Function return Char array C

Question

I try to return char array from function. I am new in C and try to learn function return value. This is my code:

int main()
{
unsigned int nr;
unsigned int mask=32;

char *outString;

printf("Enter Nr:\n");
scanf("%u",&nr);

outString = getBinary(nr,mask);
printf("%s",outString);
//getch();
return 0;
}

char * getBinary(int nr,int mask)
{
static char outPut[sizeof(mask)]="";
 while(mask>0)
{
 if((nr&mask)==0)
    {
        strcat(outPut,"0");
    }
    else
    {
        strcat(outPut,"1");
    }
    mask=mask>>1;
  }

//printf("%s",outPut);
return outPut;
}

I can't make program work! With two error on function call.

Answer 1

The major problem is, sizeof(mask) is not doing what you think it does. This is equivalent to sizeof(int) which is not what you want there.

You should better stick to a pointer and memory allocator function for this purpose.

FYI, you don't see issues currently with

 static char outPut[sizeof(mask)] "";

as sizeof is a compile time operator, so this outPut is not VLA. As soon as you try to change it to

static char outPut[mask] = "";

you'll face problems, as

• VLAs are local scope and incomplete type, static storage is not allowed.
• you cannot initialize VLAs.

Also, you must provide the prototype (forward declaration) to getBinary() if you intend to define it after main() .

Answer 2

you can change program like following:

#include <stdio.h>
#include <string.h>
char * getBinary(int nr,int mask); // add function header, it necessary to avoid compilation error 
//otherwise you can move getBinary function before your main function, because the compilator cannot recognize your function when it is defined after the call.
int main()
{
unsigned int nr;
unsigned int mask=32;

char *outString;

printf("Enter Nr:\n");
scanf("%u",&nr);

outString = getBinary(nr,mask);
printf("%s",outString);
//getch();
return 0;
}

char * getBinary(int nr,int mask)
{
static char outPut[sizeof(mask)]="";
 while(mask>0)
{
 if((nr&mask)==0)
    {
        strcat(outPut,"0");
    }
    else
    {
        strcat(outPut,"1");
    }
    mask=mask>>1;
  }

//printf("%s",outPut);
return outPut;
}