How do you return a string from a function correctly in Dynamic C?

Question

I have a program I am trying to debug, but Dynamic C apparently treats strings differently than normal C does (well, character arrays, anyway). I have a function that I made to make an 8 character long (well, 10 to include the \0 ) string of 0s and 1s to show me the contents of an 8-bit char variable. (IE, I give it the number 13, it returns the string "0001101\0" )

When I use the code below, it prints out !{happy face] 6 times (well, the second one is the happy face alone for some reason), each return comes back as 0xDEAE or "!\x02.

I thought it would dereference it and return the appropriate string, but it appears to just be sending the pointer and attempting to parse it. This may seem silly, but my experience was actually in C++ and Java, so going back to C brings up a few issues that were dealt with in later programming languages that I'm not entirely sure how to deal with (like the lack of string variables).

How could I fix this code, or how would be a better way to do what I am trying to do (I thought maybe sending in a pointer to a character array and working on it from the function might work, but I thought I should ask to see if maybe I'm just trying to reinvent the wheel).

Currently I have it set up like this:

this is an excerpt from the main()

display[0] = '\0';
for(i=0;i<6;i++)
{
     sprintf(s, "%s ", *char_to_bits(buffer[i]));
     strcat(display, s);
}
DispStr(8,5, display);

and this is the offending function:

char *char_to_bits(char x)
{
     char bits[16];
     strcpy(bits,"00000000\0");
     if (x & 0x01)
         bits[7]='1';
     if (x & 0x02)
         bits[6]='1';
     if (x & 0x04)
         bits[5]='1';
     if (x & 0x08)
         bits[4]='1';
     if (x & 0x10)
         bits[3]='1';
     if (x & 0x20)
         bits[2]='1';
     if (x & 0x40)
         bits[1]='1';
     if (x & 0x80)
         bits[0]='1';
     return bits;
}

and just for the sake of completion, the other function is used to output to the stdio window at a specific location:

void DispStr(int x, int y, char *s)
{
     x += 0x20;
     y += 0x20;
     printf ("\x1B=%c%c%s", x, y, s);
}

Answer 1

char * func()
{
  char *str = malloc (string_size * sizeof(char));
  strncpy (str, "Hello World", string_size);
  return str;
}


main()
{
  char *str = func();
  printf ("%s\n", str);
  free(str);
}

Answer 2

Your problem is that char bits[16]; is local to the char_to_bits function, so on return it is pointing to an invalid stack location. You should pass in the buffer you want filled with the string. Like so:

char * char_to_bits(char x, char * bits)
{
     strcpy(bits,"00000000\0");
     ...
     return bits;
}

and the call like so:

 char bits[16];
 sprintf(s, "%s ", char_to_bits(buffer[i], bits));

In this regard, Dynamic C (this?) is behaving just like C here. It's a common mistake made by people used to Java/C++ strings. A C "string" is merely pointer to data, not an object.

Answer 3

The problem might be the fact that your bits[] array from char *char_to_bits(char x) is declared locally, declare it globally.

Please try this program, I tested it using both Mingw and Borland/CodeGear compilers and it works:

#include <stdio.h>
#include <string.h>

using namespace std;

//Declare bits[] globaly
char bits[16];

//Your char to bit funtion
char* char_to_bits(char x)
{

     strcpy(bits,"00000000\0");
     if (x & 0x01)
         bits[7]='1';
     if (x & 0x02)
         bits[6]='1';
     if (x & 0x04)
         bits[5]='1';
     if (x & 0x08)
         bits[4]='1';
     if (x & 0x10)
         bits[3]='1';
     if (x & 0x20)
         bits[2]='1';
     if (x & 0x40)
         bits[1]='1';
     if (x & 0x80)
         bits[0]='1';
     return bits;
}

int main()
{
    //Testing char_to_bits() by writing word "Test" in binary

    char test[] = "Test";

    for(int i = 0; i < strlen(test); i++)
    {
        printf("%s ", char_to_bits(test[i]));
    }

    return 0;
}

Answer 4

Here are the problems I can see:

• Function char_to_bits returns the char array bits which is local. So its memory will be lost once the function returns. To solve this allocate memory for bits dynamically using malloc and later free it using the function free or you can make it static or declare bits globally.

• You need not dereference the call to char_to_bits in the sprintf. So change
  sprintf(s, "%s ", *char_to_bits(buffer[i]));

  to

  sprintf(s, "%s ", char_to_bits(buffer[i]));