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.

Implicit declaration of function. And assignment make pointer rom integer with out a casting. — Ali Abdulhussein
– Ali Abdulhussein, Commented Nov 14, 2016 at 10:19
Try to put a prototype char * getBinary(int nr,int mask); before your main — tdao
– tdao, Commented Nov 14, 2016 at 10:19
@AliAbdulhussein: Well, it probably works as long as you put no more than 3 digits in the outPut array (assuming sizeof(int) is 4 on your machine. You should make it larger. Also, you will see a problem if you call getBinary more than once, because the new number's digits will be appended to the old number's digits. You should set output[0] = '\0'; before the loop. — Ian Abbott
– Ian Abbott, Commented Nov 14, 2016 at 10:36

Sourav Ghosh · Accepted Answer · 2016-11-14 10:31:10Z

2

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().

edited Nov 14, 2016 at 10:31

answered Nov 14, 2016 at 10:20

Sourav Ghosh

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6 Comments

Gopi Over a year ago

Returning a local variable(array) outPut leads to UB even if it is static?

Gopi Over a year ago

His problem is he doesn't have a prototype for the function getBinary()

Sourav Ghosh Over a year ago

@Gopi there's a bigger one, hidden,

Sourav Ghosh Over a year ago

@Gopi also, what makes you think that sizeof(mask) is not an issue?

Gopi Over a year ago

When did I say it is not an issue?

|

Embedded Software Engineer · Accepted Answer · 2016-11-14 10:29:53Z

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;
}

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