Running concurrent processes using pipe in C - c

Question

I'm working on an assignment in C aimed at using a pipe to pass variables between two processes. Both processes must be forked from the parent, and they must run concurrently to pass one character at a time (sort of demonstrated below).
The issue I'm having is that the fork()ed processes are not running concurrently. The sender seems to go first, and after running for ~26 seconds the receiver begins. Here is the code I have written:
#include <stdio.h>
int ret;
int pipearray[2];
char buffer[26];
void mysender();
void myreceiver();
int main()
{
int pid = 0;
int i = 0;
ret = pipe(pipearray);
while (i < 2) {
pid = fork();
if ( pid == 0 && i == 0 ) /* child process execution (receiver) */
{
myreceiver();
printf("Your receiver is done\n");
exit(0);
}
else if ( pid == 0 && i == 1 ) /* now executes sender */
{
mysender();
printf("Your sender is done\n");
exit(0);
}
++i;
}
close(pipearray[0]);
close(pipearray[1]);
sleep(30);
printf("Parent function has finished.\n");
return 0;
}
void mysender()
{
char c;
int index = 90;
close(pipearray[0]);
while (index > 64) /* loop for all values of A-Z in ASCII */
{
c = (char) index;
open(pipearray[1]);
write(pipearray[1], c, sizeof(c)); /* Sends letter to pipe */
--index;
sleep(1);
}
close(pipearray[1]);
}
void myreceiver()
{
int index = 0;
close(pipearray[1]);
while(buffer != 'A') /*loop runs until 'A' is handled */
{
sleep(1);
open(pipearray[0]);
read(pipearray[0], buffer, 1);
printf("%s", &buffer);
index++;
if ( index == 26 ) { break; }
}
close(pipearray[0]);
}
Expected Result:
ZYXWVUTSRQPONMLKJIHGFEDCBA
Your sender is done
Your receiver is done
The parent function has finished.
My result:
Your sender is done
The parent function has finished.
Your receiver is done
I'm very new to C programming but I've been banging away at this for a while. Any tips to why these might not be running simultaneously would be very appreciated.

Answer 1

There is many errors in your code. Don't try to open the pipe after fork, it is already open and has no name. Write should use the address of c. Read must read into the right place. A flush must be done after write to stdout. Your conditional must be slightly modified to be guaranteed correct. The parent process must wait its children. Here is the modified code :
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <stdlib.h>
int ret;
int pipearray[2];
char buffer[26];
void mysender();
void myreceiver();
int main()
{
int pid = 0;
int i = 0;
ret = pipe(pipearray);
while (i < 2) {
pid = fork();
if ( pid == 0 && i == 0 ) /* child process execution (receiver) */
{
myreceiver();
printf("Your receiver is done\n");
exit(0);
}
else if ( pid == 0 && i == 1 ) /* now executes sender */
{
mysender();
printf("Your sender is done\n");
exit(0);
}
++i;
}
close(pipearray[0]);
close(pipearray[1]);
// Don't sleep, but wait until the end of the two children
wait(NULL);
wait(NULL);
// sleep(30);
printf("Parent function has finished.\n");
return 0;
}
void mysender()
{
char c;
int index = 90;
close(pipearray[0]);
while (index > 64) /* loop for all values of A-Z in ASCII */
{
c = (char) index;
// try to open a anonymous pipe is a non-sense
// open(pipearray[1]);
// Send a buffer by its address
write(pipearray[1], &c, sizeof(c)); /* Sends letter to pipe */
--index;
sleep(1);
}
close(pipearray[1]);
}
void myreceiver()
{
int index = 0;
close(pipearray[1]);
// Ensure condition is entered first
buffer[index] = 0;
// This is not the best condition ever, but ok.
while(buffer[index] != 'A') /*loop runs until 'A' is handled */
{
sleep(1);
// Don't open an anonymous pipe
// open(pipearray[0]);
// Read at the right position
read(pipearray[0], buffer+index, 1);
// print and flush, could also be printf("%s"...); flush(stdout);
printf("%s\n", buffer);
index++;
if ( index == 26 ) { break; }
}
close(pipearray[0]);
}
Now, consider to remove the sleeps in the reader, as it will be synchronized with the writes such that no read is possible if no write has been done. Alos consider to read more that one byte, because there is no concept of message, so that you can read as much byte you consider necessary to read, and as usual it is better to try reading a bunch of bytes when you can.