Conversion char[] to char*

Question

may be this is a sizzle question but please help

void Temp1::caller()
{
char *cc=Called();  
printf("sdfasfasfas");
printf("%s",cc);
}

char *Temp1::Called()
{
char a[6]="Hello";
return &a;
} 

Here how to print Hello using printf("%s",cc);

Answer 1

2 things

• You need %s, the string format specifier to print strings.

• You are returning the address of a local array variable a[6], it will be destroyed after the function returns. The program should be giving you a segmentation fault. You should be getting a crash. If you are on a linux machine do ulimit -c unlimited, and then run the program. You should see a core dump.

Answer 2

You are returning address of local variable, which exploits undefined behaviour. You need to make a static inside Called, or global, or allocate memory for it.

And use %s as format for printf

Answer 3

Firstly, this function:

char *Temp1::Called()
{
char a[6]="Hello";
return &a;
} 

is returning a local variable, which will cease to exist once the function ends - change to:

const char *Temp1::Called()
{
 return "Hello";
} 

and then, the way to print strings using printf() is to use "%s":

void Temp1::caller()
{
const char *cc=Called();  
printf("sdfasfasfas");
printf("%s",cc);
}

Answer 4

%c is for printing a single character. You need to use %s for printing a null terminated string. That said, this code is likely to crash as you are trying to return the address of the local variable a from function Called. This variable memory is released as soon as Called is returned. You are trying to use this released memory is Caller and in most of the cases it will crash.

Answer 5

Few things:

• Change return &a; to return a;
• You are returning the address of a local array which will cease to exist once the function return..so allocate it dynamically using new or make it static.
• use %s format specifier in printf in place of %c

Answer 6

Problem 1: %c is the specifier for a single char, while you need to use %s, which is the format specifier for strings (pointers to a NUL-terminated array of chars).

Problem 2: in Called you are returning a pointer to a pointer to char (a char **): a itself is considered a pointer to the first element of the array, so you don't need that ampersand in the return.

Problem 3: even if you corrected the other two errors, there's a major flaw in your code: you're trying to return a pointer to a local object (the array a ), which will be destructed when it will get out of scope (e.g. when Called will return). So the caller will have a pointer to an area of memory which is no longer dedicated to a; what happens next is undefined behavior: it may work for a while, until the memory where a was stored isn't used for something else (for example if you don't call other functions before using the returned value), but most likely will explode tragically at the first change in the application.

The correct method for returning strings in C is allocating them on the heap with malloc or calloc and return this pointer to the caller, which will have the responsibility to free it when it won't be needed anymore. Another common way to do it in C is to declare the local variable as static, so it won't be destructed at the return, but this will make your function non-reentrant neither thread-safe (and it may also give other nasty problems).

On the other hand, since you are using C++, the best way to deal with strings is the std::string class, which has a nice copy semantic so that you can return it as a normal return value without bothering about scope considerations.

By the way, if the string you need to return is always the same, you can just declare the return type of your function as const char * and return directly the string, as in

const char * Test()
{
    return "Test";
}

This works because the string "Test" is put by the compiler in a fixed memory location, where it will stay during all the execution. It needs to be a const char * because the compiler is allowed to say to every other piece of program that needs a "Test" string to look there, and because all the strings of the application are tightly-packed there, so if you tried to make that string longer you would overwrite some other string.

Still, in my opinion, if you are doing errors like those I outlined, it may be that you're trying to do something a bit too complex for your current skills: if may be better to have another look at your C++ manual, especially at the chapter about pointers and strings.

Answer 7

char a[6] is a local variable and you can not return it from function. It will be destroyed when your code will go out of scope.

You can use STL fot this:

#include <stdio.h>                                                                                                                                             
#include <string>                                                                                                                                              
using namespace std;                                                                                                                                       
string Called()                                                                                                                                            
{                                                                                                                                                          
    string a=string("Hello");                                                                                                                              
    return a;                                                                                                                                              
}                                                                                                                                                          

int main()                                                                                                                                                 
{                                                                                                                                                          
    string cc=Called();                                                                                                                                    
    printf("sdfasfasfas\n");                                                                                                                               
    printf("%s",cc.c_str());                                                                                                                               
}