C unnnamed pipes and fork for calculation - c

Question

So I'm trying to create program that accepts user input (price for example 50) and then first child passes it to second, second one add 10 (price is now 60), third one then 50 (price is now 110) and 4 one just prints/returns final price. I have fork in loop and I'm creating pipes, but price is always the same, only 10 is added in each child. What is wrong or how to fix so that it's going to work as I want to.
My code:
int main(int argc,char *argv[])
{
int anon_pipe[2];
int n,N=4;
char value_price[100];
if(argc>1)
{
int price=atoi(argv[1]);
printf("%d\n",price);
if(pipe(anon_pipe)==-1){
perror("Error opening pipe");
return -1;
}
for(n = 0; n < N; n++){
switch(fork()){
case -1:
perror("Problem calling fork");
return -1;
case 0:
close(anon_pipe[1]);
read(anon_pipe[0],value_price,100);
price+=10;
sprintf(value_price,"%d \n",price);
printf("Price: %d\n",atoi(value_price));
write(anon_pipe[1],value_price,sizeof(value_price));
_exit(0);
}
}
close(anon_pipe[0]);
sleep(1);
close(anon_pipe[1]);
}
return 0;
}

Answer 1

You seem to think that forking makes the child start from the beginning of the program. This is not the case, forking makes the child start at the same line when the fork() was called
For instance look at this code here:
read(anon_pipe[0],value_price,100);
price+=10;
sprintf(value_price,"%d \n",price);
printf("Price: %d\n",atoi(value_price));
See you increase the value of price but you never read that value form the pipe. So all children will always output +10 to their respective pipe.

Answer 2

You should check the return values of your function calls for error codes. If you had done, you would have detected the error arising from this combination of calls:
close(anon_pipe[1]);
// ...
write(anon_pipe[1],value_price,sizeof(value_price));
Very likely, you would also have detected that many of these calls ...
read(anon_pipe[0],value_price,100);
... signal end-of-file without reading anything. At the very least, you need read()'s return value to determine where to place the needed string terminator (which you fail to place before using the buffer as a string).
As a general rule, it is mandatory to handle the return values of read() and write(), for in addition to the possibility of errors / EOF, these functions may perform short data transfers instead of full ones. The return value tells you how many bytes were transferred, which you need to know to determine whether to loop to attempt to transfer more bytes.
Moreover, you have all of your processes using the same pipe to communicate with each other. You might luck into that working, but it is probable that at least sometimes you'll end up with garbled communication. You really ought to create a separate pipe for each pair of communicating processes (including the parent process).
Furthermore, do not use sleep() to synchronize processes. It doesn't work reliably. Instead, the parent should wait() or waitpid() for each of its child processes, but only after starting them all and performing all needed pipe-end handling. Waiting for the child processes also prevents them from remaining zombies for any significant time after they exit. That doesn't much matter when the main process exits instead of proceeding to any other work, as in this case, but otherwise it constitutes a resource leak (file descriptors). You should form the good habit of waiting for your child processes.
Of course, all of that is moot if you don't actually write the data you mean to write; #SanchkeDellowar explains in his answer how you fail to do that.