 0x0 is not stack'd, malloc'd or (recently) free'd

aRand returns a pointer to memory that you have freed using delete[], or a null pointer if line 24 is uncommented. Trying to dereference that pointer, like you do on line 38, is an error.

[dbachito]

Thank you so much Peter87. I also notice that valgrind is a better tool, I will try it.

My intention with the expression delete[] inside the function was trying to avoid a "memory leak", so that that bit of dynamically allocated memory could not be accessed or reallocated or deleted afterwards.

I have deleted that line and added it inside the test driver. Now it seems to give right values every time I run the program. I am pasting the tweaked code here:

#include <iostream>
#include <ctime>
#include <cstdlib>


int * aRand(const int &nL_bound, const int &nU_bound, const int &nSize, const unsigned int &nSeed = time(NULL)) {
//return a pointer to an array of size N of p-random numbers.
//parameters: boundaries, size and seed (by default set to time(NULL).


//initialise seed
	srand(nSeed);

//declare range and array to store p-random numbers.
	int nRange = nU_bound - nL_bound + 1;

	int * pnRandom = new int[nSize];

//loop over size to assign random number to each element of array.
	for (int i = 0; i < nSize; i++) pnRandom[i] = nL_bound + (rand()/100%nRange);

	return pnRandom;
}

//*******test driver

int main () {

	int * p;

	p = aRand(1,100,10);

	for ( int i = 0; i < 10; i++ ) {
		std::cout << "*(p + " << i << ") : ";
		std::cout << *(p + i) << std::endl;
	}

	delete[] p;

	return 0;
}

Does it seem more correct? Is the delete[] p; in line 38 appropriate?

Thanks again for the advice... MUCH appreciated!!

[Peter87]

The program seems to work OK but there is no need passing the int arguments by reference to the function. If you don't want to worry about delete[] you could use a vector instead.
http://www.cplusplus.com/reference/vector/vector/

[dbachito]

Magnificent, that's brilliant! I need to get to grips with vector though. Thanks and have a nice day Peter87!