Returning an array from function

Question

I am writing a function which takes in the integer value and the pointer to a character. the function converts the integer value into binary and stores it in the char pointer. the char pointer is 16 bytes long.

Snippet of code:

void int2bin(u_int16_t addr_IP, char *Binary)
{
    int count;
    printf("IP1add = %d \n", Binary);
    for (count = 0; count < 16; count++) {
        if(addr_IP>0)
            *(Binary + 15-count) = addr_IP & 0x1 ? '1':'0';
        else
            *(Binary + 15-count) = '0';

          addr_IP>>=1;
    }   
}

int main(int argc, char *argv[])
{
    u_int16_t senderIP_16[], u_int16_t receiverIP_16[];
    char  sender_IP_hi[16], sender_IP_low[16];
    int2bin(senderIP_16[0], &sender_IP_hi);
    int2bin(senderIP_16[1], &sender_IP_low);
}

In the first call to the function, it returns correct values. But in the second pass, the value of first pass is appended to the second pass, i.e length of sender_IP_low becomes 32.

How can I resolve this?

Thanks

Few things: 1/ what is that comma in the first line of main? 2/ why the length of arrays are not specified? — Peyman
– Peyman, Commented Feb 8, 2011 at 3:58

Ateş Göral · Accepted Answer · 2011-02-08 03:55:47Z

3

It looks like you're printing sender_IP_low as a string, and since it is not null-terminated, the print routine continues to print the adjacent buffer, sender_IP_hi. And you're probably just lucky that the print routine finds a zero and stops before a segmentation fault.

One quick fix is:

void int2bin(u_int16_t addr_IP, char *Binary) {
    ...

    Binary[16] = 0; // terminate the string before returning
}

...

char  sender_IP_hi[17], sender_IP_low[17]; // +1 for null terminator

Although, there are a few other things that could be fixed in your implementation, I just wanted to focus on an answer to your original question.

answered Feb 8, 2011 at 3:55

Ateş Göral

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2 Comments

Ritesh Over a year ago

Thanks Ates! It worked! You mentioned about fixing other things in the code. Could you pls tell me what else could be done to improve the implementation. Thanks!

Ateş Göral Over a year ago

@Ritesh Banka: You could simply increment the Binary pointer instead of offsetting it with a counter; and use a bitmask that starts at 0x8000 and gets right-shifted each iteration so that you can output bits hi-to-low instead of using the 15 - count trick. Anyway, your code is functionally correct and what I'm suggesting is just a matter of personal taste regarding style.

Murilo Vasconcelos · Accepted Answer · 2011-02-08 03:58:00Z

If you are printing the arrays with printf():

void int2bin(u_int16_t addr_IP, char *Binary)
{
    int count;
    printf("IP1add = %d \n", Binary);
    for (count = 0; count < 16; count++) {
        if(addr_IP>0)
            *(Binary + 15-count) = addr_IP & 0x1 ? '1':'0';
        else
            *(Binary + 15-count) = '0';

          addr_IP>>=1;
    }   
    // Put the NULL char in the last position
    Binary[16] = '\0';
}

int main(int argc, char *argv[])
{
    u_int16_t senderIP_16[], u_int16_t receiverIP_16[];
    // One more char for storing the terminator character
    char  sender_IP_hi[17], sender_IP_low[17];
    int2bin(senderIP_16[0], &sender_IP_hi);
    int2bin(senderIP_16[1], &sender_IP_low);
}

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