Sending data between threads using pipes - c

Question

I have 3 threads - I have to read something in the first one, count characters in the second, and make an output in the third. So I'm using two pipes here; for the 1st - 2nd threads, and 2nd - 3rd.
However, it doesn't work at all. I mean, I can type the string in the console, but then nothing happenes, there's no output.
What's wrong here?
Also, is this possible to add something like "Type string here: " in the first thread somewhere? Adding it in the while loop seems to produce strange results - it displays randomly after running the program :P
Here's the code:
#include <stdio.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <unistd.h>
#include <linux/stat.h>
#include <pthread.h>
#include <string.h>
int first[2];
int second[2];
void *input(void *ptr)
{
while(1)
{
char str[100];
int result;
result = read(STDIN_FILENO, str, sizeof(str));
if(result <= 0)
{
if(result == -1)
perror("read");
close(first[1]);
exit(2);
}
if(write(first[1], str, result) != result)
{
perror("write");
exit(2);
}
}
}
void *countChars(void *ptr)
{
int result, result2, count = 0;
char str[100];
while(1)
{
result = read(first[0], str, sizeof(str));
if(result <= 0)
{
if(result == -1)
perror("read");
close(first[0]);
close(second[1]);
exit(2);
}
if(write(STDOUT_FILENO, str, result) != result)
{
perror("write");
exit(2);
}
while(str[count] != '\0') count++;
write(second[1], &count, sizeof(count));
}
}
void *output(void *ptr)
{
int result2, count = 0;
char str[100];
while(1)
{
result2 = read(second[0], str, sizeof(str));
if(result2 <= 0)
{
if(result2 == -1)
perror("read");
close(second[0]);
exit(2);
}
if(write(STDOUT_FILENO, str, result2) != result2)
{
perror("write");
exit(2);
}
while(str[count] != '\0') count++;
printf("Writer: %d\n", count - 1);
}
}
int main()
{
pthread_t t1, t2, t3;
if(pipe(first) == -1)
{
printf("First pipe error");
exit(1);
}
if(pipe(second) == -1)
{
printf("Second pipe error");
exit(1);
}
pthread_create(&t1, NULL, input, NULL);
pthread_create(&t2, NULL, countChars, NULL);
pthread_create(&t3, NULL, output, NULL);
pthread_join(t1, NULL);
pthread_join(t2, NULL);
pthread_join(t3, NULL);
return 0;
}

Answer 1

You want the countChars function to count until a newline and write the int "count"
void *countChars(void *ptr)
{
int result, result2, count = 0;
char str[100];
while(1)
{
result = read(first[0], str, sizeof(str));
if(result <= 0)
{
if(result == -1)
perror("read");
close(first[0]);
close(second[1]);
exit(2);
}
if(write(STDOUT_FILENO, str, result) != result)
{
perror("write");
exit(2);
}
//while(str[count] != '\0') count++;
while(str[count] != '\n') count++;
write(second[1], &count, sizeof(count));
count = 0;
}
}
And the output function to read the sizeof an int into an int variable.
void *output(void *ptr)
{
int result2, count = 0;
//char str[100];
while(1)
{
//result2 = read(second[0], str, sizeof(str));
result2 = read(second[0], &count, sizeof(count));
if(result2 < sizeof(count))
{
close(second[0]);
exit(2);
}
printf("Writer: %d\n", count);
}
}
This works ok because only countChars is writing to the 2nd pipe. A pipe write/read is atomic up to PIPE_BUF chars and an int is way less than that so the reads are predictable.