What would be the issue here with the pointer?

Question

What would be the issue here with the pointer?

I've been playing around with pointers and i have encountered my process terminating after it's done with the traditional windows "is not responding" and exit code -1073741819 (0xC0000005).

quickfix suggested by my IDE was to initialize the *ptr with __p__fmode().

However I read that, "The __p__fmode function is for internal use only, and should not be called from user code."

If you would be so kind to help me with this problem pretty please :)

#include <iostream>
    using namespace std;
    // Code written in Jetbrains's CLion with mingw-w64 compiler


int main() {
    int r, c, *ptr = __p__fmode(); // No idea what's that __p__fmode(), but without it process terminates
    // with code "Process finished with exit code -1073741819 (0xC0000005)"
    cout<<"Enter number of rows for Matrix A:";
    cin>> r;
    cout<<"Enter number of columns for Matrix A:";
    cin>> c;
    int A[r][c];
    A[0][0] = 0;
    A[0][1] = 1;
    A[1][0] = 2;
    *ptr = A[0][0];
    cout<<*(*(A+0)+0)<<"\t";
    cout<<*(*(A+0)+1)<<"\t";
    cout<<*(*(A+1)+0)<<"\t";

}

c++

arrays

pointers

matrix

multidimensional-array

asked on Stack Overflow Feb 26, 2021 by

Reneq

2 Answers

However I read that, "The __p__fmode function is for internal use only, and should not be called from user code."

If you would be so kind to help me with this problem pretty please :)

The solution is to not call the function. In general, don't call functions unless you know what they do.

P.S.

cin>> r;
cin>> c;
int A[r][c];

The size of an array must be constant at compile time. The example program is ill-formed in C++.

answered on Stack Overflow Feb 26, 2021 by

eerorika

When ptr is uninitialised, the statement

*ptr = A[0][0];

has undefined behaviour. One possible (not guaranteed) consequence of undefined behaviour is an abnormal program termination - such as you are seeing.

The function __p__fmode() returns a pointer to a global variable according to Microsoft documentation. The function returns an int * which suggests (but does not guarantee) that the global variable involved as type int.

So, initialising ptr with the value returned by __p__fmode() means that the assignment *ptr = A[0][0] overwrites that global variable.

The Microsoft documentation also states "The __p__fmode function is for internal use only, and should not be called from user code.". Which means using it, as you are, is inadvisable.

There is a secondary concern (at least, one you haven't asked about) with your code - a definition int A[r][c]; where r and c are variables, is not valid C++. If your compiler allows this, then it is as a non-standard extension.

answered on Stack Overflow Feb 26, 2021 by

Peter

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