Segmentation Fault in Vertical Redundancy Check - c

Question

I am trying to make program in C for vertical redundancy check. The Code is given below :
#include<stdio.h>
#include<sys/types.h>
#include<fcntl.h>
#include<unistd.h>
int main()
{
int fd,i;
char *data = "01010101010111110101010101011111";
int count = 0,bit_count=0;
char *parity_bit_array = NULL;
char *data_to_send = NULL;
char *stream_name = "Named Stream";
do
{
if(*data == '1' && bit_count <= 8)
{
count++;
if(bit_count == 8)
{
if( count % 2 == 0)
{
*parity_bit_array = '1';
count = 0;
bit_count = 0;
}
else
{
*parity_bit_array = '0';
count = 0;
bit_count = 0;
}
}
}
bit_count++;
data++;
} while( !data);
do
{
if(bit_count <= 8)
{
*data_to_send++ = *parity_bit_array++;
}
*data_to_send++ = *data;
} while( !data );
printf("%s \n",data_to_send);
mkfifo(stream_name,0666);
fd = open(stream_name,O_WRONLY);
write(fd,data_to_send,sizeof(data_to_send));
close(fd);
unlink(stream_name);
return 0;
}
The file shown below is the sender file of which data is to be read by the receiver.
By using sized array it is working properly but i like to use it with Pointer.
Main Variables in this code :
data : Data on which VRC to be implemented
count : Counting 1 for Even Parity Bit
bit_count : Counting 8 Bits
parity_bit_array : To Collect Parity Bit for Every Single Byte present in data
data_to_send : Combination made by data + parity_bit_array
Ex:
data : 01110000
parity_bit_array : 1
data_to_send : 011100001

Answer 1

You are not allocating memory for your char pointers and you are trying to write to them which will lead to undefined behavior hence segmentation fault.
*parity_bit_array = '1';
There are multiple such cases in this code.
char *data_to_send = NULL;
data_to_send pointer is never allocated memory and you try to write to it
*data_to_send++ = *parity_bit_array++;
Allocate memory to char pointers like
char *data_to_send = malloc(20);
While writing to this array if you see 20 bytes is already written just do realloc() for the same memory
char *temp = realloc(data_to_send,40);
if(temp != NULL)
data_to_send = temp;

Answer 2

There are multiple issues:
} while( !data); is wrong, you should use : } while( *data != 0);
This can indirectly cause segmentation fault(If you are lucky) making the code loop indefinably.
Memory is not allocated to *parity_bit_array and *data_to_send.
Accessing un-allocated memory is undefined behavior and can cause anything including segmentation fault.
write(fd,data_to_send,sizeof(data_to_send)); should be write(fd,data_to_send,sizeof(*data_to_send)); Or something like that as per your logic.