How do I transfer const char* from function to function?

Question

Here is an example of what I am trying to do:

#include <stdio.h>

FILE* f;
const char* getstring()
{
    f = fopen ("hello.txt", "r");
    char x[200];
    for (int i = 0; i < 200; i++) x[i] = 0;
    for (int c = getc(f), i = 0; (c != EOF) && (i < 200); c = getc(f), i++)
        x[i] = c;
    fclose(f);
    printf ("getstring(): x = %s", x);
    const char* y = x;
    printf ("getstring(): y = %s", y);
    return y;
}

void printstring (const char* string)
{
    printf ("%s", string);
}

int main()
{
    printstring(getstring());
    printf ("\nprintf: %s", getstring());
    return 0;
}

and the output is:

getstring(): x = Hello World
getstring(): y = Hello World
���getstring(): x = Hello World
getstring(): y = Hello World

printf: ��������

I don't know why the printstring() function is outputting nothing and printf is outputting random data or why there is a bit of random data at the end of the string when I use the printstring() function.

Is there any way to fix this and what am I doing wrong?

Answer 1

The problem

The problem is that getstring() returns a pointer to a local array. This array gets destructed when the function returns, so you have an dangling pointer. Using this pointer is then undefined behavior. Anything can happen: for example you can get garbage random value, you can get the old unchanged value, or the system could crash.

The solution

Since this question is labelled c++, just use std::string instead of char*, and this kind of nightmare will vanish for good.

Note that Using std::string in prinf() would require you get a pointer with .c_str().

If for an obscure reason, you are required to use char* you'd have to to use strdup() or llocate some memory for the c string and return a pointner to that memory. But the caller must then delete this pointer if you don't want memory to leak.

Answer 2

The C string is stored in a function local char array. This array is destroyed when the function is left. Since the question is tagged as C++ use std::string

#include <iostream>
#include <string>
#include <fstream>

std::string getstring()
{
    std::ifstream f("hello.txt");
    std::string x;
    x.resize(200);
    for (int i = 0; i < 200; i++) x[i] = 0;
    for (int c = f.get(), i = 0; (c != EOF) && (i < 200); c = f.get(), i++)
        x[i] = c;
    std::cout << "getstring(): x = " << x;
    const std::string& y = x;
    std::cout << "getstring(): y = " << y;
    return x;
}

void printstring (const std::string& string)
{
    std::cout << string;
}

int main()
{
    printstring(getstring());
    std::cout << "\nprintf: " << getstring();
    return 0;
}